Some doubts from "Ace the Technical Pilot Interview".
 

Hello everyone,

Now I know that the book "Ace the Technical Pilot Interview" contains many mistakes and is not very reliable but I have still read it and corrected the errors checking the explanations with my ATPL manuals. There are a few questions which I'm unsure of and would appreciate if I could get some help on:

Q1) Why does a jet aircraft climb as high as possible?
The book mentions the following two: minimum cruise drag and best engine SFC. I will quote the minimum cruise drag from the book as I'm not sure if it is totally correct:
Q2) Why is the risk per flight decreased with a reduced-thrust takeoff?
The book gives three reasons but it's only the first one that seems confusing:
Q3) What is the wind direction around a high-pressure system?
I have no problem in understanding this but the books says:
I've researched the ATPL manuals I have and it basically says that in the atmosphere we have "compartments" of areas of convergence and divergence, which I would take to be as low and high pressure systems, and that between them you have a level where divergence/convergence is zero, which occurs at the 500 hpa pressure surface (FL180). Now when I look at current surface weather charts and upper level weather charts, I do not see this theory confirmed.

Thanks in advance and happy holidays!

Hi

Q1) The statement if not true, to begin with.
But I understand it as "Why do jets fly high?". The reason you mention is basically true but not fully explained. Thrust matches drag. At high altitudes you have more TAS for a given EAS. Drag is proportional to EAS squared. But specific range is directly proportional to airplane's speed, so at high altitude we have good value for money (better speed for a given drag than at low altitude).
To sum up, at high altitudes we have a better specific range (more miles per ton of burnt fuel).
However, MN also influences drag, so the whole thing is more complicated, and in a set of conditions there is an optimum level. Flying higher or lower than that means flying less efficiently.

Q2) I have no idea if it is right or wrong

Q4) the convergence-divergence theory is not applicable everywhere and it is not true that there is an upper high above a lower low, or the opposite. Wind behaves in a very different manner depending on latitude and other variables.

A major part of the reason we fly high, aerodynamic issues aside, is engine efficiency. A turbine engine is most efficient at higher rotational speeds; typically around 90% of the engine's 100% speed. At lower altitudes, the engine must be operated at lower power settings in order to meet speed restrictions (both regulatory, and airframe). The higher the aircraft goes,the more power is required,and we reach a point where the engine can be operated at optimum power; it's being operated at it's most efficient state.

This altitude depends on the amount of thrust required, which of course depends on the amount of drag the aircraft experiences...and this depends on the speed flown, weight, A0A, temperature, and other factors we consider when computing our optimum altitude. We're seeking an altitude where the combination of weight, speed, and thrust required occur where the engine is most efficiently operated.

If, for example, the engine is most efficient at 90% of it's maximum speed, we operate at an altitude where using 90% will give us the speed we desire, considering weight and temperature. As the airplane gets lighter, less thrust is required (meaning the engine is now operating at a lower, less efficient speed). We climb. As we climb, more thrust is again required, allowing the engine to be operated at it's most efficient speed.

The key to understanding that concept is understanding that the engine is most efficient at a given speed. Think of it like driving a car with a manual transmission up a hill (or a bicycle, if you will). Try using too high a gear and running the engine at too low a speed, the engine bogs down, the car bogs down, and the climb up the hill is inefficient. Instead, we climb the hill in a lower gear, and the engine runs at a higher speed, allowing it to produce the power necessary to get us up the hill. In a sense, the turbine engine is a little the same...it's most efficient at higher operating RPM's, and we achieve this at cruise speed at higher and higher altitudes as the airplane weighs less and less.

This book is riddled with errors - you do need to be careful about using it as a sole source.

I never ceased to be amased at the knowledge on this forum:ok:

I'd be interested in point Number 2, as I cant see how reducing the amount of takeoff thrust could reduce the takeoff run?

Very interesting discussion, agree with SOPS, sometimes pprune gets a bashing for all the silly discussions, but the information that can be gleaned on technical subjects is astounding.

It says reduces compared to full thrust and max weight.

The reason is when you make an assumed temp calculation you correct the thrust for density but you get the full benefit of the actual rho over the wing so performance at assumed temp x is better than performance at actual temperature x .

