LAST MINUTE performance questions.. [Archive]

spitfire747

19th Oct 2003, 03:06

Hello all, i have a few last minute performance questions from a big school and do not understand if the answer is correct or a mistake..

IN WET CONDITIONS, WHAT EXTRA PERCENTAGE OVER THE CALCULATED LANDING DISTANCE MUST BE AVAILABLE FOR A TURBOJET.

A - 43%
B - 92%
C - 67%
D - 15%

The school say 92% but I reckon its 15% !

Just wondered as this question is shown 3 times in my feedback bundle and each time given this answer of 92%

WHEN IN A GLIDING MANOEUVRE, IN-ORDER TO ACHIEVE MAX ENDURANCE THE AIRCRAFT SHOULD BE FLOWN AT ?

A - speed for max lift
B - speed for min drag
C - speed for max lift to drag
D - speed fo rmin power

Answer given is D.. anyone explain why.. ?

ROC = 1000 ft/min
TAS 198KTS
WHAT'S THE AIRCRAFTS GRADIENT ?

A - 5.08%
B - 3%
C - 4%
D - 4.98%

I thought grad was ROC divided by TAS and this gives 5.05%
any ideas... ?

WHICH OF THE FOLLOWING WOULD GIVE GREATEST GLIDING ENDURANCE ?

A - Vmd
B - 1.32 Vmd
C - best Cl to Cd ratio
D - Close to CL MAX

Answer given is D, I thought it was C

If anyone can put my mind straight i would appreciate it
Thanks
Spitty

Keith.Williams.

19th Oct 2003, 04:16

The trick in the first question is to distinguish between the calculated Landing Distance (LD) and the Landing Distance Required (LDR). For Turbojets on a dry runway the LDR is 1.67 x the LD. If the runway is wet this must be increased by a further 15%. Multilpying 1.67 x 1.15 givers 1.92.

So the LDR dry is 67% greater than the calulated landing distance and the LDR wet is 92% greater than the calculated landing distance. But the wet LDR is 1.15% of the dry LDR (or 15% greater).

You questions aks,

IN WET CONDITIONS, WHAT EXTRA PERCENTAGE OVER THE CALCULATED LANDING DISTANCE MUST BE AVAILABLE FOR A TURBOJET.

In effect the question is asking for the minimum acceptable landing distance available (LDA). The LDA must be at least equal to the LDR, so the answer is 92%. This means that the minimum aceptable LDA is 92% greater than the LD.

But had it said,

IN WET CONDITIONS, WHAT EXTRA PERCENTAGE OVER THE DRY LANDING DISTANCE REQUIRED MUST BE AVAILABLE FOR A TURBOJET.

The answer would be 15%

Be very careful with this type of question LD, LDR and LDA are all different.

The answer to your second question is as follows,

When the engines of an aircraft fail, the aircraft possesses a store of kinetic energy due to its velocity, and potential energy due to it height. Throughout the subsequent glide this energy is consumed in doing work, pushing the aircraft forward against the drag force. When the aircraft is stopped on the ground all of the stored energy will have been consumed.

Because this energy store cannot be replenished, the maximum glide endurance will be achieved by flying at the speed requiring the lowest energy consumption rate. But energy consumption rate is power required, so maximum glide endurance is achieved by flying at the minimum power speed.

Power required is equal to drag multiplied by TAS and although VMD produces the lowest drag force, it does not produce the lowest power required. This occurs at VMP, which is lower than the minimum drag speed VMD for all aircraft. So best glide endurance occurs at VMP.

It is also worth noting that best glide endurance gives minimum sink rate, whereas best glide range gives minimum glide angle. These terms are often used in exam questions.

Your third question asks,

ROC = 1000 ft/min
TAS 198KTS
WHAT'S THE AIRCRAFTS GRADIENT ?

To answer this you must go to page 28 of your CAP 698.

This gives the equation,

still air gradient = ROC/TAS x 6000/6080 %

You answer is,

% = 1000 / 198 x 6000/6080 = 4.98%

You original equation % = ROC/TAS gives a reasonable approximation. But the options in this question are very close together so the more accurate equation from the CAP is the one required for this question.

This might all be a bit confusing, but both of the above equations are in fact simply approximations. In reality % gradient = ROC (in fpm) /Ground Speed (in Kts) x 6000/6080

This question also illustrates the value of knowing the CAP inside-out. If you do nothing else before the exam, study your CAP 698 very carefully. It is full of very useful information including the answers to lots of common questions.

For example any question concerning the effects of variables on the V speeds, can usually be answered simply by examining the trends on pages 64 and 65. This is far better than guessing. And even if you believe you know the answers, it is always useful to use any spare time at the end of the exam by checking your answers using the CAP.

You final quesion is about the speed for best glide endurance. VMP is not available this time, so must choose the option closest to it. VMP is close to VS so aption d is best.