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Performance

Performance

Welcome to the Performance section. All questions and answers are unverified regarding correctness. Please report any errors in our Pilot Forum.

  • How does a wet or contaminated runway or downwind affect V1?
  • Initially on a wet runway, what is the most effective method of braking?
  • What is the rejected take-off drill?
  • Your flaps don’t extend on approach to land. What do you do?
  • Would you depart if there were still ice on the aircraft?
  • How would you remove ice from the aircraft?
  • You arrive at work and the conditions are conducive to ice. What would you check?
  • Why do they have more excess thrust?
  • Have you ever been to an airport and noticed that the big twin-engine jets climb at a steeper angle than the four engine jets. Why is this so?
  • Define Vs…
  • How does a wet runway affect V1?
  • What is a balanced field length?
  • Can a stopway be a clearway?
  • Can you taxi on a stopway?
  • What is a clearway?
  • How does flap affect the TORR?
  • How is ASDR calculated?
  • How is TORR calculated?
  • What is Vimd?
  • What is Vdf?
  • What is Vxse?
  • What is Vx?
  • What is Vyse?
  • What is Vy?
  • What is Vmo/Mmo?
  • What is Vne/Mne?
  • What is Vno/Mno?
  • Define Va?
  • Define V2….
  • Define Vr….
  • How would you reduce Vmcg if it were greater than V1?
  • What is V1 dependant on?
  • Define V1?
  • What are the limits of V1?
  • What changes the AoA when an aircraft stalls?
  • Define Vmcg….
  • How does flap affect Vmca?
  • How does C of G affect Vmca?
  • How does weight affect Vmca?
  • What is the configuration for Vmca?
  • Define Vmca…
  • Define Vmu….
  • Tell me what you know about LROPS.
  • What do you know about ETOPS?
  • How do you fly a descent efficiently?
  • What is an ‘optimal cruise….’?
  • What can you say about the ‘optimal climb profile’?
  • How does the landing technique’s affect the total landing distance?
  • What is the ‘Maximum Structural Landing Weight ‘?
  • what is the ‘Landing Runway Weight Limit’?
  • Effective Runway Length for landing – what is it?
  • What is the Obstruction Clearance Plane?
  • Please tell us something about Reverse Thrust.
  • Describe Go-Around Thrust…
  • What is the Vref?
  • What is the ‘Landing Runway Weight Limit’?
  • What is the ‘Landing Climb Weight Limit’?
  • What is the ‘Climb Weight Limit’?
  • What is the ‘Landing Distance’?
  • Describe the Maximum Landing Weight…
  • What is the Enroute Performance Limit?
  • Please explain ‘Gross Take-Off Flight Path’…
  • Clutter and Take-Off Performance. What do you know about it?
  • How does Obstacle Clearance affect the Performance Calculation?
  • What is the effect of wind when calculating T/O performance?
  • The runway gradient (slope), how does it affect the a/c perf.?
  • What is the ‘Effective Takeoff Length’?
  • What is the Take-Off Runway Limit?
  • The maximum Take-Off Weight, with what does it have to comply?
  • What is a clearway, what is a stopway?
  • Tell me about net/gross climb gradients.
  • What is balanced/unbalanced field length?
  • Tell me about the anti-skid operation
  • What is the Take-off Climb Limit?
  • Definition: The Take-Off Path
  • Definition: Required Take-Off Distance
  • Overview Thrust Definitions
  • Rejected Take-off Brake Cooling Chart…
  • Overview Speed Definitions
How does a wet or contaminated runway or downwind affect V1?

A wet V1 may be up to 10kts less than the same dry V1. This provides a longer stopping distance in the event of an RTO, due to the reduced brake effectiveness. Unless a corresponding weight reduction occurs the aircraft will now not be able to make the 35′ [for an EFTO continued] screen height and this may now be reduced to 15′.

Initially on a wet runway, what is the most effective method of braking?

Reverse thrust.

What is the rejected take-off drill?

Maintain directional control
Thrust levers to idle [activated auto-brake and spoilers]
Select reverse thrust
Brake as required

Your flaps don’t extend on approach to land. What do you do?

Follow company failure management procedures.
Conduct missed approach, advise tower of intentions
Check CB’s
Run the checklists
Recalculate LDR by adding 72%
Consult TOLD card for new Vref
Press GPWS override [Config. Warning]

 

Note: If the failure has resulted due to a hydraulics failure, associated failures may include, Nose wheel steering and Landing gear extension

Would you depart if there were still ice on the aircraft?

No.

How would you remove ice from the aircraft?

Tepid water and/or deice fluid.

You arrive at work and the conditions are conducive to ice. What would you check?

Check for ice on the airframe and flying surfaces.

Why do they have more excess thrust?

They need to still meet performance requirements with 50% of their thrust gone.

Have you ever been to an airport and noticed that the big twin-engine jets climb at a steeper angle than the four engine jets. Why is this so?

Have you ever been to an airport and noticed that the big twin-engine jets climb at a steeper angle than the four engine jets. Why is this so? More excess thrust available

Define Vs…

Power on stall speed in the clean configuration.

How does a wet runway affect V1?

A wet runway reduces acceleration performance, for a take off and deceleration performance, for a rejected take-off. Companies may have a policy to reduce V1 on a wet runway allowing the TODR calculation to reduce the 1.2Vs screen height to 15’.

What is a balanced field length?

When ASDR=TODR for an engine failure at V1.

Can a stopway be a clearway?

Yes.

Can you taxi on a stopway?

No.

What is a clearway?

