Performance
Margins
Paul Schmid
Boeing Aerodynamics Engineering
When an aircraft is dispatched in accordance with certification and operational
regulations, there are some inherent margins included in the calculated takeoff
and landing performance. Although it is not permitted to take advantage of these
margins in order to increase the aircraft performance limit weight, it is of interest
to be aware of the magnitude of these margins. This paper quantifies some of the
margins in accelerate-stop distance, takeoff distance, minimum rate of climb,
obstacle clearance, and landing distance on a 737-800, 747-400, and 777-200.
Takeoff performance requires the consideration of a critical engine failure, the
airport pressure altitude and ambient temperature, the runway slope, and the wind
component along the runway (50% of the headwind or 150% of the tailwind).
Accelerate-Stop Distance
Prior to FAR Amendment 25-42 (JAR 25 Change 14) and FAR Amendment 25-92
(JAR 25 Change 16), the AFM accelerate-stop distance only considered an RTO
associated with an engine failure on a dry runway, and no credit for the use of
reverse thrust was permitted. The first amendment introduced the requirement to
consider an RTO with all engines operating. The second amendment introduced the
requirement for wet runway accountability. However, the wet runway acceleratestop distance can take credit for reverse thrust.
Figure 1 outlines the calculation basis for the accelerate-stop distance. A mandatory
1-second interval between engine failure speed VEF and V1 is assumed, followed by
a transition segment of approximately 3 seconds during which brakes are applied,
throttles retarded, and the spoilers extended. The total time for this transition
segment consists of the flight test–demonstrated pilot actions plus 2 seconds. If
This article is presented as part of the 2007 Boeing Performance and Flight Operations Engineering Conference,
providing continuing support for safe and efficient flight operations.
Article 10
2007 Performance and Flight Operations Engineering Conference
reversers are used in the calculation, a 1-second delay is assumed for the selection
of the reversers followed by the demonstrated time for the reverser to deploy and to
spin up to the commanded reverse thrust. The reverser deployment and spin-up time
is airframe-engine dependent but is in the order of 5 to 8 seconds. Furthermore, it is
assumed that reverse thrust is reduced to idle between 60 knots and 30 knots.
Acceleration
BR
Transition*
VEF
V1
1 sec
Stopping
VB
Stop
~3 sec
Engine failure RTO
Select Rev Full
rev deploy rev
C/B to
idle at
60 kt Stop
1 sec
Engine dependent
BR
V1
All-engine RTO
Figure 1. Accelerate–stop
distance calculation basis
* Transition distance calculation is impacted by FAR amendment
25-42 and 25-92 (JAR 25 change 14 and 16).
VB
Stop
Select Rev Full
rev deploy rev
C/B to
idle at
60 kt Stop
1 sec
Engine dependent
The manner in which the additional 2 seconds of time in the RTO transition segment
has been accounted for has changed over time and is summarized in Figure 2. Prior
to 1981 one second was added to the second and third demonstrated pilot action
(throttle cut and spoiler extension respectively), during which time the aircraft
decelerated. In 1981 the FAA clarified that the intent of their policy was to provide a
two-second distance allowance at the speed reached at the end of the demonstrated
transition (VB). In Amendment 25-42 this distance allowance was redefined to be
2 seconds of continued acceleration after V1. The current certification regulations
reflect Amendment 25-92 in which the distance allowance has become equal to 2
seconds at the V1 speed. The distance associated with this 2 seconds time for the
737-800, 777-200, and 747-400 is approximately 500 to 600 feet.
The requirement to factor the headwind by 50% and the tailwind by 150% in takeoff
performance calculations is intended to provide a margin for the possible variation
of wind speed and direction during the takeoff. The magnitude of the acceleratestop distance margin associated with factoring a 10 knot headwind or tailwind is
illustrated on Figure 3 for the 737-800, 777-200, and 747-400 at their respective
maximum takeoff weight and sea level/30°C conditions. Relative to the scheduled
AFM distance, shown in parenthesis, the actual distance is reduced by 5 to 6%. It
is not uncommon for operators to conservatively use zero wind performance when
operating into a headwind; this practice increases the distance margin to 10 to 12%.
