Another MET-question concerning pressure levels

[austrianpilot]

Hey guys,

There's another question in the test bank, where I can't understand the "correct" answer:

"An aeroplane takes off from A (elevation 800 ft, qfe 1000 hPa) and flies to B (elevation 80 ft, qfe 1020 hPa). The highest ground between A&B is 2550 ft. If the aeroplane flew from A to B at 3000 ft indicated with A's QFE on the subscale, and if the regional QNH was 1026 hPa, the A/C would have (assuming that 1 hPa = 27 ft):
A crashes on the hill 690 ft below the top
-> B flown over the hill at least 1230 ft above the top <-
C flown over the hill at least 960 ft above the top
D crashes on the hill 240 ft below the top"

My way to solve the problem was the following:
QNH at A can be calculated using QFE and elevation -> QNH at A = 1030 hPa. So the A/C flies at a altitude of 3800 ft AMSL. Since the QNH at the hill is 1026, say lower than the QNH at A, the A/C would fly at an altitude of 3692 ft AMSL. This gives a terrain clearance of (3692 ft - 2550 ft) of 1142 ft. So the A/C would fly over the hill at least 960 ft above the top, and not as the software states 1230 ft above the top.

Where's the mistake?

Best regards,
austrianpilot

[Transsonic2000]

Servus:

1. It is always helpful to draw a sketch to answer this type of question.

2. Pressure decreases with increasing altitude.

3. determine the pressure difference between the altimeter setting used during the flight (QFE = 1000hpa) and the regional QNH (1026hpa)

1026 - 1000 = 26hpa x 27 = 702 feet

Hint: No need to calculate the QNH of airfield A, since the aircraft uses the QFE (1000hpa) setting as the primary pressure/altitude reference during the flight!

4. now we have determine if the 702 feet must be added or subtracted from our cruising altitude (3000ft) - this is where the (above mentioned) sketch comes into play!
If the sketch is drawn correctly it'll indicate that the 702 feet must be added (3702 ft) since the difference between our cruising altitude (OFE 1000hpa) and the regional pressure (QNH 1026hpa) equals 26hpa (x 27ft = 702ft) meaning that the aircraft is cruising 702ft above the regional QNH.

5. now subtract 2500ft (mountain) from 3702ft (cursing alt) will give a clearance altitude of 1202ft (which comes pretty close to answer B)

Hint: These type of questions always ask for a clearance altitude, meaning "crashes into the hill" is never an option (I've never encountered a question where this option indicated the correct answer)! That's not what the JAA/EASA wants hear from a future Airline Pilot!

[hvogt]

I would opt for answer C as well, however with a slightly different approach. I think calculating the QNH at A is not necessary.

If you fly at 3000 ft indicated on a subscale setting of 1000 hPa and the local QNH is 1026 hPa your true altitude will be (1026 - 1000) * 27 = 702 ft higher than the indicated altitude. So, the true altitude is 3000 ft + 702 ft = 3702 ft. Hence, you will clear the mountain by 3702 ft - 2550 ft = 1152 ft.

Transsonic2000, the mountain is 2550 ft, not 2500 ft.

[RichardH]

However, using old money of 30ft to 1hpa the numbers work exactly.

26 * 30 = 780 + 3000 = 3780 - 2550 = 1230.

Would be interesting to know the original date and source of this question.

[austrianpilot]

If I calculate via QNH(A) with the exaact values, I get the same solution as hvogt. But even if the solution is 1202ft or 1152ft, answer B is not correct...

@RichardH: would be nice, easy and correct, but the hint in the questions tells you to use 27ft/hPa, not 30ft/hPa...

[austrianpilot]

The question is taken out of the EXAM syllabus 050 01 06 03, question 274 (60834)

[RichardH]

austrianpilot. You miss my point. You originally asked why you can't get the numbers. I am telling you with over 35 years experience it was accepted prior to JAA that there was 1 Mb/HPa to 30 feet and that is how questions were worked.

I suspect this question was around years ago, looks very much like a UK PPL/IMC question. Someone has copied this into the "database" but forgot to change the figures to work for 27ft. Just saying that a question comes from x & y does NOT prove its origin. There are still a few of questions around like this.

On 30ft to 1Hpa B is correct. On 27ft to 1 Hpa B can't be correct with C being the better answer.

You should report this question to your database provider.