V1 VR VLOF V2/35 ft
Recognition Time Decision Time Brake
Release Full Stop
Accelerate Stop Distance Required
This is the Decision Time
The aircraft is braked to a full stop. The distance between the Brake Release point and the Full Stop Point is the Accelerate Stop Distance Required.
All Engines Operating The aircraft accelerates with all engines operating until 2 seconds after V1.
After this 2 second delay the engines are throttled back and the aircraft braked to a full stop. The distance between the Brake Release point and the Full Stop Point is the Accelerate Stop Distance Required.
The differences between the two cases are:
¾ In the Engine Failure case the distance to V1 is longer when the critical engine
has failed.
¾ In the All Engines Operating case:
¾ The distance covered by the 2 seconds after V1 is longer, and
¾ The speed of the aircraft will be higher than in the Engine Failure case The longer distance can be found by trials and this is taken as the Accelerate Stop Distance Required. To assist in the calculation the following assumptions are allowed:
¾ Means other than wheel brakes may be used provided that they are safe and reliable
¾ Consistent results must be obtained under normal operating conditions ¾ Exceptional piloting skills are not required to control the aircraft
Surprisingly, thrust reversers are not found to satisfy the other means of braking requirements. In all approved flight manuals, the Accelerate Stop Distances calculated are not based on the use of thrust reversers.
2 Second Delay
V1
Accelerate Stop Distance Required Brake
Aircraft Rotation
The calculation of both TORR and TODA assume that the aircraft is rotated at the correct speed. Early and late rotation changes the TODR data as shown in the table below.
Rotate Early Rotate Correct Rotate Late
Rotate Too Slow TODR Increase TODR Increase TODR Increase Rotate Correctly TODR Increase Minimum TODR TODR Increase
Rotate Too Fast TODR Increase TODR Increase TODR Increase If the aircraft is rotated early:
¾ Additional induced drag is induced which,
¾ Causes a reduction in acceleration and increases the ground run
¾ The lift-off speed will be lower than normal which gives the aircraft less climb ability
If the aircraft is rotated late the ground run is increased but the climb ability is good.
When rotation is made early or late the TOR and TODR are increased. Rotating early is the worst case scenario because:
¾ The ground run increases ¾ The climb gradient decreases
Where an aircraft is rotated too slowly to the correct angle then the TODR will increase. Rotating too quickly increases the profile drag resulting in an increase in TODR.
Margins are always built into performance calculations. The TOD with one engine failed using a rotation speed 5 knots below the correct speed must not exceed the corresponding TOD using the correct rotation speed.
Remember that V2 must be achieved within the required TOD. V2 is the minimum speed that
gives the aircraft the required climb ability when one engine becomes inoperative. Balanced Field Operations
V1 Range The decision speed, V1, is selected within a speed range which is between:
¾ VMCG Minimum control speed on the ground, and
The length of the range between VMCG and VR depends on how large the take-off mass is with
respect to the runway limited take-off mass. The V1 selected is the highest speed at which the
take-off may be aborted in case of an engine failure. Where an engine failure is recognised above V1 the take-off must be continued.
Where V1 has a low value then the ASD will be low.
This is because braking will be started at a relatively low speed. Less distance is needed to accelerate to V1 which in turn gives the relatively short ASD. During braking, less kinetic
energy has to be dissipated which will also aid in giving a shorter braking distance.
Low V1 values give a relatively long TOD because the acceleration from V1 to VR has to be
carried out with one engine failed. VR is fixed at a given aircraft mass and therefore the larger
the speed interval between V1 and VR the longer the distance needed for the aircraft
acceleration.
Balanced Take-Off Field Length
A mass increase increases both TODR and ASDR. The maximum take-off mass for any given runway length is calculated by selecting a V1 so that TODA and ASDA are the same.
Where TODR and ASDR are the same then we have a Balanced Field Length Take-Off. The balanced field length take-off method is used as it allows the highest take-off mass for the available runway.
