View Full Version : need help with ATPL performance.

2nd May 2009, 08:27
Hi guys.

I hope some one in here can help me out here.

I am to teach ATPL performance and have run into a question I can't solve or understand.

The question is:

For a turboprop powered aeroplane, a 2200 m long runway at the destination aerodrome is expected to be "wet". To ensure the wet landing distance meets the requirements, the "dry runway" landing distance should not exceed:

a) 1540 m
b) 1147 m
c) 1339 m
d) 1771 m

The rules says:

Landing distance required on a dry runway for destination and alternative aerodromes, from 50' to full stop must not exceed :

70 % of LDA for turboprop aeroplanes.

If the runway is forecast to be wet at the estimated time of arrival the landing distance available must be 115% of landing distance required.

I have the answer but I dont understand why because of the way the rules are descriped.

Can anyone give me a good explanation ??

Thanx for your help !


2nd May 2009, 11:08
It does seem to be €urobabble designed to confuse.....

As I read it,

If the Dry RW LDA is 2200m, the turboprop must have a LDR of not more than 2200 x 0.7 = 1540m

If the RW is expected to be wet, the 1540m LDR would be too high as there is a further factor to apply. So it must be not more than 1540 / 1.15 = 1339m as that will be safe wet or dry.

Is the answer (c)?

If so, why don't they just say:

LDR must be < 0.7 LDA if dry
LDR must be < 0.6 LDA if wet (OK, 0.6086957 if you must!)

2nd May 2009, 17:34
Very simple Math Question

The wet runway length required = landing distance required *1.43*1.15

1.43 = the 70% safety factor

1.15 = the wet factor

The question simply asks;

If 2200 = landing distance * 1.43*1.15 then what is the landing distance

I get 1337.79 which allowing for my rounding of the 70% factor makes (c) 1339m the answer.


To explain the rules,

The landing distance required is as per the flight manual and caters for pressure altitude and temperature etc

The rules say that the landing distance required can not exceed 70% of the published landing distance available (LDA).

1 divided by 0.7 is 1.43.

The 1.43 safety factor has to be applied to all landings to achieve the 70% requirement.

So when dry, you get the landing distance from the flight manual and multiply this by 1.43 to get the minimum LDA for the conditions.

When the runway is wet then as well as the standard 1.43 factor you also have to apply a factor of 1.15 for the wet.

This means that the wet LDA must not be less than the dry figure from the flight manual * 1.43 * 1.15.



what next
2nd May 2009, 19:03

If so, why don't they just say:

LDR must be < 0.7 LDA if dry
LDR must be < 0.6 LDA if wet (OK, 0.6086957 if you must!)

Because there are really only three factors to be remembered: 60% for jet aircraft, 70% for propeller aircraft and +15% for wet/contaminated runways. Combining these in all possible ways (as done in the propeller example above) alreday would result in four figures to memorise. Plus the additional factors from the operating manual for slope, tailwind, surface,... will result in endless combinations. These "eurocrats" were not so stupid after all, when they only gave us those three figures to remeber...

Greetings, Max

2nd May 2009, 20:19
Hi guys.

Thank you for your answer. Yes C is the correct answer and after your explanations it makes senses.

What confused me is the wording 115% of landing distance required. Which made me think I should add 15% to the 70% . But of course one thing is a required distance and the other is available.

Thank you for your help.

Best regards Jic.