10.2
Paul Schmid | Performance Margins
1 sec
“Pre 1981”
707, 727, 737-100/-200
747-100/-200/-SP/-300
VEF
“Post 1981”
757, 767, 747-400
737-300/-400/-500
Throttle
Speed
brakes
Flt test
transition
2 sec
at VB
VEVENT
2 sec
continued
acceleration
Flt test
transition
VEVENT
2 sec
at V1
VEF
Amendment 25-92
737 NG, 757-300, 767-400,
777-300ER/-200LR,
Typically
–400 ft
V1
Brakes
Amendment 25-42
777-200/-300
3 sec (typical)
Typically
–100 ft
Typically
150 ft
Flt test
transition
Baseline
Acceleration
Transition
Stopping
Figure 2. Accelerate–stop
transition segments
SL/30°C
No slope
737-800
CFM56-7826
MTOW (172,200 lb) F5
(AFM distance, ft)
430
390
790
777-200
Trent 892
MTOW (656,000 lb) F5
800
700
1,350
747-400
PW 4056
MTOW (875,000 lb) F10
800
700
1,400
Tailwind (9,130)
Headwind (7,630)
Headwind assumed
zero (8,000)
Tailwind (13,600)
Headwind (11,150)
Headwind assumed
zero (11,740)
Figure 3. Accelerate–
stop margins; wind
Headwind assumed accountability (10 knots),
zero (11,900)
dry runway
Tailwind (13,540)
Headwind (11,389)
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2007 Performance and Flight Operations Engineering Conference
As stated earlier, the regulations do not allow credit for the use of reverse thrust
in determining the required accelerate-stop distance on a dry runway. The
distance margin provided by the use of idle or maximum reverse thrust relative
to the scheduled AFM distance on a dry runway is illustrated in Figure 4. For the
737-800 the effect of detent reverse is also included. The distance margins are
2 to 3% with idle reverse and 4 to 7% with maximum reverse. Note that the scheduled
distance for the 737-800 and 777-200 is defined by the all-engine RTO, whereas the
scheduled distance for the 747-400 is defined by the engine-failure RTO because the
certification basis for the 747-400 did not require the consideration of an all-engine
RTO.
SL/30°C
No wind/slope
737-800
CFM56-7826
MTOW (172,200 lb) F5
777-200
Trent 892
MTOW (656,000 lb) F5
Figure 4. Accelerate–stop
margins; thrust reversers,
dry runway
747-400
PW 4056
MTOW (875,000 lb) F10
8,000
Idle 160
2 Rev at
Detent 290
Max 300
11,740
2 Rev at
4 Rev at
All-engine RTO,
no reverser
Idle 350
Max 860
11,900
2 Rev at
(AFM distance, ft)
All-engine RTO,
no reverser
Idle 250
Max 410
Idle 220
Max 720
Engine fail RTO,
no reverser
On a wet runway, reverse thrust may be used to determine the accelerate-stop
distance, but the calculated distance may not be less than the dry runway distance
without reverse thrust. The effect of reverse thrust on the wet runway acceleratestop distance is illustrated in Figure 5. Note that the scheduled AFM distance for
the 737-800 and 777-200 is defined by an engine-failure RTO and assumes credit
for one reverser. The distance margins shown are therefore applicable only if there
is no engine failure during the RTO. For the 747-400, the scheduled acceleratestop distance is defined by the dry runway distance, and distance margins relative
to that dry runway distance are provided for an engine-failure RTO (two symmetric
reversers) and all-engine RTO (four reversers) on a wet runway.
By regulation the takeoff analysis must assume that the RTO occurs at V1. However,
a high-speed RTO (greater than 100 knots) is rare and the decision to abort a takeoff
earlier results in significant distance margins. Figure 6 illustrates that the acceleratestop distance is reduced by approximately 13% when the takeoff is aborted 10 knots
prior to V1.