VMCG V1 VR
V1 VR
Contaminated and Wet Runways Regulations and Definitions JAR 25 requires that:
¾ The approved flight manual contains information regarding the field length required for take-off. The data has to be based on take-off data from a dry hard surfaced runway.
¾ The operator has to supply the documentation which shows how to correct the approved flight manual for other runway conditions such as wet runways or runways covered with ice or snow.
In the UK regulations require the manufacturer to provide guidance information for the operator to enable the calculations to be made.
¾ The manufacturer is required to provide guidance information concerning take-off from wet and contaminated runways
JAR 25 contains advisory material to help the manufacturer present the material The main distinction made in JAR 25 is that between wet and contaminated runways:
Wet Runway A wet runway is a runway well soaked with water, but without significant areas of standing water
Required Runway V1 ASD TOD TOD<ASD TOD>ASD TOD = ASD V1<V1B V1B V1>V1B
Contaminated Runway A contaminated runway is where more than 25% of the runway surface area, within the required length and width being used is covered by:
¾ Surface water more than 3 mm deep, or loose snow or by slush equivalent to more than 3 mm of water
¾ Snow compressed into a solid mass which resists further compression and will hold together or break into lumps if picked up
¾ Ice, including wet ice, causing a low braking action Wet Runways
Calculation of the field lengths for take-off from wet runways are based on the following data: VSTOP Defined as the highest V1 from which the aircraft can be stopped within the
ASDA.
VGO Defined as the lowest V1 from which a continued take-off is possible within
the TODA.
Where an engine failure occurs between VSTOP and VGO then the speed will be:
¾ Too high for the take-off to be aborted and the aircraft stopped before the end of the runway
¾ Too low for a continued take-off for the aircraft to reach 35 ft at the runway end Where V1 and VSTOP are the same, this leads to a longer TOD for a given mass. There is a
possibility of compensating for this by reducing the required 35 ft screen height to 15 ft at the end of the available TOD.
For a wet runway some safety margins in the TOD are reduced or even sacrificed in order to avoid large reductions in take-off mass. For an aborted take-off the margins are not reduced. The aircraft will still stop on the remaining runway should a take-off be rejected at V1, a speed
designated as VWET.
If the screen height is lowered to 15 ft for a wet runway it is possible that the allowable take- off mass may be higher than that for a dry runway. JAR states that the maximum allowable take-off mass be that calculated for a dry runway. In addition, the All Engines Take-Off Distance must still meet the requirements for a dry runway.
1.15 x TOD to 35 ft The required TOD is the longer of:
¾ All engines dry runway TOD Contaminated Runways
Where an aircraft takes-off from a contaminated runway there will be a reduction in the safety margins. Because of this, take-off from a contaminated runway should be avoided if at all possible.
Contamination can cause:
¾ Reduced braking friction ¾ Increased rolling resistance
¾ Risk of aquaplaning/hydroplaning where there is standing water
The operator has to develop data for operations from wet and contaminated runways. The JAR requirements do not require specific flight testing to determine take-off data. Guidance material is published to enable the necessary calculations.
Optimum Take-Off
When the optimum speed and flap setting are chosen for take-off then the maximum take-off mass for a given runway is increased. For this given mass the aircraft’s take-off performance in the all engines and one engine inoperative condition will also improve.
Optimum Speed Optimum speed use gives the following performance improvements:
¾ Higher TOM, or
¾ Better climb gradient when climb limited TOM or obstacle limited TOM is below runway limited mass
To improve climb performance a higher V2 and consequently a higher V2/VS ratio is
selected in order to obtain more excess thrust. The higher V2 is obtained by higher
acceleration. The runway limited mass must be reduced in order to achieve this. The optimum V2/VS ratio is the value which gives a runway mass equal to the climb
requirement mass or obstacle mass.
Optimum Flap Optimum flap setting also optimises the take-off performance. The importance of the optimum flap setting is shown in some modern aircraft where the setting is accurate to 1/
10 th of a degree. This is achievable over the whole range
available.
