3. SCREEN 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
4. DEFINITIONS
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.
5. DEFINITIONS
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
7. DECLARED 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)
9. TORA
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).
11. TODA
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
12. ASDA
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
13. DECLARED 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.
22. REQUIRED DISTANCES
Field lengths required for takeoff are:
TORR: Takeoff Run Required
TODR: Takeoff Distance Required.
ASDR: Accelerate-Stop Distance Required.
23. TORR
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.
26. ASDR
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.
28. TODR
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.
30. CONSIDERATIONS
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).
31. BALANCED 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 ft
Balanced V1
34. UNBALANCED 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.
35. UNBALANCED 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
36. UNBALANCED 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
37. UNBALANCED 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.