A section at the end of a runway, which can be used for calculating TODA, from lift off to 35’.

How does flap affect the TORR?

TORR increases due to the increase in drag.

How is ASDR calculated?

It is the distance from a standing start to a failure of the critical engine at V1, plus recognition time, to close the thrust levers and bring the aircraft to a complete using full braking. (No reverse thrust is to be taken into account when calculating ASDR)

How is TORR calculated?

It is the distance from a standing start to a failure of the critical engine at V1, multiplied by 1.5 times the take off distance to lift off plus half the distance to 35’.

What is Vimd?

The speed for minimum drag

What is Vdf?

Maximum flight dive speed [highest speed during certification]

What is Vxse?

Single engine best angle of climb

What is Vx?

Best angle of climb

What is Vyse?

Single engine best rate of climb

What is Vy?

Best rate of climb

What is Vmo/Mmo?

Maximum operating speed

What is Vne/Mne?

Never exceed speed. Usually a structural limit.

What is Vno/Mno?

Normal operating speed

Define Va?

The maximum speed at which full and/or abrupt control deflection may be applied without exceeding the limiting load factor.

Define V2….

Take-off safety speed
Achieved at 35’ following a critical engine failure at V1 and rotation at Vr.
Minimum of 1.2 Vs or 1.1 Vmca

Define Vr….

The speed at which the pilot commences rotating.
Minimum of 1.05 Vmca, 1.1 Vs, 1.1 Vmu
The speed at which allows the greater of 1.1 Vmca or 1.2 Vs to be achieved at 35’ with a critical engine failure at V1.

How would you reduce Vmcg if it were greater than V1?

A Reduced thrust take off reduces yaw resulting from an engine failure.

What is V1 dependant on?

ASDA, TODA, aircraft weight, ambient temperature, obstacle clearance, runway slope and runway surface conditions.

Define V1?

Decision speed. The speed at which TODA=ASDA if an engine failure were to occur.

What are the limits of V1?

V1 must be greater than Vmcg and less than Vr.

What changes the AoA when an aircraft stalls?

The centre of pressure moving forward until the stall and then rapidly moving aft and producing a nose down pitching moment at the stall.

Define Vmcg….

The minimum speed at which it is possible to maintain directional control, whilst on the ground, during the take-off roll and following a failure of the critical engine whilst employing full rudder deflection.

How does flap affect Vmca?

Flap increases lift and therefore drag. The wing on the operating engine/s side has more resistance to motion and increases Vmca.

How does C of G affect Vmca?

An aft C of G decreases the moment arm of the rudder and thus it’s effectiveness consequently increasing Vmca.

How does weight affect Vmca?

Nil effect. Vmca and Vmcg is dependant on pressure altitude and temperature.

What is the configuration for Vmca?

Aircraft at MTOW
Most rearward C of G Max 5ï‚°
AoB towards the live engine
Max continuous power on operating engine
Critical engine failed and propeller windmilling or feathered if auto-feather fitted
Full rudder deflection

Define Vmca…

The minimum airspeed at which it is possible to maintain directional control, when airborne and following a failure of the critical engine and whilst employing full rudder deflection.

Define Vmu….

Minimum Unstick Speed. The Minimum airspeed at which airplane can safely lift off ground and continue take-off.

Tell me what you know about LROPS.

Under ETOPS, twin-engine aircraft must use the nearest available diversion airport in case of an emergency. If this is closed, the pilot must fly on to the next closest diversion airfield. Diversions under ETOPS are not normally allowed to exceed 180 minutes in duration.

However many diversion airports in remote and inhospitable areas have poor medical, communication and accommodation facilities to cope with an emergency. They may have extremely challenging runways – especially in poor weather conditions – or they may simply not be equipped to deal with a sudden influx of passengers requiring food, accommodation, medical supplies and clothing suitable for sub-zero temperatures. In a number of areas such as the Polar Regions, there is simply no airport at all.

The new LROPS rules are based on the idea that a flight crew must be able to choose the safest, most appropriate airport and not necessarily the closest. This will ensure a high safety level to all passengers and crews, even in the most extreme regions of the globe. LROPS rules will apply to all flights with diversion times beyond three hours, regardless of the number of engines on the aircraft, and to any flight over a designated ‘extreme’ region.

LROPS-certified twins will be able to exceed the maximum 180-minute diversion time, subject to special permission. However, due to the quantity of reserve fuel necessary to handle a diversion at low altitude with an inoperative engine, a four-hour limit seems the natural economic diversion time for two-engine LROPS aircraft. To achieve such certification, these aircraft will have to be equipped with powerful communication systems, advanced medical equipment and sophisticated fire-fighting capabilities.

For routes across the Himalayas, Airbus’ On Board Oxygen Generation System [OBOGS], now under development for LROPS, will be particularly useful. It will enable a depressurised aircraft to maintain an altitude higher than 10,000ft [3,000 metres]. Since aircraft burn less fuel at higher altitudes, this will require somewhat lower fuel reserves, which in turn will reduce operating costs.

A long-range means of communication, called Aircraft Dependent Surveillance [ADS] system, which uses Global Positioning Satellites [GPS] and radio links, is also being developed. It will provide pilots with a more reliable means of keeping in touch with air traffic controllers in remote and inhospitable areas.

For four-engine aircraft such as the A340 and the A380, there will be no limit on diversion times, while aircraft without LROPS certification will be banned from flying in extreme areas.