10.4
Paul Schmid | Performance Margins
SL/30°C
No wind/slope
737-800
CFM56-7826
MTOW (172,200 lb) F5
777-200
Trent 892
MTOW (656,000 lb) F5
747-400
PW 4056
MTOW (875,000 lb) F10
(AFM distance, ft)
Engine fail RTO,
8,290
1 rev at max
2 Rev at max
90
11,800
2 Rev at max
230
11,900
2 Rev at max
4 Rev at max
Engine fail RTO,
1 rev at max
Engine fail RTO,
dry runway
420
Figure 5. Accelerate–stop
margins; thrust reversers,
wet runway
830
SL/30°C
No wind/slope
737-800
CFM56-7826
MTOW (172,200 lb) F5
777-200
Trent 892
MTOW (656,000 lb) F5
747-400
PW 4056
MTOW (875,000 lb) F10
8,000
V1 = 139
1,070
11,740
V1 = 152
All-engine RTO,
V1B = 162
1,600
11,900
V1 = 148
(AFM distance, ft)
All-engine RTO,
V1B = 149
Engine fail RTO,
V1B = 158
1,600
Takeoff Distance
Figure 6. Accelerate–stop
margins; RTO 10 knots
prior to V1B, dry runway
FAR Amendment 25-92 (JAR 25 Change 16) introduced the requirement to consider
a wet runway for takeoff performance analysis. With an engine failure, the takeoff
distance on a wet runway is defined to a 15-ft screen height rather than a 35-ft screen
height. However, the wet runway takeoff distance cannot be less than the dry runway
takeoff distance. Furthermore, with an engine failure the takeoff run is equal to the
takeoff distance (i.e., no clearway credit).
The requirement to factor the headwind and tailwind provides similar margins in
takeoff distance as was shown for the accelerate-stop distance. Figure 7 illustrates
that the wind factors provide approximately a 4% distance margins for a 10-knot
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2007 Performance and Flight Operations Engineering Conference
headwind/tailwind. If zero wind conditions are conservatively used to represent 10knot headwind conditions, the margin doubles to approximately 8%.
SL/30°C
No slope
(AFM distance, ft)
737-800
CFM56-7826
MTOW (172,200 lb) F5
340
320
650
777-200
Trent 892
MTOW (656,000 lb) F5
500
450
950
Figure 7. Takeoff
distance margins; wind
accountability (10 knots),
dry runway
747-400
PW 4056
MTOW (875,000 lb) F10
500
460
930
Tailwind (9,130)
Headwind (7,240)
Headwind assumed
zero (8,000)
Tailwind (13,600)
Headwind (11,150)
Headwind assumed
zero (11,740)
Tailwind (13,540)
Headwind (11,380)
Headwind assumed
zero (11,900)
Minimum Rate Of Climb
The minimum climb capability of an aircraft during the takeoff phase is generally
established by the regulatory minimum climb gradient at V2 speed with the
critical engine inoperative and gear retracted (second segment). These minimum
required gradients are 2.4, 2.7, and 3.0% for two-, three-, and four-engine aircraft
respectively.
Figure 8 quantifies the rate of climb for the 737-800, 777-200, and 747-400 at
maximum takeoff weight, an ambient temperature of 30°C, and the pressure altitude
at which they reach the minimum climb gradient. The left column for each model
quantifies the rate of climb with the critical engine failed and V2 speed. The higher
rate of climb for the 747-400 reflects the higher minimum gradient requirement for
a quad (3%) versus a twin (2.4%). The second column illustrates the improvement in
rate of climb if the critical engine fails sometime after V1 by which time the speed has
increased to V2+15. The third column illustrates the rate of climb achieved without
an engine failure. The relatively large increase in rate of climb of the twins relative to
the 747-400 reflects a doubling of thrust rather than only a 33% increase in thrust.
