1st Interview Q's

[MLS]

Hello working my way through the XYZ book for an interview in Aus this coming week.
Does anyone have some answers to the following questions?

How would you teach a student about Vmca?

Why do you teach Vmca in the aircraft vice the simulator?

Thanks in Advance!!

Anyone else here got a stage 1 in Aus?

[WaldoPepper]

How about something like:

What is Vmca ? (definition)

What happens when reaching Vmca ?

Factors affecting Vmca ?

How does the manufacturer determine Vmca ?

Recovery method

As far as why not teach it in the sim as opposed to an aircraft, i guess the sim cannot simulate the yaw sensation followed by an uncontrollable roll toward the failed engine. Also, doing it in the aircraft shows the student exactly what it feels like in that particular aeroplane, and that it's reasonably scary enough to not want to do it in real life. After experiencing it for real, most people will stay away from slow speed flight on 1 engine.

Hope this helps

WP

[MLS]

Thanks WP,

I was hunting somewhere around that answer but I wasn't 100% confident. Thanks for taking the time out and replying.

MLS

[Altyre]

Hi MLS
" Ace the technical pilot interview" by Gary Bristow has a good explaination of VMC and how you should teach it. It also has a lot of interesting material they will ask you about at your CX interview, I found it to be a big help for mine.

[betaboy]

I found Bristow's ACE book pretty good generally, but, unless I'm missing something, he seems completely incorrect on the answer to the question "What are the advantages of a wide chord fan blade" (p. 67).

He writes "The chord is the LENGTH of the fan's blade from its center mounting to its tip. It can be said, therefore, that as a basic principle, the wider the fan chord, the higher the blade tip speed, the greater is the generated airflow speed and mass, and the greater is the discharged air pressure, resulting in an increase engine thrust."

I am quite certain than the word chord refers the WIDTH of the fan blade from its leading edge to the trailing edge. Googling "wide chord fan blade RB211" leads to many hits which point to benefits such as: lighter weight than a conventional fan with more strength, less mechanical complexity, better ability to handle FOD and bird strikes, higher fan throughput and better aerodynamic efficiency, etc. So what is Bristow talking about?

Also, while we're on the subject, does any body know how anti-icing is accomplished for the fan blades? Is it not required because the blades just don't tend to accumulate ice?

[Pointer]

Betaboy: it has my understanding that blades don't need anti icing due to high speed, when any accumilation takes place center of gravity rule makes it go away.

Just recently had interview; did not have the impression that it was so technical; more situations and how to judge them. it took about 20 min for the 30 Q's

Different operational statements and responses. I hope they liked what i had to say.

Pointer :E

[Smokie]

Bristow also gives the incorrect critical engine during X wind T/O's
It is of course the upwind engine that is the critical one not the downwind.

[MLS]

Thanks all,

I agree with the wrong answers on the Wide cord fan blades. The rest of the book is pretty good. Saves time digging out heaps of answers from everywhere.

Most of the answers are pretty basic.

One question that I’m still not 100% happy with is at what stage in flight is lift the greatest?

I have two answers so far which seem to make sense; one is on takeoff – due, hight lift devices, hight weight. The other is in during straight and level flight? Not sure which one is correct. If anyone else had some insight it would help as I can see arguments for both cases.

Thanks all

MLS

[Canadian Beech]

I've always taught that the most lift is created at highest angle of attack you can have (just before the stall). Any further increase will decrease the lift. This is where your Coefficient of lift is the greatest.
anyone else?

[betaboy]

How about: at any point in a flight, the main purpose of lift is to counteract weight, thus, lift is greatest at takeoff because that is the point in the flight at which you weight the most.

[404 Titan]

Yep, Take off you will have the greatest lift because of greatest weight. Lift is also greatest in a turn, particularly steep turns. Angle of attack (except turns and steep turns and other high G maneuvers) and high lift devices are irrelevant as they only increase the co-efficient of lift, not lift itself, when the aircraft is in steady level flight, i.e. they allow the aircraft to fly slower and produce the same lift to counteract weight. If this weren’t the case the aircraft would climb or descend. Also what do you think happens to lift in a steady climb or descent? It actually decreases. If you haven’t already got it, I suggest you get a copy of NAVWEPS, Aerodynamics for Naval Aviators. It is a US publication. The reading is hard going but it is a wealth of information.

[treholer]

Hmm, That very same question was in my 1st interview tech quiz (July). I answered in Descent. The interviewing officers never even commented on this. I still believe I am correct. Will happily debate this

TH

p.s. My line of thinking was when is lift greater than weight

[404 Titan]

treholer

I can assure you that you are incorrect with that answer. I haven’t got one readily available to me but if you can get yourself a copy of a detailed forces diagram of a aircraft in flight, you will see the lift slowly decreases to zero the closer an aircraft reaches a vertical climb or descent. In a vertical climb the only forces acting on an aircraft is thrust, weight and drag. In a descent it is weight and drag.