While twin-engine aircraft will be capable of flying very long-range routes, they will keep relying heavily on the diversion airports available along these routes. They will also have to demonstrate to the authorities that these airports can offer safe landing and evacuation of occupants as well as their subsequent accommodation and transport to a commercial airport in the worst winter conditions. Therefore Airbus has chosen to develop twin-engine and four-engine aircraft to ensure an optimised solution for each type of operation. Four-engine aircraft will be free from these constraints and able to fly to much more distant airports.

All A340 and A380 aircraft will have LROPS certification with options of up to eight hours of diversion time. Airbus will be offering special LROPS design packages for customers who want to operate such routes with the A340 and A380. Retrofits will also be available for aircraft already in service. The aircraft will operate under a ‘no diversion time’ philosophy and will only be limited on long-range routes by the capacity of their hold fire extinguishers.

What do you know about ETOPS?

Extended Twin Engine Operations
Provision for twin engined to be within a certain flying time of alternate aerodrome:

Suitable: Weather above alternate minima [Engine failure planning]

Adequate: Above landing minima [Depressurised planning]

120-138 min [FAA 1985]
180 min [FAA 1988]
207 min [15% extension to 180 min] [more direct routing (B777)]

180 min ETOPS certification requirements:
Engine system maturity and reliability
Operator experience
Additional maintenance requirements
ETOPS pre-flight inspection [special airworthiness dispatch requirements]
Airworthiness assessment of aircraft type

How do you fly a descent efficiently?

The Descent Problem
A properly timed descent can yield a substantial fuel savings over flight plan values. Once again into a headwind, proceed faster and vice versa. If you can time the descent use idle power all the way to the 500′ AGL point VFR or to the final approach fix for IFR, and don’t spend much time at low altitude with flaps and/or gear extended, you will achieve the most efficient use of the fuel. ATC vectoring, icing conditions, or a leaky pressure vessel could preclude using idle power.

In general, given a descent speed profile, when flying into a headwind, fly a faster TAS by approximately half the headwind for maximum range, and when flying with a tailwind behind you, fly a slower TAS by approximately half the tailwind for maximum range. Typically a profile would consist of maintaining a constant mach number [perhaps the same as the cruise mach number] until the “crossover altitude” after which a constant IAS is used until approach manoeuvring.

Drag
Induced Drag
Parasite Drag – D = 0.5pVS2CD
Compressibility

Thrust Required [D] vs. TAS curve Weight increase

  • curve moves up and minimum moves to the right Altitude increase
  • curve lays back toward the higher speeds and minimum moves to the right Dirty
  • curve moves up and minimum moves to the left Critical Mach number
  • at a slightly lower TAS than the knee where compressibility drag takes off
  • Performance in climb = f [Thrust Available – Drag][V/W] or excess power available at a given TAS
  • Minimum on curve is Max L/D point and minimum Drag and max endurance
  • Max Range at tangent from origin or from a point removed from the origin by the value of the wind, causing a increase [HW] or decrease [TW] in the speed for max range of about 1/2 the HW/TW component.

Specific Range = NAM/1000# of fuel = TAS/FF

  • Specific Range vs. Mach Number
  • Max range is at the peak of curve
  • At a given weight this curve & it’s peak move higher and to the Right
  • Curve is relatively flat at the peak for a relatively large airspeed range
  • Long Range Cruise move along curve until Spec. Range drops 1% and you get a 2%- 10% increase in speed and, therefore, a timesavings for minimal fuel cost.
What is an ‘optimal cruise….’?

The Cruise Problem

Maximum Cruise Thrust – below METO thrust and intended to provide required cruise performance while maximalising engine life Optimum Cruise – the maximum range will be optimum at a certain altitude where winds and engine performance are optimum – once again as in the climb, if you are proceeding against a headwind a faster TAS by approximately 1/2 the wind component yields better range and vice versa for a tailwind.

Long Range Cruise – move along curve until Specific Range [nm/lb of fuel] drops 1% and you get a 2% – 10% increase in speed and, therefore, a time savings for minimal fuel cost increase.

Buffet Boundaries – depending on your weight, the maximum altitude that you can attain will be determined by the buffet boundaries at various G-levels due to turbulence, normally 1.2 to 1.6 1.

1.Low speed buffet – stall or g-buffet -caused by separation of flow over the top of the wing due to high angle of attack – remember that your IAS at stall increases with g-loading [essentially increased weight] when you encounter varying degrees of clear air turbulence. 2. High speed buffet – mach buffet – caused by separation of airflow over the top of the wing or any portion of the airframe as the speed of sound is approached by the aircraft – shock formation can begin as low as Mach .77 on some aircraft – it is annoying to passengers and causes a huge increase in drag which causes higher fuel usage – the higher the altitude, the lower the IAS at which it buffet occurs for a given weight aircraft, since the speed of sound decreases for a higher altitude where the temperature is lower and TAS is higher for a given IAS – Weight also has an effect as it requires higher AOA and therefore faster airflow over the top of the wing or fuselage.

Therefore, as altitude is increased or weight is increased, the spread between low & high-speed buffet decreases. For a given weight, there will be an altitude restriction. For a heavy aircraft flying a long distance, the desired altitude may only be reached later in the flight after a series of “step climbs” as fuel is burned off.

What can you say about the ‘optimal climb profile’?

The Climb Problem
Maximum Climb Thrust
Best Climb Angle
Best Rate of Climb
Optimum Climb Speed – balance between 3 competing goals

  • Getting to altitude as quickly as possible – jet engines burn less fuel at the higher altitudes in cruise
  • Using the minimum fuel in the climb
  • Travelling as far as possible in the climb

Note: most airlines try to minimize costs by considering crew costs and maintenance costs [time on the airframe] as well as minimizing fuel usage. By flying faster, we can increase fuel costs but decrease time on the airframe and engines and how much they have to pay the crew.

