Performance Calculation Safety Margins

[crj705]

I am looking for a source of information for what kind of safety margins are built in to performance calculations for a FAR/JAR 25 aircraft.

For instance, is there a way to quantify how much of a margin you have between the assumed performance numbers and the actual temp when doing a flex takeoff?

Also, is the size of the margin based upon the likelihood of the event happening?

[wmelvin]

In 1953 the ICAO Standing Committee on Performance, composed of experts from a number of nations published what became known as the SCP-53 report. This report set out to establish safety margins for expected new aircraft, principly jets. It established a method of calculating the variation of parameters for particular operations. As I recall the critical condition for the takeoff case was engine failure and there were about 13 parameters, including the variability of pilot control, density altitude, airfoil roughness, etc. Then for a certain desired incident probability, say 1 in 10 million, a certain margin could be determined. Thus for a twin engine aircraft with an engine failure, there should be a 2.4% gross climb gradient to allow for the variables which might occur and clear a 1% net gradient. A 4 engine aircraft required a 3.0% gradient because it had twice the probability of an engine failure--there were no 3 engine aircraft expected at the time.

For the accelerate stop case, margins where applied to demonstrated stopping distance which I think were 10% for 2 engine aircraft and 15% for 4 engine aircraft. This was not done and the present evolved and very marginal system is what we have; but this is well beyond the scope of your question.

So, for a reduced thrust takeoff, if the thrust is set to have the above stated margins, then theoretically, if an engine fails you have the risk of penetrating a 1% slope of 1 in 10 million; not certain that is the level that was chosen, but you get the idea.

There is an issue of the DC-9 having been allowed to have density altitude correction to 400 feet which erodes the gross gradient margin a little. This was discovered by Boeing which asked for similar treatment on the B-727. I suppose all subsequent aircraft got the same deal. This was called by some: "Southern California Thrust".

[rudderrudderrat]

Hi crj705,

For instance, is there a way to quantify how much of a margin you have between the assumed performance numbers and the actual temp when doing a flex takeoff?

The engines would be producing the equivalent thrust if the actual conditions were at the assumed temperature. The aircraft is however flying in air which is denser than the assumed temperature, so it will perform much better than if the density conditions actually existed.

I would guess the improvement margin is proportional to the density difference (between actual and assumed temperatures in degrees absolute).

So If I used a flex of 70 degs C, when the actual conditions were 20 degs C, then my performance would be better by a factor proportional to (70-20)/(273+70).