3,000
• MTOW
• OAT = 3°C
2,500
• PA = Altitude for minimum
climb gradient
2,000
Rate
of
climb, 1,500
ft/min
1,000
500
Figure 8.Rate of climb
capability second segment
limit
10.6
0
737-800/CFM56-7B26
172,200 lb
777-200/Trent 892
656,000 lb
747-400/PW4056
875,000 lb
EO V2 +15
EO V2 +15
AE V2 +15
Paul Schmid | Performance Margins
Obstacle Clearance
By regulation the net flight path, which begins at 35 ft above the takeoff surface,
must clear all obstacles by 35 ft. This net flight path is the actual flight path reduced
by a climb gradient equal to 0.8%, 0.9%, and 1.0% for two-, three-, and four-engine
aircraft respectively.
The difference between the gross and net flight path provides an ever increasing
obstacle clearance margin with distance from brake release. This is illustrated for a
737-800 in Figure 9. If the engine were to fail 10 knots after V1 the gross flight path
would improve as indicated. The gross flight path without an engine failure is also
shown.
1,800
Nett FP, One-Eng-Inop, V2
Gross FP, One-Eng-Inop, V2
1,600
Gross FP, all Eng op, V2 + 15
1,400
Gross FP, engine fail @ V1 +10
1,200
Altitude, ft
1,000
800
• OAT = 30°C
600
• No runway slope, no wind
• Alt = sea level
• A/C off
400
• Forward CG, flaps 5
200
0
• Takeoff wt =172,200 lb (MTOW)
0
10,000
20,000
30,000
40,000
50,000
Figure 9. 737-800/CFM567B26 takeoff flight path
60,000
Distance from brake release, ft
The increased obstacle clearance margin provided by factoring a 10-knot headwind
is illustrated in Figure 10.
Gross FP, tailwind unfactored
1,600
Nett FP, tailwind factored
1,400
Gross FP, tailwind factored
1,200
1,000
Altitude, ft
800
• OAT = 30°C
600
• Alt = sea level
400
• A/C off
• No runway slope, 10 knot tailwind
• Forward CG, flaps 5
200
0
• Takeoff wt =172,200 lb (MTOW)
0
10,000
20,000
30,000
40,000
50,000
Distance from brake release, ft
60,000
70,000
Figure 10. 737-800/
CFM56-7B26 takeoff flight
path
Note that on a wet runway, the takeoff distance may be defined by the engine-failure
takeoff distance to a 15-ft screen height. In such a condition the obstacle clearance
will be reduced by 20 ft.
10.7
2007 Performance and Flight Operations Engineering Conference
Landing Distance and Runway Length Required
The landing distance is defined by FAR/JAR 25.125 to be the horizontal distance to
come to a complete stop from a point 50 ft above the landing surface, assuming
• Airspeed at the runway threshold is VREF.
• A level, smooth, dry, and hard surfaced runway.
• Airport altitude and standard day temperature.
• 50% of the headwind or 150% of the tailwind.
• Maximum manual braking.
• No reverse thrust.
If the runway at the destination is forecast to be dry at the time of dispatch, FAR
121.195(b) and JAR-OPS 1.515(a) specify that the aircraft must come to a stop within
60% of the available runway length. Therefore, the dry runway length required is
1.67 (1/0.6) times the landing distance calculated per FAR/JAR 25.125. The landing
distance provided in Boeing AFM/AFM-DPI includes this operational factor.
If the runway at the destination is forecast to be wet at the time of dispatch, FAR
121.195(d) and JAR-OPS 1.520(a) specify that the runway length required is 115%
of the required dry runway length. The landing distance provided in Boeing AFM/
AFM-DPI includes this operational factor. Note that FAR 121.195(d) and JAR-OPS
1.520(c) provide for a reduction in the 115% factor subject to additional flight test
and operational approval on wet runways.
The FAR do not differentiate between wet and slippery runway length requirements
at the time of dispatch. However, JAR-OPS 1.520(b) specifies that the runway length
be the greater of the required wet runway length or 115% of the contaminated runway
landing distance.