[Omark44]

For a given power setting I'm inclined to think that lift will be greatest when drag and gravity are the least, that is lift when compared to thrust, drag and gravity.

Lift at it's greatest will be a product of least drag, least effect of gravity? i.e. the maximum amount of lift that can be obtained for the minimum amount of effort?

Happy to be shot down!:D

[medflyer]

To quote 404 Titan: "In a vertical climb the only forces acting on an aircraft is thrust, weight and drag. In a descent it is weight and drag."

As I understand it: In flight, there are 4 forces acting on the aircraft, Lift, Thrust, Weight and Drag....they all have to be in equilibrium...In a climb, lift IS less than weight, but the THRUST component acting in the vertical plane (due to the inclined angle created by the thrust line relative to the flightpath) creates a component of lift which contributes to the net lift and will EQUAL weight. The aircraft climbs due to an excess of thrust.

Now, point the nose downhill...assuming a power off glide (ie: no thrust) the weight still (always) acts towards the centre of the earth and will equal lift; therefore if the a/c weighs 300,000 pounds you will have lift of 300,000 pounds; however you must now picture a component of weight acting forward along the declined flight path; it is THIS component that will counteract the drag acting on the aircraft; ie: Drag of 50,000#, weight of 300,000# (acting downward to the centre of the earth; ie: gravity), lift of 300,000#; thrust nil, component of weight acting FORWARD perpendicular to the flight path 50,000#.

To reiterate: All forces strive to stay in equilibrium.

[Pointer]

Why take such a difficult approach to an easy awnser....?

if you take the lift formula there are (if we assume weight is constant, we do because its the flight parameter wich are important)

You can see lift is proportionate to V(square), or better proportionate to the amounth of air flowing over your wing.

then if the portion of air is slow we need to create more lift out of the same 'parcel' of air (less air in same time)then when need to do when there is more air (faster) flowing in same 'parcel' (same time). therefore we need more lift enhancing facility's and milking it (more deflection of air) means more drag. but the drag was not the question.

so during T/O the lift generated out of the same volume of air is greatest. :ok:

fire away :\

Pointer :E

[404 Titan]

medflyer
Now, point the nose downhill...assuming a power off glide (ie: no thrust) the weight still (always) acts towards the centre of the earth and will equal lift; therefore if the a/c weighs 300,000 pounds you will have lift of 300,000 pounds; however you must now picture a component of weight acting forward along the declined flight path; it is THIS component that will counteract the drag acting on the aircraft; ie: Drag of 50,000#, weight of 300,000# (acting downward to the centre of the earth; ie: gravity), lift of 300,000#; thrust nil, component of weight acting FORWARD perpendicular to the flight path 50,000#.
I strongly suggest you don’t give an answer like that at your CX interview because it is dead wrong. Aerodynamics is something that you should have a good understanding of. I suggest you get a copy of the book I quoted, as it will quite clearly show you that I am right. To clarify my point, imagine an aircraft that has an excess power to weight ratio. What do you think happens to the angle of attack as you approach the vertical? Before you answer the question, keep in mind where the relative airflow is coming from. The angle of attack drops to zero. Now if we have a zero angle of attack what happens to the CL? It drops to zero. If CL of lift is zero what happens to lift? I’ll leave that for you to work out. To make it easier I have provided the lift formula for you, just in case you have forgotten it.

An aircraft's lift capabilities can be measured from the following formula:

L = (1/2) d v2 s CL

· L = Lift, which must equal the airplane's weight in pounds
· d = density of the air. This will change due to altitude. These values can be found in a I.C.A.O. Standard Atmosphere Table.
· v = velocity of an aircraft expressed in feet per second
· s = the wing area of an aircraft in square feet
· CL = Coefficient of lift , which is determined by the type of airfoil and angle of attack.


PS: Have a look at some of the other posts I have put on this sight for wannabes. I don’t have a reputation for leading people up the garden path. There are a few of us here that put in a considerable amount of time and effort to help people like you with the CX interview process. Bighting off the hand that is trying to help you isn’t a good idea.

[MLS]

Titan,

I think my answer will be

Lift would be greatest during the acceleration level segment (after takeoff) due to the high weight and the fact that the aircraft is in level flight at that very high weight.

What do think?

I agree with the fact that lift is reduced during a climb and descent also.

Directly in replying to my posts and indirectly in replying to other peoples posts your answers have help my preparation greatly. I would like to thank you and others who have taken time and effort to give real information. I have always been told in aviation to get the information straight from a text book, but getting someone to point you in the right direction is a great start.

Interview is Tuesday so better get back to the study!!