I will disagree. Engine is designed to be most efficient for the cruise portion of the flight. Cruise, for natural reasons consume the most fuel and time. During cruise we would like the aircraft to be faster and engine to be efficient.

I may risk some inaccuracy, but I will put it like this. The jet engine will like to fly in dense air (Low altitudes), the denser the medium better the jet action. The jet aircraft on the other hand will like to fly high, in thinner air. Thinner the air higher the speed.

So here we are with two conflicting demands, and as always there is a point of balance. In order to determine the point of balance, we need to have a qualifier, such as fuel economy. Turns out that from this fuel economy point of view a jet should fly at an optimum altitude, which is higher than turboprop or piston prop aircrafts.

They don't fly high they fly at an optimum altitude. So if the interviewer asks you this questions and you start with politely rephrasing the question to "Why do jet aircrafts have an optimum cruise altitude higher than turboprop or piston prop aircrafts?", you may win the interviewer without even answering.





You enumerated everything but missed the single most important factor which is density altitude. It would have made things a lot easier from point of view of explaining.

Circumlocution in its worst form.


hmm... leaves me speechless.. we could better understand aircraft engines once we agree to put such analogies in deep freeze.

Q 2)
 

Q2) Why is the risk per flight decreased with a reduced-thrust takeoff?

The Book says:


@ Mohit, since you stated that you found only the first reason confusing, I will presume that you don't understand what is and why do we need reduced thrust take off. I may stand wrong, but you can correct me on this.


Reduced thrust at take-off, why on God's earth should I reduce thrust at takeoff? Thats the big question. Why don't we teach such things at flight schools? Why don't we do it with C-152s?

Take-off is a time at which the engine is at maximum stress. As safe pilots, we should try to reduce stress on engine whenever possible. Engine is your friend. By allowing an engine to operate at lower thrust during takeoff, we are reducing max thrust operation times, such times degrade engine life quiet fast.

When do we do it?


1. Long runway: If you are not operating from a short field, you can reduce your takeoff thrust and save on some engine maintenance time for your airline.

2. Less weight at takeoff: Lower weight at takeoff, lower thrust.

3. Great headwind: headwind reduces runway length required.

4. Noise abatement: Some noise abatement procedure may require reduced thrust, its better to listen to less louder noise for longer duration than a loud noise for shorter duration. (Noise is a complex territory)


A reduced thrust takeoff program, at any airline if managed with proper safety insights in mind, can save a lot of engine repair time and money. It also increases safety. An engine which is operated in reduced-thrust take off program, would have had to spend lesser time at stressful high rpms, and hence should be left with more life, before the overhaul is done. On the other hand engines used at max thrust, may have had too many revolutions at high stress and may run out of safe life before next overhaul.

Cars which race everyday have much shorter life than the one who driven gently with care. Same thing goes with aircraft engines.

Shockers in your car will have considerably shorter life if you drive fast on speed breakers. Short term but high stress operation eats fatigue life much faster than average stress operations.

This is the reason why reduced thrust take-off reduces risk per flight. You know you are flying an engine which nobody stressed too much. Its gotta be safer.

Lets come to the books explanations here.
Assumed/flexible temperature method
Jet engines have limitations on turbine temperature, you can't let it go high. If temperature goes too high the engine may get internal damage. When the air outside is hot, they have an inbuilt mechanism to reduce max RPM to make sure the temperatures at the turbine don't exceed the limits.

So if you fool the engine by telling that outside temperature is 40c while actual temperature is 20c, the engine computer will reduce max rpm when TO/GA switch is pressed. You can feed this assumed temperature in the FMC. Some manufacturers call it flexible temperature, the FMC is allowing to let you operate the engine at the chosen flexible temperature you want. Hence the assumed/flexible method designator.

You can still get the max thrust by moving throttle to full position, this will override the assumed/flexible temp setting.

The answer in book is cofusing, seems like it has been verbatim copied from an interviewee's reply. The response shows rote learning rather than understanding.

De-Rating

This is not an actual reduced thrust takeoff. I listed here to remove confusion. De rating is done to by maintenance staff to limit maximum thrust, for example from 25kn Flat Rating to 23 kn De-rated thrust, this can be done for several reason, such as engine life and making two slightly different engines have same thrust. Pilot cannot control this during flight or take-off.



