Performance Margins
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Joined: Apr 2008
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From: nowhere
Performance Margins
According to an article I am reading on performance margins,
"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 accelerate stop 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%."
So take a look at a particular aircraft that departs from a particular runway three days in a row.
On day one, the aircraft is at exactly its max allowable weight for the runway when the 10 headwind is taken into account.
On day two all conditions are the same except that the aircraft is much lighter. But there is a 10 knot tailwind and once again, it turns out to be at its exact maximum weight.
On day three there is no wind but all other conditions are the same. The aircraft is at a weight that is between what it was on the first two days but it turns out once again(very fortunately for the airline) to be able to takeoff and once again by coincidence happens to be at its maximum limiting weight.
In terms of performance margin, would it be correct to day that on the calm wind day, the aircraft had the least margin? In other words, the greater the tailwind or headwind in a performance-limited(acceleration-stop) takeoff, the greater the safety margin.
"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 accelerate stop 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%."
So take a look at a particular aircraft that departs from a particular runway three days in a row.
On day one, the aircraft is at exactly its max allowable weight for the runway when the 10 headwind is taken into account.
On day two all conditions are the same except that the aircraft is much lighter. But there is a 10 knot tailwind and once again, it turns out to be at its exact maximum weight.
On day three there is no wind but all other conditions are the same. The aircraft is at a weight that is between what it was on the first two days but it turns out once again(very fortunately for the airline) to be able to takeoff and once again by coincidence happens to be at its maximum limiting weight.
In terms of performance margin, would it be correct to day that on the calm wind day, the aircraft had the least margin? In other words, the greater the tailwind or headwind in a performance-limited(acceleration-stop) takeoff, the greater the safety margin.
Only half a speed-brake
Joined: Apr 2003
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From: Commuting not home
I think we may need two specific terms to chew through this one.
margin_type_1: how much does the A/C outperform the calculated figures
margin_type_2: how much reserve do the calculated figures allow from the constraints of the physical world (ASDA, screen height, obstacles).
margin_type_1: how much does the A/C outperform the calculated figures
margin_type_2: how much reserve do the calculated figures allow from the constraints of the physical world (ASDA, screen height, obstacles).
Joined: Jun 2002
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From: Wor Yerm
All things being equal you will have a greater margin with a 10 knot tailwind because the basic performance would be calculated using a 15 knot tailwind giving five knots worth of tailwind performance margin PLUS any other margins. At zero wind you will be be credited with just the normal margins.
PM
PM
Thread Starter
Joined: Apr 2008
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From: nowhere
Thanks,
The margins are for consideration of an engine failure at V1 and the stopping distance or height over the end of the runway. I would think that the margin(as a percent) for the aircraft with the 10 knot tailwind would be the same as for the aircraft with a 10 knot headwind. Less so for the aircraft with zero wind.
The margins are for consideration of an engine failure at V1 and the stopping distance or height over the end of the runway. I would think that the margin(as a percent) for the aircraft with the 10 knot tailwind would be the same as for the aircraft with a 10 knot headwind. Less so for the aircraft with zero wind.
Joined: Mar 2006
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From: England
An interesting question; perhaps an alternative view might help.
For landing, if the wind adjustment is considered as representing the likely spread of touchdown points (distribution), then the on-ground distance in a range of winds would be similar; further safety margins (factors) are applied according to regulation / advise.
For an RTO this is not quite the same, the distance variable with wind might be most noticeable in un braked period from failure, recognition, until retarding action. Thereafter the wind effect on braked distance might be similar to landing, not being very significant.
The main difference is that for an RTO there are few if any additional margins added
For landing, if the wind adjustment is considered as representing the likely spread of touchdown points (distribution), then the on-ground distance in a range of winds would be similar; further safety margins (factors) are applied according to regulation / advise.
For an RTO this is not quite the same, the distance variable with wind might be most noticeable in un braked period from failure, recognition, until retarding action. Thereafter the wind effect on braked distance might be similar to landing, not being very significant.
The main difference is that for an RTO there are few if any additional margins added