Low flap setting gives excess thrust and a better climb performance. There is a reduction in:
¾ Runway performance, and
¾ Performance versus close in obstacles
The optimum flap setting is where the two limiting TOM curves meet. Where the optimum flap is used the V2/VS ratio is usually left constant.
Reduced Take-Off Thrust
In most cases, where an aircraft has an engine failure on take-off the performance is much better than that required. Where the actual TOM is lower than the performance limited TOM derated or reduced take-off thrust can be used. This helps in saving engine life. The preferred method of calculation is to use the assumed temperature method.
Assumed Temperature Method
The assumed temperature is obtained by calculating the limiting temperature where: ¾ The actual TOM would use all of TODA (FLL)
¾ The aircraft would be limited by climb requirements (WAT) at actual TOW ¾ Maximum permitted thrust reduction occurs
The lowest of the three values is taken as the assumed temperature. This temperature is then used to calculate the thrust setting to be used for take-off.
Thrust reduction is limited top 25% of the maximum take-off thrust. Where one engine fails after take-off, the one engine performance requirements must be satisfied. Go-around thrust can always be selected to give a better one engine performance. Flexible take-off thrust is allowed where runway mass limitation tables are available for the runway in use.
The assumed temperature method is a reduced thrust method. For this reason it must not be used when the runway is contaminated. The runway must not be:
¾ Contaminated by snow, slush standing water or ice ¾ De-icing or anti icing fluids must not have been used ¾ Airframe anti-ice must not be in use
¾ All EPR gauges must be operative Noise Abatement
ICAO Document 8168 PANS-OPS details the information required for departure and approach procedures regarding noise abatement.
Aircraft are noisy and in the modern era where an airport is close to a built-up area then procedures are designed to reduce the noise as much as possible. Where special departure procedures are designed then it is possible that the TOM may be limited in order to achieve the requirements of the two noise abatement procedures. Two procedures are outlined below. Note that both procedures are not to be initiated at less than 800 ft above aerodrome level
Noise abatement procedures in the form of reduced power take-off should not be required in adverse operating conditions such as:
¾ If the runway surface conditions are adversely affected (eg snow, slush, ice or other contaminants)
¾ When the horizontal visibility is less than 1.9 km (1 nm)
¾ When the crosswind component, including gusts, exceeds 15 kt ¾ When the tailwind component, including gusts, exceeds 5 kt ¾ When wind shear has been reported or forecast, or
¾ Thunderstorms are expected to affect the approach or departure Noise Abatement Departure Procedure 1 (NADP 1)
This procedure is intended to provide noise reduction for noise sensitive areas in close proximity to the departure end of the runway. The procedure involves a power reduction at or above the prescribed minimum altitude and the delay of flap/slat retraction until the prescribed maximum altitude is attained.
¾ The initial climbing speed to the noise abatement initiation point is not less than V2 + 10 knots
¾ When at or above 800 ft above aerodrome elevation the engine power/thrust is adjusted in accordance with the noise abatement schedule in the aircraft operating manual
800 ft 3000 ft
Take –off Thrust
V2 + 10 to 20 kt (or V2 + 20 to 40 kmh)
Initiate power reduction at or above 800 ft
Climb at V2 + 10 to 20kt
Maintain reduced power
Maintain flaps/slats in the take-off configuration
Maintain positive rate of climb
Accelerate smoothly to en-route climb speed At no more than 3000 ft retract flaps/slats on
¾ A climb speed of V2 plus 10 to 20 knots is maintained with the flaps/slats in the
take-off position
¾ At no more than 3000 ft above aerodrome elevation while maintaining a positive rate of climb the aircraft is accelerated and the flaps/slats retracted
¾ At 3000 ft above aerodrome elevation accelerate to en-route climb speed Noise Abatement Departure Procedure 2 (NADP 2)
This procedure is designed to alleviate noise distant from the aerodrome. The procedure involves the initiation of the flap/slat retraction on reaching the minimum prescribed altitude. The flaps/slats are to be retracted on schedule while a positive rate of climb is maintained. Power reduction is performed with:
¾ The first flap/slat retraction, or
¾ When the zero flap/slat configuration is attained
At the prescribed altitude the transition to normal en-route climb procedures is made.