Which aircraft will have better accelerate stop distance:
A) the one with 50kn thrust
B) the one with 45 kn thrust

The answer to above question, reveals the in-correctness of the reason 2. Or may be I don't understand what author is trying to say. Can any one on the firum help here?


This seems to me a sentence which is out of context. Apparently copied from some text and inserted in the book without thinking about the relevance. I don't know how this ability reduces risk per flight as compared to having the full thrust already.


I hope this helped.

You'd have best remained speechless, given your poor understanding of turbine engine efficiency, and the purpose for flying high.

Simply put, a turbine engine can't be run at it's most efficient state at lower altitudes, due to excess thrust and excess airspeed. Only when we are required to run at higher power settings at higher altitudes are we able to operate the powerplant at it's most efficient state.

Obviously.

You throw out this rubbish and then go on to whine about "circumlocution in its worst form?" The answer to why a turbine engine is best operated highest...is "because its' optimum altitude is higher than turboprop or piston prop aircrafts?" Not much of an answer. You might as well go with the Acing the Technical Interview responses. You'll get nearly as much mileage.

Optimum altitude is a function of aircraft weight, ambient temperature, desired cruise speed, etc.

Hi, pruners I would like to know if somebody have a web page or link to get acces to the content of this book I remember in this forum some months a go I see a link to this info, so may be some one has it

I really appreciate if you can give me this info

many tks

toby :ok:

I think the theory is overtly simplistic to enable examiners to set solvable questions. The basic theory as I understand it is that the wind will follow lines of constant pressure (isosomthingelses) at mid to upper levels. Same law of NH wind on back with low pressure to left. So upper level convergence and divergence is really a bit of a red herring insofar that all upper level pressures are relative and a surface low does not necessarily translate to an upper high if the temperature gradients and relative heights of the various columns are taken into account.

Thanks for all the feedback guys. Very helpful. Just a last couple of questions I need help on:

I can't find any sources which mention these conditions from my ATPL manuals. Any suggestions?

Now I know 0000 is when transponder malfunctions but this "200 ft or greater difference betwen the aircraft and ATCs radar height readings" I find no source to confirm it from.

Thanks again.

When can you override a GPWS warning?

You should never ignore a GPWS warning, especially EGPWS/TAWS.
Although you may think that you are safe, most terrain encounters are due to human errors (yours or someone else) in misunderstanding the situation / location.
Always commence a climb and reassess the situation at a safe altitude, not just when the warning stops.
Remember that nothing is ‘so obvious’ in aviation as the scene of a CFIT accident.

why do we fly high in jets?

It's to get the maximum miles per gallon.

This is know as the Specific Air Range or SAR

the formula for SAR is TAS/GFC

GFC is Gross fuel consumption (gallons per hour)

GFC = Thrust*SFC

SFC is pound of fuel per pound force of thrust per hour.

We can derive the following formula for SAR if we realise that in steady flight Thrust = Drag.

SAR = (1/SFC)*(TAS/Drag)

Now we have 2 factors that we need to optimise to get the maximum SAR:

1/SFC is purely engine related.

TAS/Drag is purely airframe related.

To get the highest value of 1/SFC we need SFC to be as low as possible. This means we need the engine to be producing it's maximum thrust per pound of fuel and this occurs by design when the engine is operating around max continuous. Any lower trust and you are wasting fuel.

So 1/SFC is largest at @max cont

now lets look at TAS/DRAG

that's pretty easy really isn't it? If we think about the drag curve then this must be @1.32Vimd (max range speed)

so now we have 2 conflicting requirements

a) max cont thrust

and

b) 1.32 Vimd

so how can we match the two?

the last thing we have at our disposal is air density so we can climb to reduce the density of air entering the engine, and therefore the mass flow of air at a fixed RPM and intake size. This of course means we are reducing the thrust. We climb until the thrust is such that we equal the drag at +.32Vmd and we have found our optimum altitude.

