Those in the know!

[Trigger Happy]

G'day all,
Found a list of QF Interview questions and am interested to see if I'm on the right track.

1. If you had to go from Syd to Darwin what height would you go. Why?

2. Why not fly as high as possible?

3. What happens to the speed of sound as you get higher?

4. If 743 flying at HIGH altitude (0.84M) and another one is flying at LOW altitude (0.84M), which would have the higher TAS?

5. Reconfirm how high you would fly to Darwin and why?

Go get em' tiger! Cheers in advance..

[Notso Fantastic]

1. Most efficient altitude for ground distance per unit fuel used fitting in with ATC requirements.
2. ATC requirements & fuel efficiency above.
3. Speed of sound = 644 x 1.2T kts (T= deg C). So as C gets colder and more negative, speed of sound goes down.
4. At low alt, air warmer, therefore Speed of Sound is higher, so M.84 will have a higher TAS
5. The whole point is to make operation most economical in relation to fuel consumption/aircraft time cost/commercial requirements/curfews. A pragmatic approach to all the above is required 'on the day'.

[oicur12]

1 the opt as specified by the fmc. why, cos the book says its optimum. you dont have to know the particulars.

2 go back to q1. do what the fmc suggests. who cares why you dont fly as high as possible

3 who cares. cant control it.

4 who cares. call him up and ask him. ask atc. watch him on tcas.

5. cos the fmc told you to. not rocket science ya know.

[BOAC]

Beg to differ with PT 1 of 3

Speed of sound is 660kts at sea-level ICAN (ie 15deg C). It is then proportional to the square root of the absolute temperature.

Eg SoS at an SAT of -40deg C = 660 x sqr rt(233)/sqr rt(288)

Not-so - your's makes the SoS at ICAN sea-level 1159.2 kts and you are going backwards at negative SATs!!:D :D

Mike Lithgow would have been a bit pushed...................

[Trigger Happy]

Notsofantastic,
Cheers for taking the time. Appreciate your help.

[Capt Pit Bull]

ditto.

plus, Oicur, if your MEL allows dispatch (ours does) with the FMC inop, what are you going to do then?

d'oh!

[Tinstaafl]

Re Q.1: in addition to the efficiency aspects you also have to consider what cruising levels are available ie the hemispherical rule (even '000s one way, odd '000s the other).

[FlyUK]

LSofS is calculated using the following formula:

38.94 X Sqrt of temp in kelvin.

Eg. At ISA 38.94 x sqrt of +288

= 38.94 x 16.97

=660.8 Kts

Therefore the higher you go the colder is gets, hence the LSoS decreases. :8

[Old Smokey]

If you had to go from Syd to Darwin what height would you go.

The optimum level offering the minimum fuel, minimum cost, or best Fuel/Time ratio, i.e. according to the current Cost Index.

The level chosen would be evens up to F/L 400 for a RVSM aircraft, or up to F/L 280 for a non RVSM aircraft. Above these 2 levels ICAO levels (Odds every 4000 feet after F/L 310 applies)

Why not fly as high as possible?

As high as possible would probably be ABOVE the optimum level, unless a favourable tailwind existed, and that would be very unlikely SYD-DRW.

What happens to the speed of sound as you get higher?

It decreases proportional to the square root of the absolute temperature UNTIL you reach the Tropopause, and then it remains constant. (Don't forget Tropopause in ISA =36089 ft, 11000 M).

If 743 flying at HIGH altitude (0.84M) and another one is flying at LOW altitude (0.84M), which would have the higher TAS?

The lower one would have the higher TAS because of the higher temperature, as long as it was below the Tropopause. If both were above the Tropopause, both would have the same TAS.

Reconfirm how high you would fly to Darwin and why?

No change to the above.

Don't forget that QANTAS will want to see someone who can see the total picture, not one who can 'zero in' on one aspect only. QANTAS don't hire F/Os, only potential Captains. Good luck future captain.

[Notso Fantastic]

Hello BOAC! You're not RTFQ! 644 + 1.2T (T=deg C) gives ISA SL SOS 644+18= 662kts.
At -40C, SOS=644 + (-48)=596kts (or 594 kts by your method which involves dividing square roots by square roots and multiplying by 660- easy upside down in a Canberra if you're good at mental calcs!).

I think a simple mental calculation using a basic easy formula accurate within better than 1/3% might be the better option!

[BOAC]

Sorry, NSF, but it is CTFQ (as in 'check'). Seen your typo?:D

Never need to know in that much detail except when I were an aeronautical student. Now-a-days I just reads the Mach meter and knows it gets lower as I gets higher - to the trop, of course.

Actually I flew with a co last week who really wanted to know - only just had room for the blackboard and easel. I think he was impressed that the old git remembered (anything) :D

[Keith.Williams.]

Trigger Happy,

Please forgive me if I am misreading between lines here, but it looks as if you expected readers to change their answers to questions 1 and 2, after they had considered question 5. If this was your expectation then it might be helpful to look a bit more closely at what the optimum altitude is.

To get maximum still air range we must achieve two things simultaneously:

Firstly we must operate our airframe at its best TAS to Drag ratio. This is achieved at the speed where a tangent from the origin touches the drag to TAS curve. This is typicaly at about 1.32 Vmd.