¾ The initial climbing speed to the noise abatement initiation point is V2 + 10 to 20
knots
¾ On reaching 800 ft above aerodrome elevation the body angle/angle of pitch is decreased while still maintaining a positive rate of climb. The aircraft is accelerated to VZF and:
¾ Power is reduced with the initiation of the first flap/slat retraction, or ¾ Power is reduced after flap/slat retraction
¾ A positive rate of climb is maintained and the aircraft is accelerated to a climb speed of VZF plus 10 to 20 knots to 3000 ft above aerodrome elevation
¾ At 3000 ft the transition is made to normal en-route climb speed
(or V2 + 20 to 40 kmh)
800 ft 3000 ft
Take –off Thrust V2 + 10 to 20 kt
On reaching 3000 ft transition smoothly to en-route climb speed
Not before 800 ft with a positive rate of climb accelerate to VZF and reduce power with the initiation of the first flap/slat
retraction or
When flaps/slats are retracted with a positive rate of climb reduce power and climb at VZF + 10 to 20 knots
Best Rate of Climb
In the case of an engine failure let us assume that the power available has been reduced by 50% yet the power required remains the same, as shown in the diagram below. As can be seen the excess power is reduced dramatically.
Best rate of climb is now referred to the Best Single Engine Rate of Climb, VYSE . For a JAR
23 aircraft this speed is marked with a blue line on the ASI. This is then referred to as the Blue Line Speed.
In summary:
¾ The maximum rate of climb will be reduced
¾ The best single engine angle of climb, VXSE will increase yet the corresponding
rate of climb will be less than the maximum rate of climb. Weight Effect on Optimum and Service Ceiling
Where there is a high gross mass there will be a decrease in rate of climb which will result in a lower absolute and service ceiling. With jet aircraft the initial gross mass is high (because of the fuel required) and hence initial cruising levels are low.
When fuel is consumed the gross mass decreases and a higher altitude becomes available. This allows the aircraft to step climb. The maximum cruise level can be calculated from graphs provided in the aircraft manual such as the one produced below.
VY VYSE 2 Engine 1 Engine Speed Power
Buffet Onset Speed
Where a jet aircraft operates at a high altitude the speed range is restricted by:
¾ Performance factors
¾ Aerodynamic performance
With increasing altitude the:
¾ Indicated stall speed increases slowly ¾ Buffet onset speed increases slowly
This means that the minimum speed will increase with altitude.
At high TAS and high altitude the resultant Mach effects make the boundary layer more turbulent on parts of the aircraft. This is called high speed buffet and indicates that the pilot will have control problems if the speed is increased further. IAS for the high speed buffet onset decreases as the altitude and weight increases.
The low speed buffet onset increases and the high speed buffet onset decreases with increasing altitude. This results in the available speed range becoming narrower. At a certain
altitude and weight the two speeds will be the same. This gives rise to the term “Coffin Corner”, where the speed is both too high and too low at the same time.
This situation can occur during manoeuvres where the load factor is increased giving the same result as an increase in Mass.
Example A 40° turn in a 15 ton aircraft equals level flight in the same aircraft with a gross mass of 19.5 tons.
The associated 1.3 G load factor (15 x 1.3 = 19.5) is adopted as a manoeuvring safety factor when cruise flight level is selected. This allows the aircraft a bank angle of 40° before stalling. Buffet Onset Boundary Chart
The chart on the following page is particular to the type of aircraft. The relationship is shown between: ¾ Altitude ¾ Load Factor ¾ Cruise data ¾ Weight ¾ Mach Number ¾ IAS
The chart can be used to find:
¾ The manoeuvre margin in terms of load factor and bank angle ¾ The low and high speed buffet for 1G flight.
Example Given the following, calculate the manoeuvre margin and the high and low speed buffet speeds:
Airspeed M0.72 Flight Level FL 350
CG 10% MAC
Manoeuvre margin