As the aircraft weight reduces with fuel burn we could continue to climb to match thrust to the reducing drag (less lift required means less induced drag). This cruise climb is the theoretically optimum way to operate a jet but as the ATC environment generally prohibits cruise climbs we step climb instead.


in summary we fly high because it allows us to operate both the engines and the airframe in their respective optimum bands.

afterthought:

In my old days on maritime patrol we had a spanner in the works when it came to optimising fuel use. We were often required to fly around at low level.
Solution: shut down some engines to keep those running as close to optimum as possible.

Sometimes we were more interested in loitering rather than getting from A to B and for this the reference speed was a gnats chuff above Vmd rather than max range.



p.s. Jimmygill is wrong to say de-rate is done by maintenance staff. It can be done by the pilots through the FMC/MCDU before each flight on most modern FADEC engines.

If the question were" When can you overide a GPWS warning" that is completely different to " when can I ignore a GPWS warning".. Nuisance warnings do occur, and NO WARNINGS are ignored, however, warnings can be overidden. Examples would be circle to land with terrain inhibit, non-normal landing configs, flap inhibit and so on. As a rule, you can only overide the warning if you know why it has occurred or you are visual, or both. The non-normal sections of both the Airbus and Boeing FCTM tackle this issue.

FE Hoppy

Excellent post indeed!

Regarding the engine shutting down for fuel saving, is de efficiency gain by the live engines so significant that it overcomes the extra drag of a windmilling engine? Does it depend on the number of engines the airplane has?

In the 320, with an engine out we burnt approximately a 33% more fuel, according to the FCOM.

Microburst2002

Yes the gains easily overcome the windmilling drag. Of course we had 4 spey 250s embedded in the wing roots. Once we started flying around at 1000' agl or below 4 engines were way too many. We could almost always shut one down straight away. As the weight reduced we would bring the second back to idle firstly. And only when we had a certain amount of single engine performance (we're talking a 4 jet here) would we actually shut down the second engine. Now if we happened to have a problem with one of the two remaining engines the first action was to relight one of the shutdown ones.

Next time you have your holding tables in front of you compare two engine fuel flow when holding at 1500' at typical landing weights with single engine holding at the same weight. You may be surprised.

Many ETOPS critical fuel scenarios are decompression with 2 engines running rather than decompression with engine failure.

Kirks gusset, re #16.
There are situations where the Enhanced functions of EGPWS can / should be inhibited; as you state these are in the AFM. As the systems installations improve with GPS embedded in the EGPWS, the use of geometric altitude, and the terrain database expands and is updated, there are few if any situations where the terrain functions need to be inhibited.

A critical and mistaken belief is that EGPWS suffers nuisance warnings. Whilst any warning system might fail or react inappropriately in extreme situations, the proven reliability of EGPWS is now better than 0.03 alerts per 1000 flights (egpws.com), at least 100x better than GPWS (2003 data).
In some comparisons, this level of reliability is better than the error rate of most human performance. Thus, it could be more likely that the human has made a mistake and not the system.

Pilots should always react to a warning. The debate whether it was warranted or not, can and must be made at a safe altitude before continuing the flight. The essence of the situation is that you don’t know if a warning is valid or not.
The industry does not appear to suffer the same level of skepticism with ACAS or windshear alerting; perhaps this is because GPWS had a relatively poor history.
EGPWS should be considered as something completely new, highly reliable, and extremely effective.

Re: As a rule, you can only override the warning if you know why it has occurred or you are visual, or both.
See TAWS ‘Saves’ - some real life experiences, which indicate that ‘as a rule’ there should not be any rule – other than always pull up.

Safetypee, we will have to agree to differ on this one. The realiability of EGPWS is not in question but cannot override pilot situational awareness. For instance, A circle to land in CMF we may get a warning due terrain on the base turn, but we know this, it's in the brief, the SOPS cover this,and are visual. With Airbus, again the circle to land can cause an issue if the secondary flight plan is not active and the "wrong" runway is active, again a visual situation. If the Airfield is not in the FMS, the EGPWS does not give the full warnings. In all these situations, we would certainly not pull up otherwise we would never land.. I agree if the cause is unknown then there is no question of a pull up. Incidently, as far as I am aware, Boeing still inhibit the Flap warnings/ gear warnings when s/e.. the current QRH reflects this.
" The essence of the situation is that you don’t know if a warning is valid or not" Not entirely correct, we do in the above circumstances, and it is covered in the brief