2. DISTANCES2. DISTANCES

Performance JAR 25Performance JAR 25

DISTANCESDISTANCES

Screen height

Definitions

Declared distances

Required distances

Balanced field length

Unbalanced field length

SCREEN HEIGHTSCREEN HEIGHT

Screen height is different depending on the type of aeroplane:

JAR 23: 50 ft

JAR 25: 35 ft

JAR 25 (WET RUNWAY): 15 ft

35 ft

JAR 25 Screen height

DEFINITIONSDEFINITIONS

RUNWAY: Defined rectangular area on a land airport that is prepared for the landing and takeoff run of aircraft along its length.

STOPWAY: An area beyond the takeoff runway that is at least as wide as the runway, is centred upon the extended runway centreline, and is able to support an airplane during an aborted takeoff without causing structural damage to the airplane.

Stopway must not be used for landing calculations.

DEFINITIONSDEFINITIONS

CLEARWAY: An area beyond the takeoff runway that is under the control of airport authorities where terrain or fixed obstacles may not extend above specified limits. It is not required to be able to support the weight of the aeroplane, it must only be free of obstacles.

min 250 ft

α = 1.25º26 inches

DEFINITIONSDEFINITIONS

500 ft

α = 1.25º

DECLARED DISTANCESDECLARED DISTANCES

Field lengths available for takeoff and landing are known as declared distances:

TORA: Takeoff Run Available

TODA: Takeoff Distance Available. (TORA + CWY)

ASDA: Accelerate-Stop Distance Available. (TORA + SWY)

LDA: Landing Distance Available. (TORA – DT)

DECLARED DISTANCESDECLARED DISTANCES

RWY

CWY

SWYDT

TORA

ASDA

TODA

LDA

TORATORA

TORA: The length of runway which is declared available by the appropriate authority and suitable for the ground run of an aeroplane taking off.

TORA is equal to the distance from the threshold to the Departure End of Runway (DER).

TORATORA

TORA

TODATODA

TODA: The length of the takeoff run available plus the length of the clearway available.

The maximum length of the clearway is half the TORA.

TORA CWY (1/2 TORA MAX)

TODA

ASDAASDA

ASDA: The length of the takeoff run available plus the length of the stopway, if such stopway is declared available by the appropriate authority and is capable of bearing the mass of the aeroplane under the prevailing operating conditions.

TORA SWY

ASDA

DECLARED DISTANCESDECLARED DISTANCES

According to JAR-OPS 1.490, “[…] an operator must take account of the loss, if any, of runway length due to alignment of the aeroplane prior to takeoff.”

Line-up corrections should be made when computing takeoff performance, anytime runway access does not permit positioning the airplane at the threshold.

DECLARED DISTANCESDECLARED DISTANCES

Effective ASDA

Effective TODA

Runway length

A B

A: Adjustment to takeoff distance

B: Adjustment to accelerate-stop distance

DECLARED DISTANCESDECLARED DISTANCES

There are different corrections based on 90º turns and 180º turns.

180º turn

DECLARED DISTANCESDECLARED DISTANCES

DECLARED DISTANCESDECLARED DISTANCES

DECLARED DISTANCESDECLARED DISTANCES

The line-up distance will increase if the runway is not wide enough.

DECLARED DISTANCESDECLARED DISTANCES

Depending on the length of the stopway and/or the clearway (if available), ASDA may be equal, longer or shorter than TODA.

DECLARED DISTANCESDECLARED DISTANCES

DECLARED DISTANCESDECLARED DISTANCES

Declared distances can be found in the AIP, although they are included in the charts.

REQUIRED DISTANCESREQUIRED DISTANCES

Field lengths required for takeoff are:

TORR: Takeoff Run Required

TODR: Takeoff Distance Required.

ASDR: Accelerate-Stop Distance Required.

TORRTORR

Takeoff Run Required is the longest of:

a) The distance from start of ground roll to a point halfway between the lift-off point and the point at 35 ft, assuming that the critical engine has failed at Vef.

b) 115% of the distance from start of ground roll to a point halfway between the lift-off point and the point at 35 ft, with all the engines running.

Therefore, the TOR will be only limitative when using a clearway.

The maximum usable clearway is then half the distance between the lift-off point and the point at 35 ft.

TORRTORR

TORRTORR

ASDRASDR

Accelerate-stop Distance Required is the longest of:

a) The distance taken to accelerate to Vef, continue accelerating to V1 with one engine inoperative, accelerate for 2 more seconds, and come to a full stop.

b) The distance taken to accelerate to V1, accelerate for 2 more seconds, and come to a full stop. (With all engines operating)

Reverse thrust is NOT taken into account.

The most effective braking sequence is: brakes, thrust idle, spoilers.

ASDRASDR

TODRTODR

Takeoff Distance Required is the longest of:

a) The distance from the start of ground roll to the point where the aeroplane reaches a height of 35 ft and the V2 speed with engine failure having occurred at Vef.

b) 115% of the distance between start of ground roll and the point where the aeroplane reaches a height of 35 ft and the V2 speed with all engines operating.

Distance “a)” will be usually longer with twin-engine aeroplanes.

TODRTODR

CONSIDERATIONSCONSIDERATIONS

If V1 is reduced ASD will decrease and TOD will increase, and vice versa.

In general terms, acceleration is more affected than braking when the same condition affects both of them (for instance, runway slope).

BALANCED FIELD LENGHTBALANCED FIELD LENGHT

If the V1 is calculated in such a way that both the ASDR and the TODR are of equal length, the takeoff is said to be a balanced field length takeoff. This situation is given when neither stopway nor clearway exist, or if both are available.

In that case the V1 is known as balanced V1.

TOD = ASD

35 ftBalanced V1

BALANCED FIELD LENGHTBALANCED FIELD LENGHT

V1

m

TODRASDR

V1 BALANCED

BALANCED FIELD LENGHTBALANCED FIELD LENGHT

V1

m

TODRASDR

V1 increases with weight

100.000 Kg

120.000 Kg

UNBALANCED FIELD LENGTHUNBALANCED FIELD LENGTH

If the TORA is not enough to takeoff with our ATOW and a stopway or a clearway is available, we can take advantage of them, changing the balanced V1 value.

Thus, ASDR <> TODR, but the situation will meet the requirements because of that clearway or stopway.

Even with enough TORA, we may operate with unbalanced V1 if the balanced V1 is below the Vmcg or if it is above Vmbe.

UNBALANCED FIELD LENGTHUNBALANCED FIELD LENGTH

CLEARWAY AVAILABLE (NO STOPWAY)

If we decrease the balanced V1 value, the ASDR will decrease and the TODR will increase.

In this case TODR > ASDR, and ASDR = ASDA = TORA.

TORA = ASDA = ASDR CWY

TODR

V1 < Balanced V1

35 ft

UNBALANCED FIELD LENGTHUNBALANCED FIELD LENGTH

STOPWAY AVAILABLE (NO CLEARWAY)

If we increase the balanced V1 value, the ASDR will increase and the TODR will decrease.

In this case ASDR > TODR, and TODR = TODA = TORA.

TORA = TODA = TODR SWY

ASDR

V1 > Balanced V1

35 ft

UNBALANCED FIELD LENGTHUNBALANCED FIELD LENGTH

We have seen how changing the value of V1 we can increase our OTOW, if a clearway or a stopway is available.

But if BOTH of them are available, then we can operate with a balanced V1, since both ASDA and TODA are increased with those clearway and stopway.