In general, given a climb speed profile, when flying into a headwind, fly a TAS faster by approximately half the headwind for maximum range, and when flying with a tailwind behind you, fly a TAS slower by approximately half the tailwind for maximum range. Typically a profile would consist of maintaining a constant IAS until the “crossover altitude” after which a constant mach number is used until level off.

How does the landing technique’s affect the total landing distance?

Threshold crossing height – The target is 50′ over the end of the effective runway length. If you are 100′ above instead, this will add approximately 100′ to the landing distance when using the typical 3-degree glide slope.

Flare Technique – trying to make the perfect grease job landing could increase the landing distance considerably more than the high approach as well as increase the chances for a tail strike. 3. Touchdown speed faster than VREF – Since the object is to dissipate kinetic energy, the faster the touchdown speed, the longer the stoping distance. The equation for kinetic energy is [mV2]/2. A 10% increase in weight or mass will yield about a 10% increase in landing roll, whereas a 10% increase in landing speed will cause about a 20% increase in the landing roll.

Compounding the problem – If on the typical airliner, you were to cross the threshold 30′ high and 5 kts fast, you would use up 1/2 of the 40% of the runway that is your margin of safety

What is the ‘Maximum Structural Landing Weight ‘?

The aircraft maximum certified landing gross weight is set by the manufacturer based on structural considerations.

what is the ‘Landing Runway Weight Limit’?

Landing Runway Weight Limit – Turbojets must land within 60% of the effective runway length at both the destination and at the alternate if the runway is dry and braking action is good or better. Landing distance is a function of weight, temperature, pressure, wind, runway slope and approach speed as well as runway braking action. Speed not less than 1.3 Vso at 50′ height above runway. Flaps, speed brakes and wheel brakes used [reverse thrust not considered].

The landing distance increased 15% if landing runway on wet or slippery.

Typically a weight penalty is taken when the runway is wet or when the visibility is low.

Effective Runway Length for landing – what is it?

Effective Runway Length for landing – measured from the point that the obstruction clearance plane intercepts the runway near the approach end to the far end of the runway. Usually it is the entire runway length. However, if there is an obstruction that rises high enough near the approach end, the effective runway length would be shortened from the entire runway length.

What is the Obstruction Clearance Plane?

The Obstruction Clearance Plane – a 20:1 sloped plane that usually intercepts the end of the runway and through which no obstacle protrudes. The aircraft on approach maintains a minimum of 1.3 Vso at least 50′ above this obstruction clearance plane. It must begin 1500′ prior to the intersection with the runway.

Please tell us something about Reverse Thrust.

Through the use of blocking doors and deflectors the jet engine’s exhaust flow is directed at an angle forward to slow the aircraft after touchdown. On a turboprop aircraft, the same effect is accomplished by changing prop blade angle to a negative angle. Reverse thrust is not considered in performance calculations for either landing or accelerate-stop.

Describe Go-Around Thrust…

Basically the same as Take-off thrust except that the EPR or N1 value is different based on the fact that the aircraft is at a much higher velocity.

What is the Vref?

VREF – Landing Reference Speed is the minimum CAS at the 50′ height in a normal landing. This speed is equal to 1.3 times the stall speed in the landing configuration [VSO]. There are typically two landing flap settings, the greater of which is typically not used due to the high fuel burn associated with it.

What is the ‘Landing Runway Weight Limit’?

Landing Runway Weight Limit – Turbojets must land within 60% of the effective runway length at both the destination and at the alternate if the runway is dry and braking action is good or better. Landing distance is a function of weight, temperature, pressure, wind, runway slope and approach speed as well as runway braking action.

Speed not less than 1.3 Vso at 50′ height above runway.

Flaps, speed brakes and wheel brakes used [reverse thrust not considered].

The landing distance increased 15% if landing runway on wet or slippery

Typically a weight penalty is taken when the runway is wet or when the visibility is low.

Maximum Structural Landing Weight – The aircraft maximum certified landing gross weight is set by the manufacturer based on structural considerations.

Structural Limit -The aircraft maximum certified take-off gross weight is set by the manufacturer based on structural considerations.

What is the ‘Landing Climb Weight Limit’?

Landing Climb Weight Limit – [Go-around in approach configuration with all engines] – Typically not limiting. – In the landing configuration a go-around can accomplished with:

  • Three engines operating.
  • Thrust that is available 8 seconds after throttle movement from idle to take-off position.
  • Climb gradient not less than 3.2%. Climb speed not to exceed 1.3 VS.
What is the ‘Climb Weight Limit’?

Climb Weight Limit [or Landing Performance Weight Limit] – ability to perform a missed approach with loss of the most critical engine and:

Go-around thrust on remaining engines, gear up, flaps at go-around setting.

Go-around speed:

  • < 1.5 VS [go-around flap setting
  • < 1.1 VS [approach flap setting]
  • Minimum climb gradient of 2.1% for 2-engine, 2.4% for 3-engine, and 2.7% for 4-engine aircraft
  • This is a function of weight, temperature, pressure, and engine bleeds as well as approach flap position
What is the ‘Landing Distance’?

Landing distance – begins at 50′ over where the effective runway length begins, and continues to the point where the aircraft touches down and then continues on to the point at which the aircraft comes to a complete stop using maximum braking and speed brakes only. No credit was taken for reverse thrust, which can be used for an additional margin of stopping ability.

