performance question 3

[gphillips134]

during certification testing of a two engine jet aircraft, the true take off distances were found to be:

1547m with the power units operating
1720m with the critical engine inoperative at V1

the distance used for certification purposes would be?

explaination on how the answer is arrived at please

[peb]

1.15 multy1547m with the power units operating = 1779.05


Take the grater answer of 1779.05 and the critical engine
inoperative at V1


In this case 1779.05

[gphillips134]

where does the 1.15 come from?

[peb]

It is a safety factor. Thats what i have been told for certification testing of a two engine jet aircraft.

[pugzi]

This comes straight out the CAP manual, as does the answers to a lot more questions besides, if you know where to look that is. This manual is never talked about often enough in performance, if you know it and know where stuff is, it makes life a lot easier, espeially for Class A v speed questions and additional take off procedure questions. They literally GIVE you the answer!.
So in response to this question, I'd say turn to page 53.
In fact the "golden pages" are p46 (nice load of definitions for you, inluding a very speial one for V1), p53 (as mentioned above) and p64/65 (the two best pages).
In the Oxford feedback the cap manual answers the first six or seven question on the first two pages alone!!
Anyway.

Hope this helps.

Steve

[Genome_Junkie]

gphilips

It all has to do with the probability of something happening during the take off run and making screen height at the end - 50ft. So you are looking at building in a saftey margin into the performance figures.
Take the engine out case: What is the likelyhood of an engine failing? - Quite low actually. What is the likelyhood of the engine failing just at Vef so that you will not make screen height - extremely remote.
That is why the performance figures for the engine out case use Gross performance figures - ie your plane has a 50:50 chance of meeting the performance criteria. That means also that in the really unlikely case of the engine failing at that particular speed you still have a 50:50 chance of meeting your screen height.

Now look at the all engine case:
What is the probability of having all engines working during the take off run and beyond? - Well, if that probability was low then I don't want to be on that airplane. Its the most likely case. Therefore the regulations (JAR25, I think) require the gross performance figures to be adjusted in such a way that you are very likely to make the screen height at the end of the runway. So you adjust the gross figures by a certain factor. JAR25 says that this factor is to be 1.15. This will give you your net performance.
You need to take now the higher of the two required figures to cover your backside.

Cheers

Arnie

[sexygirl]

Don't forget he's asking about a twin jet

Therefore class A

Therefore 35ft screen height!

Don't want to split hairs but the examiner might...!!

[Genome_Junkie]

Sexygirl

You are of course right.

35ft all engines and dry runway. 50ft is the landing screen height.

Nevertheless, I still think that it comes down to the likelyhood of which condition is more likely: All engines or Engine out. The more likely case has a bigger safety factor build in. That is where the 1.15 factor comes in. So sometimes the all engine net TODR can be larger than the engine out one, especially 3 or 4 engined jets.

Cheers

Arnie