ATPL Question

[JB235]

Hi everyone,

I sat ATPL Performance and Loading yesterday and sadly 68% was the best I could do.

In my KDRs was one of those minor questions that could make or break you.
I can't seem to find the correct answer in my study material.

I went for ( B ) and got it wrong.

Question, Holding Altitude for lowest SFC

A) as low as possible

B) as high as possible

C) Lower rather than higher

D) Higher rather than lower

[ConfigFull]

Hi JB235,

What notes are you using for study?

Let's look at what effects SFC:

RPM:
SFC reduces in the design RPM for a specific engine, Higgins puts it in the 92-98% range. An aircraft at a higher altitude will be holding at an engine RPM closer to the design RPM therefore a better SFC.

Temperature:
Not much of a change for a similar TAS; however

Altitude:
You'll get a better SFC with altitude up to the tropopause where the SFC will begin to become less-favourable again. This is where I think you've got the question wrong.

I'd be inclined to choose D in this case.

[JB235]

Gday ConfigFull,

I am using AFT notes.

Yep I think your right.It was one of those curly ones.
I mean who is going to be holding above the tropopause
Thats why I went for as high as possible :sad:

Cheers

[Roxy_Chick_1989]

Gee, those choices complicate what seems a relatively simple question.

[manymak]

Gee, those choices complicate what seems a relatively simple question.

Roxy, welcome to CASA ATPL examinations.. Well any of their exams for that matter! :ooh:

[Feather #3]

From practical experience, it's [D].

G'day ;)

[Tinstaafl]

I wouldn't have answered 'as high as possible'. That implies the aircraft's absolute ceiling is a possibility, which does not equate with the most efficient use of fuel. 'Higher rather than lower' however doesn't include a particular altitude that is inefficient, leaving the exact altitude open to suit the circumstances.

[Roxy_Chick_1989]

Altitude:
You'll get a better SFC with altitude up to the tropopause where the SFC will begin to become less-favourable again. This is where I think you've got the question wrong.


could someone explain this?

[cavok123]

Seen that q before and went D

[oz in dxb]

I would have thought that SFC was more applicable for the cruise rather than in a holding pattern. More thrust for less fuel flow per hour.

In a holding pattern your best altitude will depend on your aircraft weight as you will want to fly at min F/F.

B777-300ER:
340T - min F/F at 10 000FT
180T - min F/F at 30 000FT

Mind you it has been a few decades since I did ATPL performance and yet again I didn't RTFQ!

Oz

[Dangnammit]

I think we are all missing the point. CASA exams are also a test on your understanding of the english language.
Ahh the joy!