Approach Climb Weight Limit [or Landing Performance Weight Limit] – ability to perform a missed approach with loss of the most critical engine and:

Go-around thrust on remaining engines, gear up, flaps at go-around setting

Go-around speed:

  • < 1.5 VS [go-around flap setting]
  • < 1.1 VS [approach flap setting]

Minimum climb gradient of 2.1% for 2-engine, 2.4% for 3-engine, and 2.7% for 4-engine aircraft

This is a function of weight, temperature, pressure, and engine bleeds as well as approach flap position.

Landing Climb Weight Limit [Go-around in approach configuration with all engines] – Typically not limiting. – In the landing configuration a go-around can accomplished with:

  • Three engines operating.
  • Thrust that is available 8 seconds after throttle movement from idle to take-off position.
  • Climb gradient not less than 3.2%. Climb speed not to exceed 1.3 VS.
Describe the Maximum Landing Weight…

Maximum Landing Weight – at the Destination and Alternate Airports. The allowable weight for take-off from the departure point must be limited so as to comply with the following approach climb limit, the landing performance limit and the structural limit at the destination airport assuming a normal rate of burn of fuel to destination without dumping fuel.

What is the Enroute Performance Limit?

To establish the maximum allowable gross weight for any given flight, the performance of the airplane must be related to the terrain over which it is to be flown. Consideration must be taken of the possibility of engine failure en route and the resulting performance deterioration, to effect a safe landing after either one or two engine failures. The aircraft in the event of a loss of an engine at any point enroute over mountainous terrain should be able to achieve a landing at an enroute or drift down alternate. Basically, the route over the mountainous area must be segmented so that each segment, in the event of an engine failure, offers an adequate airport as an alternate such that descent can be made with 2000′ obstacle clearance within 5 miles either side until arrival at the alternate with a positive climb capability 1500′ above the airport [all at METO power and assuming 100 kts headwinds no matter which direction to the alternate airport].

Method 1 Dispatch – at that weight, the aircraft has adequate engine out capability to clear all obstacles within 5 nm by 1000′ with a 300 fpm climb rate at that clearing altitude.

Method 2 Dispatch – at that weight, the aircraft with engine out cannot clear all obstacles, but by dividing the route up into several segments, on each segment there is an adequate airport with alternate weather minimums to use in case of engine loss during that segment.

Please explain ‘Gross Take-Off Flight Path’…

From 400′ height to the end of the final segment must have a climb gradient of not less than 1.5%. The Net Take-Off Flight Path – Is a profile starting at reference zero, having a gradient 0.9% below the actual take-off flight path. The net flight path must clear all obstacles by 35′ vertically or 200′ horizontally within the airport boundary and 300’ horizontally outside the airport boundary. Since there is no means for a pilot to determine his gradient of climb while in flight, it is important that he observe quite closely the prescribed techniques and airspeeds, particularly during the early stages of flight, to assure obstacle and terrain clearance in the event of engine failure.

Clutter and Take-Off Performance. What do you know about it?

Most 121 operations allow take-off with certain depths of standing water, slush [1/8″], or snow [wet 1/4″ or dry 1″]. These are referred to as clutter and affect both V1 as well as maximum weight you may take-off with. Clutter has greater and greater effect as the aircraft builds up speed. Clutter will make it harder to stop during an abort due to the slippery runway and will slow acceleration for the take-off due to the drag of the bow wave and the tires. It may also affect the climb limit on some aircraft where cycling of the gear is required after take-off. To compensate for clutter, both a weight reduction and a V1 reduction are required.

How does Obstacle Clearance affect the Performance Calculation?

The effective length of a runway may also be reduced by the presence of obstacles in the takeoff flight path. The takeoff gross weight of the airplane must be limited so that all obstacles not cleared by at least 300 feet horizontally will be cleared vertically by at least 35′ by the “net” flight path. The “net” flight path for takeoff is derived by subtracting 0.9% gradient from the actual climb out path the airplane is capable of flying, thus producing conservative data.

What is the effect of wind when calculating T/O performance?

The effect of a headwind in shortening the takeoff distance may be considered, but in doing so, only one-half of the wind component parallel to the runway may be used. For a downwind takeoff, 150% of the reported tailwind must be taken into account. Additional conservatism is provided in that wind data is measured 33ft resp. 10m above the runway, whereas the effective wind at runway level will be somewhat less due to ground friction, obstacles and so on. Since this is automatically built into Airport Analyses and performance charts, crews need use only the reported wind.

The runway gradient (slope), how does it affect the a/c perf.?

Account must be taken for the effect of runway slope on acceleration, stopping distance and climb out to 35′. Uphill grades increase the ground run to reach takeoff speed, but improve stopping distance; overall, more distance is required to reach the 35′ elevation. The reverse is true of down grades.

What is the ‘Effective Takeoff Length’?

Effective Takeoff Length – In determining the effective length for takeoff of any particular runway, many factors require consideration: Runway Length – Normally, the length available will be limited to the paved area of the runway. In some cases, however, an area at the far end of a given runway may be designated as a “stopway” which can be used for rollout in the event a takeoff is aborted. Also, some runways may have areas beyond the far end designated as a “clearway plane” which will provide obstacle clearance while accelerating to a safe climb speed while achieving 35′.

What is the Take-Off Runway Limit?