Drag is proportional to CAS and as altitude increases the TAS at any given CAS increases. So if we climb at constant CAS the drag will remain constant (if we ignore reducing mass), but the TAS will increase. This means that our TAS to Drag ratio increases with increasing altitude. So the higher the altitude the greater will be the TAS to Drag ratio (provided our mach number does not become excessive and cause an increase in drag).

The second thing we must do is to operate our engines at their best Thrust to Fuel ratio. This means getting the lowest cost in Kg of fuel flow for each Kg of thrust. To achieve this we need to be operating our engines in cold air at their optimum RPM. This is typically in the 85% to 95% RPM range.

But at low altitude the thrust in the 85% to 95% RPM band is far greater than the drag at 1.32 Vmd. So if we want to fly at 1.32 Vmd to get best TAS to Drag ratio at low altitude, we must run our engines at an inefficienly low RPM. This will not give maximum overall efficiency (best range or best economy).

If, on the other hand, we choose to run our engines at 85% to 95% RPM at low altitude, then our airspeed will be far too high to give us best TAS to Drag ratio. Neither of these options will give us maximum overall efficiency.

But as altitude increases, the reducing air density reduces the thrust at 85% to 95% RPM. Eventually we will reach an altitude at which the drag at 1.32 Vmd is equal to the thrust at 85% to 95% RPM. This is the optimum altitude. In this condition both our airframe and our engines are operating at their greatest efficiencies so we will get best range and best fuel economy.

If we continue to climb above the optimum altitude, the thrust at 85% to 95% RPM will continue to decrease. So we will need to run our engines at an inefficiently high RPM to get thrust equal to drag. We could of course choose to run our engines at the optimum 85% to 95% RPM, but then our airspeed would be less than the optimum 1.32 Vmd. Neither of these options will give best overall efficiency.

In reality of course various other factors such as ATC requirements, trip length, head/tail winds, and non-fuel costs will also play a part in determining the combination of speed and altitude that is actually used.

[Old Smokey]

Keith,
Excellent post, but I'd disagree with you on one point, namely -

So if we climb at constant CAS the drag will remain constant (if we ignore reducing mass)

If we climb at a constant CAS, the EAS will be steadily reducing, and drag will be REDUCING. Drag arising from the Low Speed Polars is related to EAS, not CAS. (This is actually beneficial to the climb as thrust is also reducing as we climb).

All of this is valid until we encounter Mcrit, as you've pointed out.

[Keith.Williams.]

You are of course correct Old Smokey, aerodynamic forces are related to EAS rather than to CAS.

I deliberately left out the difference between EAS and CAS to keep the explanation reasonably simple. Let's face it, this difference is a very small factor compared to the difference between CAS (or EAS) and TAS at high altitudes.

The heart of the matter is that just getting a high TAS is not enough to get best range. We also need best engine fuel efficiency.

[Trigger Happy]

Thanks again for everyones help. Seeing this was so successful, heres a couple more.

(1) Why is the rudder split on a 747?

(2) Why does the 747 SP have a larger tail?

(3) The duties of a SO and how they differ on the Classic?

I know they are fairly straight foreward but you know what they say about 'the Big Picture'. Cheers!

[Old Smokey]

Trigger Happy,

(1) Why is the rudder split on a 747? - Separate Hydraulic supplies allows for redundancy in the event of hydraulic failure OR jam of one rudder.

(2) Why does the 747 SP have a larger tail? - Much shorter fuselage providing less moment arm for directional stability / control, hence compensated for in having a larger Fin and Rudder surface.

(3)The duties of a SO and how they differ on the Classic? - Sorry, don't work for QF.

Keith Williams - Sorry Keith, felt kinda bad to pick at your excellent submission, but QF above any other operator that I know would pick on just such a thing. Very very Ultra big on performance, or at least they used to be.

Regards,

Smokey

[enicalyth]

Firstly QF make you think... what flight rules?... as you will be flying 310-335 degrees mag depending on the leg that's point 1. Then it's a subtle dig to see what you know about bouncy animals aicraft that actually go YSSY-YPDN. There's the world of difference in what a BAe 146 Quantas Link a/c can do and a 737. Next are you flying RVSM or following the old IFR? Then give a little elaboration; if RVSM can your aircraft get to FL400 a) at all b) profitably in the four or so hours you have to QF it. The answer might well be FL360 bearing in mind what everyone has said about FMS, wind, cost index etc. (If not RVSM then substitute FL350, FL390). Basically you have say 4hrs 15mins to get 120 happy pax YSSY-YPDN getting the most air miles for the least fuel. Fly fast enough to meet schedule but not so fast that you consume too much fuel... that's called V/c ratio. Next you need to fly with a good lift:drag ratio... that's L/D. Multiply the two together... VL/cD gives a figure of merit. When it peaks you are doing it right providing weather, wind, law and ATC permit.
By the way when you get that job lob a stubbie out passing Mudgee (MDG) on your way to Darwin and be prepared to explain the difference between a Mudgee stubbie and a Darwin stubbie. Best of Luck!!