[404 Titan]

MLS

You are welcome. I know that most appreciate the help that some of us already here give. Yes I do agree with you about lift being greatest “just at lift off” and in the level acceleration phase if for the only reason that weight is the greatest. If we ignored the fact that weight changes with time, lift is the greatest in level flight, turns and any other maneuvers greater than 1g. Lift is the least in a steady climb and descent and any maneuver less than 1g. Oh and another common belief: “Flaps increase lift”. Nope, they increase the lifting ability of a wing at lower speeds.

Good luck with your interview. Don’t stress if you can’t answer a question. Just tell them you aren’t sure. Don’t BS them as they can see that. All they are really looking for is someone that can sell himself or herself and can handle a bit of stress and is not a pain in the butt. Knowing your tech stuff just means you have put the effort in.

Good luck with the interview. I’m sure you’ll do well.

Regards
404 Titan.

[medflyer]

404 Titan;

Your posts are informative--I'm not trying to bite anyone's hand; merely debate a point. I do feel a bit foolish however, as I have reread your original post more clearly and now see you were talking about the forces in VERTICAL climbs, whereas I thought you were discussing a NORMAL climb/descent...

:O


Keep the good info coming, it is genuinely appreciated :ok:

[BlueEagle]

Sorry I can't help you with the .jpg images here. First they have to go to a host site and from there they can be linked to PPRuNe.
If you know someone with their own web page you have got it cracked!

[clohessy the claw]

the chord is definitely the width of the blade and surely the point is to keep blade tip speed to a minimum and definitely subsonic.his advantages are however valid.

could anyone resolve a conundrum for me,i cannot get my head around the question in bristow page 2 what is the effect of weight on rate of descent and the apparent contradictory answer to question on page 18,how does weight affect an aircrafts flight profile descent point?

surely if the heavier aircraft has a greater rate of descent would it not require a later descent point,not an earlier one.

maybe the answer is there,but i have been mulling over it so long that i have confounded myself.
any help appreciated.

[betaboy]

Yes, agreed, Bristow's "check the momentum" explanation is next to useless. However, I know this effect is true. The machine I fly is very sensitive to this; you have to start a barber pole descent much earlier if you're heavier.

I've perused and searched the Tech Log, and have carefully poured through about 4 aerodynamics texts (Kermode, Dole, Naval Av, etc.), and have not found a definitive answer. I found a plausible explanation in an old (borrowed) internal Air Canada document on fuel efficiency: "higher landing weights will increase the descent distance for the SAME descent speed since it takes longer to dissipate the greater potential energy."

Also, lots of confusing discussion at:
http://www.pprune.org/forums/showthread.php?s=&threadid=129617
where the concensus seems to be that that the heavier a/c would be flying closer to Vmd (min drag speed).

And, a somewhat believable explanation from Bellerophon at:
http://www.pprune.org/forums/showthread.php?s=&threadid=125401

I'd be really happy if someone could point me to a definitive reference on this.

A heavier a/c will descent faster if you DON'T fix the airspeed. I.e. fix the AOA, and the heavier aircraft will descent at a higher airspeed, leading a higher descent rate.



--------------------------------------------
Also, a link to a picture re. the excess thrust discussion above:
http://history.nasa.gov/SP-367/f119.htm

[grind king]

Greeetings Gents,

It is my understanding that for a given speed in descent the heavier a/c will descend at a higher rate. Conversely for a given rate of descent the heavier a/c will descend at a faster speed. As top of descent point is based on ground speed the heavier you are the faster you'll go hence the earlier you must start to descend.

This is my take on that. So it's only worth the paper that it's printed on. Hope it helps though.

[Pointer]

The awnser i gave is above and that's (among other things) what got me through to the next round. :D

See you in HKG...

Pointer :E

[jetset]

Hi Guys

After both doing the interviews (1st) and reading the books xyz, abc and ace, my tips.

Use the questions as an aid to research, there are mistakes in all three.

Remember that it is just required knowledge and the point of the interview is to recruit pilots for cathay not necessarily aeronautical engineers! So have a good think about the rest of it as well.

Good luck



:ok:

[Medwin]

Smokie,

Bristow is right about critical engine on x-wind. It should be the downwind because if you lose it(downwind eng.) the upwind engine is going to aggravate the yaw.

[VR-HDB]

Medwin:
Bristow is right about critical engine on x-wind. It should be the downwind because if you lose it(downwind eng.) the upwind engine is going to aggravate the yaw.

No, not again...

If you consider giving "Bristow answers" at an interview - do a search here about the book first!

[Medwin]

Dang, now I'm all confuse.
So, which one is the critical engine on 4 engine aircraft? Im just looking for a quick answer about the book, what's wrong with it?

[QNH1013]

On T/O the outboard Upwind engine will be the critical engine.
ie normally the aircraft will weathercock in to wind and you apply appropriate rudder to maintain centreline. If the upwind engine fails, due to the assymetry, you would have to put MORE rudder input, aggravating the situation.