Figure 11 outlines the calculation basis for determining landing distance and
the required runway length. The landing distance includes the flare from a 50-ft
threshold to touchdown, a transition segment during which brakes are applied
and spoilers extended, and a stopping segment. A flare time of approximately 4.5
seconds, established by flight test, results in a flare distance of 1,000 to 1,200 ft.
A transition time of 1 second is assumed when the auto spoilers are armed, and 2
seconds if the spoilers are extended manually. The deceleration during the stopping
segment assumes the maximum manual braking coefficient demonstrated in flight
test on a dry runway. As discussed previously, the required runway length for a dry
and wet runway are respectively 1.67 and 1.92 (1.67*1.15) times this calculated
landing distance. When reverse thrust is used in the calculation of advisory landing
distance, a 1 second delay to engage reversers is assumed, followed by the time it
takes the reverser to deploy and spin up to the selected level. Furthermore, it is
assumed that reverse thrust is reduced to idle between 60 knots and 30 knots.
Figure 12 shows how the calculated landing distance for the 737-800 at maximum
landing weight is affected by airplane braking coefficient (related to runway friction
capability) and the use of reverse thrust, and how this distance compares to the
scheduled AFM dry and wet landing field length. The 737-800 demonstrated an
airplane braking coefficient of 0.38 on a dry runway, and a braking coefficient of
10.8
Paul Schmid | Performance Margins
0.20 is typical for a wet smooth runway. Boeing associates braking action reports
of Good, Medium, and Poor with airplane braking coefficients of 0.20, 0.10, and
0.05 respectively. It is very evident from this data that a significant distance margin
relative to the scheduled AFM distance is available for dry runway conditions. When
the runway is wet (or braking action is good) the distance margin is less when no
reversers are used, but is approximately the same as on a dry runway if reverse thrust
is used. However, as the runway becomes more slippery, the distance margin ceases
to exist even with credit for reverse thrust.
Flare
FAR/JAR 25
Landing
Distance
50 ft
Transition
TD
~ 4 1/2 sec
Stopping
VB
Stop
1 sec (AS)
2 sec (MS)
Runway length required
• Dry runway (AFM)
* 1.67
* 1.15
• Wet runway (AFM)
Select Rev
Full
rev deploy rev
C/B to
idle at
60 kt
Stop
1 sec
Figure 11. Landing distance
calculation basis
Engine dependent
14
• Landing Weight (MLW) = 146,300 lb
• Max Manual Braking
12
No Reverse Thrust
• Flaps 30, Approach Speed = Vref30
• Sea Level, Standard Temperature (15°C)
Detent2rev
10
• No Wind, No Runway Slope
Maximum rev
Landing 8
distance,
1,000 ft 6
AFM wet LFL = 6,675 ft
AFM dry LFL = 5,805 ft
4
2
Poor
0
0
0.05
Wet/good
0.1
0.15
0.2
Dry
0.25
Airplane Braking Coefficient
0.3
0.35
0.4
Figure 12. Landing
distance, 737-800
10.9
2007 Performance and Flight Operations Engineering Conference
Figure 13 quantifies the distance margins for the dry and wet runway conditions
depicted on the previous figure. This presentation highlights the fact that some of the
potential distance margin provided by the 1.67 and 1.92 factors must accommodate
the adverse effects of higher approach and landing speed, ambient temperature
above standard day, and negative runway slope, which are not included in the AFM
landing distance. Nevertheless, adequate distance margins remain for dry and wet
runway conditions.
• MLW (146.300 lb)
• Flap 30, VREF 30
• Max manual braking
• No reverse thrust
Actual
3485
Dry runway
2 Rev @
AFM distance, ft
5805
Detent 140
Max 150
VREF 30 + 5
215
60 ISA + 10°C
35 -1% slope
5070
Wet/good runway
2 Rev @
Figure 13. Landing distance
margins, 737-800
10.10
Detent
Max
510
550
VREF 30 + 5
305
95 ISA + 10°C
95 -1% slope
6675 (1.15* dry)