Takeoff Runway Limit – In determining the maximum allowable gross weight for takeoff for any given runway, the performance of the airplane must be related to the dimensions of the airport; that is, the required takeoff distance for the gross weight must not exceed the effective takeoff length available. = f [p, T, engine bleeds, wind, runway slope, clutter, with aircraft components in correct working order according to the MEL except that the most critical engine is lost at V1]

The maximum Take-Off Weight, with what does it have to comply?

Maximum Take-Off Weight – the lowest of 5 possibilities In order to achieve compliance with the regulation, the take-off gross weight for any given flight must not exceed the lowest of the maximum weights allowed for:

  • Compliance with takeoff runway requirements

    Compliance with takeoff climb requirements

  • Compliance with en route performance requirements [drift-down in mountainous areas]
  • Compliance with maximum landing weight taking into account normal fuel burnout enroute and figuring the most restrictive of the Landing Runway Limit, the Approach Climb Limit, the Landing Climb, and the Maximum Structural Design Landing Limit
  • Maximum Structural Design Take-off Limit of the airplane
What is a clearway, what is a stopway?

Clearway – is an area beyond the runway no narrower than 500′ wide. The clearway is expressed in terms of a clearway extending from the end of the runway with an upward slope not exceeding 1.25%, above which no object nor any portion of the terrain protrudes, except that threshold lights may protrude above the plane if their height above the end of the runway is not greater than 26″ and if they are located to each side of the runway.

Note: For the purposes of establishing takeoff distances and takeoff runs, the clearway plane is considered to be the takeoff surface.

Clearway Plane – is the clearway beyond the end of the runway, up to 1/2 the length of the runway, that can be used in the accelerate-go part of the take-off during which time the aircraft [after losing the most critical engine] accelerates to V2 and reaches 35′. Therefore, the take-off distance is considered to be 1 1/2 times the length of the runway. Of course, the ground roll portion of the take-off must not exceed the length of the runway.

Stopway – is an area beyond the runway, not less in width than the width of the runway, centrally located about the extended centreline of the runway, and designated by the airport authorities for use in decelerating the airplane during an aborted takeoff. To be considered as such, a stopway must be capable of supporting the airplane during an aborted take-off without inducing structural damage to the airplane.

Note: The use of clearways and stop ways [where existing] are allowed by the regulations, but used only in special cases by the airlines.

Tell me about net/gross climb gradients.

Gross Gradient is the demonstrated ratio expressed as a percentage of [Change of Height] / [Horizontal Distance Travelled]

For Instance: A climb gradient of 3.0% means an increase in altitude of 3′ for every 100′ forward travel.

Net Gradient is the demonstrated gross climb gradient reduced by the decrement required by regulation.

What is balanced/unbalanced field length?

Balanced Field Length – The condition where the take-off distance or accelerate-go distance is equal to the accelerate-stop distance. This distance must not exceed the length of the runway. It is determined by the selection of V1 speed. For a given set of ambient conditions and aircraft weight, only one value of V1 would cause these distances to be equal and also less than or equal to the associated runway length. This is called the balanced field length and is the minimum required for take-off. Selecting a lower value for V1 reduces the accelerate-stop distance, but increases the accelerate-go distance, whereas the selection of a higher V1 would have the exact opposite effect. Most operations calculate take-off performance based on balanced field length [i.e. stop-ways and clearway planes are not utilized in most cases].

Unbalanced Field Length – The condition where the take-off distance and accelerate-stop distance are not equal. Perhaps we are using a stopway in the calculation of the accelerate-stop distance and a clearway plane in the calculation of the accelerate-go distance. Unbalanced field calculations are use in 2 cases where the stopping ability is degraded due to clutter or anti-skid inoperative. In these cases, the maximum runway limit weight is first decreased to give a balanced field distance that is much shorter than the runway length. Therefore, both the accelerate-stop and accelerate-go distance will be shorter. Next, the V1 is reduced to make the accelerate-stop shorter than the accelerate-go distance, thus unbalancing the field to the side of safety for stopping.

Tell me about the anti-skid operation

Anti-skid brakes give maximum stopping capability in the case of an aborted take-off. If they are inoperative, V1 as well as the maximum take-off weight will be affected. Also, operation in clutter is prohibited. Anti-skid braking action provides two protective features:

Anti-skid Protection – Tacho generators are used on each main wheel to sense wheel rotation speed at any given moment. A computer is used to analyse the rotational speed data from moment to moment and knows when the maximum rate of deceleration that would send that wheel into a skid is exceeded. Before this rate is exceeded, the correct amount of the hydraulic brake pressure is released to the hydraulic return line to prevent the deceleration of the wheel rotation to be too great. This occurs despite the fact that the pilot is pressing the master brake pistons with the toe brakes as hard as he can. This protection engages at about the rotational speed that gives 10-12 kts of forward motion of the aircraft.

 

Locked Wheel Protection – Each tacho generator for a wheel on the right side is paired with a tacho generator for a wheel on the left side of the aircraft. The computer compares these two tacho generator outputs continuously. If the wheel on one side goes through a puddle, the braking will easily bring that wheel to a complete stop causing hydroplaning despite the first protective feature. The locked wheel protection will remove all hydraulic pressure from the locked wheel that is stopped so that it is not “locked” by braking action when it reaches the other side of the puddle, protecting against tire damage.

What is the Take-off Climb Limit?

The maximum weight for take-off from any given runway may be limited to allow airplane performance equal to certain minimum climb gradients on two engines, assuming the critical engine to have failed at V1 speed and the take-off continued. For the climb it is assumed the aircraft will not be banked before reaching 50′ and thereafter no more than 15 degrees AOB until completion of climb segments.

