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Spot the error?

I know you’re all twiddling your thumbs with nothing better to do, so trying wrapping your neurones round this little tease:

Given:
Distance from departure to destination: 4630 NM
Safe Endurance: 12,4 h
True Track: 240
W/V: 060/80
TAS: 530 kt
What is the distance of the PSR from the departure point?

Just holler when you want the explanation, it makes interesting reading! Promise!

Bordeaux

Actually, here’s the explanation – look away now, those that like to do things the hard way

Bordeaux

and

doesn’t make sense.

This Q has to be a play on words, as much of this crap is.

Administrator
Shoreham EGKA, United Kingdom

However, calculating a PSR or ETP is fairly critical if one is engaged in long ocean crossings. Just ask Jason!

London area

However, calculating a PSR or ETP is fairly critical if one is engaged in long ocean crossings

… and your answer, as a transport jet pilot, is, Josh?

Administrator
Shoreham EGKA, United Kingdom

Why doesn’t it make sense?

Are there any pedant points for noticing that a constant track of 240 (and 060 on the return) is not a great circle route, and so the distance from departure to PSR will be less than the distance travelled?

Actually, thinking more as I write this, if you start off in the UK, you get to the Caribbean or somewhere similar, but if you start from e.g. Mt Erebus you just spiral around the South Pole!

So let me guess: there is some departure point near the South Pole where the PSR is 1 mile from departure, because of the crazy route?

:-)

Last Edited by DavidS at 21 Apr 12:53
Booker EGTB, White Waltham EGLM

Why doesn’t it make sense?

Because for a track of 240 a wind vector of 060/80 is a tail wind of 80kts. So it should increase the outbound ground speed.

EIWT Weston

So, Jojo, where is the catch? It can’t be the calculation itself, even a ten year old can figure it out.

doesn’t make sense.

You’re right, it’s a tailwind, not a headwind. However, it makes no difference when calculating the distance (= e * gs1 * gs2 / (gs1 + gs2)).

So it looks like the question writer put in a deliberate error in the GS calculation, to make the candidate worry and hopefully calculate the wrong answer or better still not select any answer?

The problem with these aviation exams is that they often use word plays to throw you off. I recall one asking about a “generator” and 2 of the 3 wrong answers used “alternator” instead but had otherwise correct wording. We all know that a generator outpus DC and an alternator outputs AC, but that is actually wrong for all cars and most GA aircraft where the alternator outputs DC (the 3 phase rectifier is built in). And there is no way in the known universe to generate DC anyway; a generator uses the commutator to do the rectification. Another widespread trick is mixing feet and metres for parameters which are always in feet in European/US aviation usage e.g. altitudes. Often a single word is inserted which subtly changes the meaning of the question, to ensure that the obvious answer is wrong

Administrator
Shoreham EGKA, United Kingdom
25 Posts
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