Aircraft Performance exam-help!

[Lister Noble]

Help please.
I've got my Aircraft Performance ground exam tomorrow morning,and I am having real trouble with the land/take off safety factors.
Letest version Safety Sense leaflet 7B,gives some figures that do not tally with the Airquiz site answers.
Do I assume the 7B figures are correct?
Lister:confused:

[FlyingForFun]

If you can give a specific example, we can say which answer is correct.

But in general, I would go with the Safety Sense leaflets.

(And out there in the real world, check your POH to see what, if any, safey factor has already been applied - many manuals don't apply one, and you have to apply it yourself, but in some manuals a safety factor has already been added, and this should be explained in the manual.)

FFF
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[Lister Noble]

When landing on dry grass on firm soil, what factor do the CAA recommend that you multiply the landing distance by?
A 1.1
B 1.15
C 1.33
D 1.2
You chose D, but the correct answer was B.

Thanks,example 7B says 1.2,which is what I said,so I also reckon stay with the leaflet and see how it compares tomorrow.

Lister:now not so confused:

[FlyingForFun]

Lister,

In this example, AirQuiz is incorrect.

However, the Safety Sense leaflet is also incorrect.

Safety Sense Leaflet 7 can be found in LASORS 2006 (http://www.caa.co.uk/docs/33/LASORS_06_WEB.pdf) - click on the link and scroll to page 417 of the PDF file.

The table of safety factors is on page 7 of the leaflet, which is page 423 of the PDF file.

This table gives, for short grass, an increase in take-off distance of 20%, a factor of 1.2. But the question asks about landing distance. And the increase in landing distance given is 15%, or a factor of 1.15, which is answer B.

(There is also a note that, in some aircraft such as those without brakes, grass can decrease landing distance, but you should factor the increase unless you are very conversant on type. Not relevant to the question, but interesting nonetheless!)

However, on the previous page, in the section labelled "Safety Factors" (which starts on page 5 and runs on to page 6), it gives an example of a take-off calculation, and it says: "The same aeroplane in still air from a dry, short- grass strip (factor of 1.2)". This is a direct contradiction of the following page.

So, to try to resolve the problem, I had a look at the place where the Safety Sense Leaflet information comes from: AIC 67/2002 (Pink 36) (http://www.ais.org.uk/aes/pubs/aip/pdf/aic/4P036.PDF). (Registration to the AIS website is required.) The answer is found on page 3, paragraph 7.6:Guide line factors: for dry grass (under 8 inches) the landing distance will be increase by 20% (a factor of x 1.20).Hope that clears that one up!

FFF
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[Lister Noble]

FFF,
Thanks for all your help,but you must admit it is all a bit confusing and I hope it works out OK tomorrow!
I will post my result when I get back from the exam.
I suppose in practical terms,one foolproof way is to take all the worst likely scenarios for the aircraft flown,multiply them all together and make sure you always have that much runway in real life.
Lister:)

[Lister Noble]

Just back and passed exam,got two wrong,one totally my own fault re runway slope,I can't believe I put down the answer I did,it pays to check answers a few times!
The other one was really confusing about tailwind effect on max range speed,I had a good discussion with the instructor afterwards and am still confused.
Lister:)

[OpenCirrus619]

LN,
Perhaps a simple example will help:
Suppose your speed for Max Range (still air) is 100kts and at the required power setting you use 10gph and have an endurance of 5 hours (50gal). This would result in a still air range of 500nm.

Also suppose your aircraft burns 9.05gph at 90kts. Now you have an endurance of 5.52 hours - which gives a range of 497.24nm.

So, in still air, Max Range is achieved at 100kts.

Now add a tailwind of 12kts. All that changes, compared to the 2 examples above, is that the groundspeed changes. Hence:
90kts airspeed give range of 563.54nm (5.52 * 102).
100kts airspeed gives range of 560nm (5.00 * 112).

Basically you've had the benefit of the 12kts tailwind for an extra 31.5 minutes - taking you another 6.3nm for free. What you lose is that THROUGH THE AIR you do 9.95nm/g at 90kts as opposed to 10nm/g at 100kts.

Does that help?

OC619

[Andy_RR]

LN - oh the irony of it! :)

Check this thread out - http://www.pprune.org/forums/showthread.php?t=211147

Mark 1's description is the most easy to understand, but it works better drawing it on a graph


edit: here's a graph! :)

http://uploads.turbosport.co.uk/GETPIC3_dfs879sdf8787hfd/5008851200602161514.gif

Congrats on the pass though!

[Lister Noble]

Andy and Open Cirrus,
Many thanks,I think I get it now.
My answer was leave the power setting unchanged,derived from the fact that it was already set at max range and the bonus free ride did not affect the actual technical performance.
Bit of a trick question,or maybe not?
I have now sat and passed the six exams just leaving R/T,but I have noticed quite a bit of ambiguity in the questions and a couple of wrong answers.
The CFI is writing to the CAA about the wrong answer ones.
Lister:)