Definition: The Take-Off Path

The Take-Off Path – is the accelerate-go path to 35′ plus the flight path climb profile on a take-off with the most critical engine failure occurring at V1 speed. The path extends from the standing start to a point in the take-off where a height of 1,500′ above the take-off surface is reached [jet airport traffic pattern altitude], or to where transition from take-off to en route configuration is complete, whichever is higher. For performance specifications, the FAA divides the climb into only 3 segments, gear down climb, gear up climb and final segment. However, the manufacturers divide the take-off flight path further into 5 segments for the certification process:

1st Segment – Starts at 35′ height [reference zero] and ends when gear retraction is complete. The weight may not be in excess of that which will allow 0.3% climb gradient.

2nd Segment – Starts when gear retraction is completed and ends at height of not less than 400′ above the take-off surface [most operations use 800′ however]. The weight may not be in excess of that which will allow a climb 2.4% for 2 engine, 2.7% for 3 engine and 3% for 4 engine gradient with the remaining engines at take-off thrust, the flaps at the take-off setting and the airplane flown at V2 airspeed

3rd Segment – Starts at not less than 400′ height [most operations use 800′ however] and continues until flaps are retracted. The climb gradient must be at least 1.2% for 2 engine, 1.5% for 3 engine and 1.7% for 4 engine.

4th Segment – Starts at end of flap retraction and continues until acceleration to V2 + 50 knots where the thrust is reduced after the 5 minute maximum time limit at take-off thrust is reached. The climb gradient must be at least 1.5%.

Final Segment – Extends to a gross height of 1500′ AFE [above field elevation] or more, at a constant speed of V2 + 50 knots, flaps up, with maximum continuous thrust. Most use 3000′ or 2500′ the typical top of class D airspace.

Definition: Required Take-Off Distance

Required Take-Off Distance – is the longer of the following distances:

3 Engine Take-Off Distance: The total of the distance required to:

– Accelerate with all engines at take-off thrust to 35′ height above the runway at V2 +10
– Plus a 15% margin.

Accelerate-Stop Distance: The total of the distance required to:

 

  • Accelerate, with all engines at take-off thrust from a standing start to Take-off Decision Speed, V1
  • Make a transition from take-off to idle thrust, decelerate, and;
  • Bring the airplane to a full stop within the length of the runway [or runway plus stopway] remaining.

Note: In the certification tests that were conducted to determine the accelerate-stop distance, stopping distance was based on the drag from the take-off wing flap setting, speed brakes, and maximum wheel braking. No credit was taken for reverse thrust, which can be used for an additional margin of stopping ability. Certification does not take into effect the runway composition or contamination with reverted rubber, or crosswind considerations.

2 Engine Take-Off Distance: The total of the distance required to:

  • Accelerate to V1, as above;
  • Continue with one engine inoperative to a rotation speed VR at which time the nose wheel is raised off the ground, then;
  • the aircraft must leave the runway by the end of the runway, and:
  • Climb out through 35′ height achieving the take-off safety speed V2.

It can be seen from the above that, except for an aborted take-off, the Take-off Distance consists of two parts, a ground run and an air distance. The ground run is the distance from the start of take-off to lift-off. The air distance may be either:

  • The distance required to reach a height of 35′ after lifting off with one engine inoperative

Or

  • 115% of the distance required to reach a height of 35′ from the lift-off point with all three engines operating.
Overview Thrust Definitions

T – Thrust. Thrust actually decreases as airspeed increases. You may add thrust with throttle advancement to reset the maximum allowed as you accelerate down the runway.

Maximum Take-off Thrust – Maximum set by the manufacturer the crew calculates this before each take-off – it has a 5-minute limit that is mandatory.

EPR – Engine Pressure Ratio – PT7 / PT2 [total pressure in the exhaust / total pressure in the intake] – used by some engine manufacturers to set thrust

N1 – The rotational speed [in % of maximum designed] of the low-pressure compressor of the jet engine – these engines idle at approximately 50% N1. – Used by other engine manufacturers to set thrust since near the high RPM range, the thrust vs. N1 curve is approximately a straight line.

Flat Rating of jet engines – jet engine thrust has a relationship with temperature that is not strictly linear. Therefore, they are certified to guarantee delivery of the maximum thrust up to a certain temperature specified in the aircraft limits section.

Reduced Thrust Take-off – On cold days, with a light load, and when using a long runway, the maximum thrust will give more than sufficient acceleration and climb out capability for the weight of the aircraft. The take-off thrust setting may be reduced to save on engine wear by selecting a higher temperature by which to calculate the thrust setting. This higher temperature is called the assumed temperature and would serve to increase the balanced field length to the length of the runway. Analysis shows that using reduced thrust for take-off has the effect of increasing the time for the take-off and therefore slightly increases fuel usage. However, the advantage of reduction in engine failures and reduction in overall engine operating costs are far more significant. A 1% average thrust reduction yields a 5% reduction in engine operating cost as well as failure rate! Remember for safety sake, that beyond V1, if the engine fails, you should increase thrust to maximum for best performance.

Note: Reduced thrust is never used when the antiskid or any other component required for stopping ability is inoperative or if clutter exists that would decrease acceleration or create a slippery runway for stopping.

METO Thrust – maximum except for take-off thrust also known as MCT or maximum continuous thrust. This is the thrust that the engine is reduced to after flaps are raised at the first power reduction. It is a mandatory maximum developed by the engineers.

Maximum Climb Thrust – below METO thrust and intended to provide required climb performance while maximalising engine life – Climb thrust is usually set at 1500 – 3000′ AGL on the climb out after the final segment. – A recommended setting by the manufacturer.

