Thanks for that
2 Liter Peter, wherein I hope you don’t mind if I pursue this a little more – if only for the enlightenment of some of the laymen in our midst, w.r.t. the thought processes and actions involved in a take-off.
NB. The following is plagiarised from various sources but is, hopefully, fairly succinct wherein it contains a revision of some of the Perf A principles w.r.t. take-off and V1 stop / go decisions.
Enjoy........
The regulations require that the risk of an event occurring is defined and wherein those definitions are as follows:
Frequent - Up to 1 / 1,000
Likely to occur often during the life of each aeroplane.
Reasonably Probable - 1 / 1,000 to 1 / 100,000
Unlikely to occur frequently but may occur several times during the life of each aeroplane, e.g. Engine Failure.
Remote - 1 / 100,000 to 1 / 10,000,000
Unlikely to occur to each aeroplane during its life but may occur several times during the life of the fleet, e.g. Low speed over-run, or Failure to achieve Net Take-Off Flight Path (NTOFP).
Extremely Remote - 1 / 10,000,000 to 1 / 1,000,000,000
Unlikely to occur in the life of the fleet but still possible, e.g. High speed over-run, Ditching, Double engine failure in twin engine aircraft, Hitting an obstacle in the NTOFP.
Nb. The use of ‘Fleet’, both in the above and what follows, refers to all the aircraft of a particular type, as produced by the manufacturer ( and not to a particular airline’s fleet of that type ).
In early stages of an aircraft’s development there will only be a few pre-production aircraft to test, all usually flown by test pilots. The data produced from these test flight is known as ‘
Measured Performance’.
Given:
- The newness of the airframe & power plant
- The skill levels of the Test Pilots
- The accuracy of the test parameters, e.g. precise measurement of aircraft weight, etc
- The need to deliver a commercially viable / saleable aircraft
- measured performance is invariably better than the fleet average.
As can be seen ( and as would be expected ) some aircraft will achieve a better than average Rate of Climb, some worse, but most cluster around the average.
From the context of aeroplanes, this
average performance of an aircraft fleet is called ‘
Gross Performance’.
Gross performance assumes that the fleet is properly maintained and flown in accordance with the techniques described in the Aircraft Flight Manual (AFM).
You are as likely to
not achieve Gross performance as you are to achieve it.
Whilst Gross performance is a good reference datum for the performance calculations, it does
not however provide an adequate safety margin for Public Transport operations as, by definition, half of the fleet will not attain Gross performance.
Putting it another way, if you were to base your performance calculations using the average of the aircraft fleet ( i.e. Gross performance ), when an engine fails during your take-off roll, you’ll have a 50:50 chance of getting away with either stopping on the runway / stopway or climbing safely away !
We could of course build and operate an aeroplane so that the risk of accident was infinitely small; this would be exceptionally safe but the cost of air transport would then be moved beyond realistic limits. That said, as safety margins are reduced profits increase ( to a degree
).
So, given that Gross Performance is not always going to be
safe enough, just what is an adequate safety margin ?
The risk level considered acceptable is that there should only be a one in one million ( 1 : 1,000,000 ) chance of a system failure that is then followed by a failure to achieve the regulated required level of performance.
This level of performance is called ‘
Net performance’.
Net performance is Gross performance that has been reduced by an amount to allow for such things as, e.g. variations in piloting technique.
If an event is statistically highly likely occur, e.g. a normal climb conducted without an engine failure, there will be a large margin between the Gross and Net performance levels ( i.e. equating to a high safety margin existing between the two levels of performance ).
On the flip-side, if an event is statistically
unlikely to occur, e.g. an engine failure at an exact point in the take-off run, then the difference between the Gross and Net performance will be very small ( i.e. a low safety margin between the two levels of performance )
It is worth noting that performance planning to ‘net performance standards’ keeps the risk of an accident to an acceptable ( read, ‘very low’ ) level, but
it does not reduce it to zero, even if the correct techniques have been followed !
This bell curve shows the difference between net & gross performance for a likely event, e.g. All Engines Rate of Climb.
Nb. As a ‘likely event’ it has a high level of safety… i.e. 1 : 1,000,000, and thus had it been drawn to scale, the (red) shaded area would represent just one millionth of the total area of the bell curve.
If an event is unlikely then the improbability of the event is used as part of the safety factor.
This following bell curve graph shows the reduced safety factor in the engine-out case:
However, if an event is
so unlikely that the probability of it happening is assessed as being already less than one in a million then the safety margin between Net and Gross performance reduces to zero.
A less obvious, though much more important, example is the engine failure on take-off, i.e. if you have an engine failure during take-off, there are no safety margins remaining !
I.e. the implication of the lack of safety factors, following an engine failure at V1, is that if you lose an engine at V1 you are now down to Gross performance, i.e. you have a 50:50 chance of making 35ft ( and / or 15ft with a wet V1 ) at the end of the Take-Off Distance Available / TODA.
This in itself this is not particularly disturbing, however looking at the factors considered when working out the Take-Off Distance Required / TODR ( and ergo the Emergency Distance Required )
- The all engines gross distance to 35ft multiplied by a factor of 1.15 ( a likely event therefore large / 15% safety margin )
- [i]The gross distance to 35ft, an engine having failed at dry V1[i] ( note no safety factor ); or
- The gross distance to 15ft, an engine having failed at wet V1 ( note no safety factor )
…… we find that, w.r.t. the Emergency Distance Required, there are no safety factors built into this; beyond the improbability of an engine failure at V1.
Ultimately this means that, if you have an engine failure at V1 at your MTOM ( RTOM ), you only have a 50:50 chance of stopping by the end of the stopway.
This is very significant and accordingly has lead to shift in philosophy to
avoid high speed aborts ( albeit except in certain
exceptional circumstances, e.g. aircraft won’t rotate / jammed controls ).
As
2 Liter Peter correctly surmises above, it’s considered much safer to take-off when close to V1 sort the problem out and then land with all of the runway in-front of you, than attempt to stop with much of the runway behind you.