Maximum Cruise Thrust – below Maximum Climb Thrust and intended to provide required cruise performance while maximalising engine life – a recommended setting by the manufacturer.

Rejected Take-off Brake Cooling Chart…

For any aborted take-off, the wheel tire assembly will be heated up. This chart provides for a cooling time to allow heat to dissipate before another take-off is attempted so that the faceable plugs will not reach their activation temperature or tires will explode. Faceable plugs are several channels arranged circumferentially around the wheel and filled with a solder-like material that is normally hard, but which turns to liquid at it’s peak temperature and flows out of the channel allowing the nitrogen to exit the tire.

Overview Speed Definitions

CAS – Calibrated Air Speed is the indicated airspeed of an aircraft, corrected for position and instrument error. Calibrated airspeed is equal to true airspeed in standard atmosphere at sea level.

EAS – Equivalent Air Speed is the calibrated airspeed of an aircraft corrected for adiabatic compressible flow for the particular altitude. Equivalent airspeed is equal to calibrated airspeed in standard atmosphere at sea level. The closer to the Transonic Region of airflow [0.75 – 1.25 Mach], the less the air around the aircraft acts like an incompressible fluid as in subsonic flow.

IAS – Indicated Air Speed is the speed of an aircraft as shown on its Pitot static airspeed indicator calibrated to reflect standard atmosphere adiabatic compressible flow at sea level, uncorrected for airspeed system errors.

VLOF – Lift Off Speed is the airspeed at which the airplane first becomes airborne.

VMBE – Maximum Brake Energy Speed is the highest speed from which the airplane [at maximum certified take-off gross weight and unfavourable conditions of temperature, pressure & winds] can be brought to a stop without exceeding the maximum energy absorption capability of the brakes. Maximum brake energy speed is compared in take-off planning to V1 speed. The certification process consists of long taxi with numerous stops to warm up the brakes and then an aborted take-off when reaching the proposed VMBE. The brakes catching fire is OK as long as the fire is contained in the wheel wells for the first 5 minutes. Presumably, the fire trucks can be used to put it out after that. Remember that the wheels are made of a magnesium alloy with the brakes recessed in one of the wheel halves and the flexing of the tire sidewalls during wheel rotation also builds up heat.

 

VMCA – Minimum Control Speed, Air is the minimum airspeed at which, when the critical engine [with a jet like the DC-10 or B-727, this would be either wing mounted engine] is suddenly made inoperative, it is possible to recover control of the airplane with that engine still inoperative, and maintain straight f1ight, either with zero yaw, or with an angle of bank of not more than 5 degrees with the remaining engines at take-off thrust. VMCA may not exceed 1.2VS.

VMCG – Minimum Control Speed, Ground is the minimum airspeed on the ground at which the take-off can be continued using aerodynamic controls alone with the critical engine failed and the remaining engines at take-off thrust. Nose wheel steering [NWS] is not allowed in the determining of VMCG.

VMU – Minimum Unstick Speed is the airspeed at and above which the airplane can safely lift off the ground and continue the take-off and not display any hazardous characteristics. This speed is below VR.

VS – Stalling Speed is the stalling speed or the minimum steady flight speed at which the airplane is controllable.

VS0 – Stalling Speed in Landing Configuration is the stalling speed or the minimum steady flight speed in the landing configuration.

V1 – Take-Off Decision Speed is the speed at which when an engine failure is recognized, the distance to continue the take-off to a height of 35′ will not exceed the usable take-off distance, or the distance to bring the airplane to a full stop will not exceed the accelerate-stop distance available.

  • V1 will not be less than VMCG or
  • Greater than VR

Or

  • Greater than maximum brake energy speed [VMBE]

Note: this is based on only average piloting skills and the call-out by the pilot not flying is usually made 2-3 kts early to account for reaction time of the average pilot.

VR – Rotation Speed is the speed at which rotation of the airplane is initiated by lifting the nose wheel off the ground. VRoccurs before lift-off, but is selected to provide lift-off and climb speeds with safe margins above the minimum control and stall speeds and will allow reaching V2 before reaching a height of 35′ above the take-off surface. By definition VR cannot be less than:

  • V1 speed, or
  • 105% of minimum control speed in the air [VMCA], or
  • A margin above the minimum speed at which the airplane can be made to lift off the ground and continue the take-off without displaying any hazardous characteristics. This speed is referred to as minimum up-stick speed, [VMU].

V2 – Take-off Safety Speed is the speed at which the airplane should be flown after lift-off in the event an engine fails at or subsequent to reaching V1 speed during the takeoff run. This speed provides the necessary climb gradient for obstacle clearance with an engine failed. V2 must be attained at or prior to the 35′ height. By definition it must not be less than:

  • 110% of the minimum control speed in the air [VMCA], or
  • 120% of the idle thrust stall speed with flaps at the take-off setting.

In the case of the four-engine take-off, the speed resulting at the 35′ height will be higher than V2 due to the greater acceleration available from the same rotation speed used to establish the three-engine take-off distance. Thus, if an engine fails during the take-off run, and the take-off is continued, the pilot is assumed to fly as close to V2 speed [never below] as possible. If no engine failure occurs, he may allow the airplane to climb out at higher speeds.

V2+10 – the target speed for a normal takeoff [without the loss of an engine]

VREF – Landing Reference Speed is the minimum CAS at the 50′ height in a normal landing. This speed is equal to 1.3 times the stall speed in the landing configuration [VSO].