Conflicting feedback

[Rubber Dagger]

can anyone help?

I am taking my ATPL exams in May this year, i have just been doing some feedback on Instruments, the papers i have are from the 2 "main" roundschool providers, and both give a different answer to the same question:

What does the "barbers pole" on an ASI indicate?

a. VMO & Altitude

b. VMO & Temperature

c. VNO

d. VNE

One gives answer a, and the other gives answr b? i have looked in my notes and can not find the answer anywhere.

Any ideas??
:confused:

[High Wing Drifter]

Answer A is the Bristol answer. I too got caught out with this when doing the feedback. As Mach is based on temp then I answered B. It is wrong becuase a machmeter gets Mach thus: M = (Pitot - Static) / Static. So the higher the altitude (the lower the pressure) lower the speed of sound. The machmeter automatically compensates for temprature but does not measure it. Well that is my understanding of it. If I am wrong I would like to know...

[silverknapper]

I think its A too.

Ah, the dangers of looking at feedback from more than one source!!!! Happened to me with Law, I ended up scrapping the other stuff!

SK

[Northern Highflyer]

The value of Mach does not relate to temperature directly.

Mach = TAS/LSS

LSS is affected by temperature, so as you climb the temperature drops, therefore the LSS drops. If you are in an isothermal layer then the LSS will remain constant as temperature remains constant.

However TAS is dependent on air density, which as you climb also drops whatever the air temperature is. Remember the formula :

0.5 rho V squared

When density decreases your TAS increases to maintain a constant CAS.

Therefore Mach depends on altitude, not temperature.
VMO relates to a Mach number. Answer A

Regards

NHF

[peb]

The device consists of an alarm connected to a barometric aneroid capsule subjected to a static pressure and an airspeed sensor subjected to a dynamic pressure. The airspeed indicator of an aircraft is provided with a moving red and white hatched pointer. This pointer indicates the maxium speed in VMO operation versus altitude.


So the right answer is A

[Keith.Williams.]

This is a well known, rather poor JAR question. Strictly speaking none of the options are true, but A is the least untrue. To understand what I mean by this we need to consider what the "Barbers' Pole" actually indicates.

The maximum speed at which an aircraft may be flown depends upon its altitude. At low altitude the limiting speed is Vmo, whereas at high altitude the limiting speed is a mach number Mmo. The purpose of the "Barbers' Pole" is to give an indication of the CAS value of whicheve of these two speeds is the most limiting. So at low altitudes it indicates Vmo.

As altitude increases, the TAS equating to Vmo increases due to decreasing air density. At the same time, the local speed of sound (LSS) decreases due to decreasing temperature. Mach number is equal to TAS/LSS. So as altitude increases, the increasing TAS and decreasing LSS cause Vmo and Mmo to move closer together. At a certain altitude the CAS equating to Vmo is equal to the CAS equating to Mmo. This is the altitude at which a climbing aircraft shifts from being limited by Vmo to being limited by Mmo.

So as an aircraft climbs above this altitude the "Barber's Pole" changes from indicating Vmo and starts to indicate the CAS equating to Mmo. This means that the "Barbers' Pole" indication remains constant up to a certain altitude then starts to decrease.

So strictly speaking the pole indicates Vmo at low altitude and Mmo at high altitude. It never actually indicates any altitude or temperature. But the dominant factor in the change-over from one limit to the other is increasing altitude, so the best answer to this question is option A.

[mad_jock]

And more to the point how does knowing this improve you being a pilot?

And how many pilots that are flying the line everyday would be able to answer the question?

MJ

[Keith.Williams.]

We are now getting into the realms of whydodecaadodat????

I'm afraid I do not have an answer to that one, but I've been advised to go for oprtion B whenever I do not know the answer.

[mad_jock]

I was told always told go for the longest answer if usure

And if its B as well, you have to be right ;)

MJ

[OneIn60rule]

You are flying at a constant Mach number at an altitude of 35000 feet and overall temperature decreases, what happens with your CAS?

I don't remember the question very well but no matter what your altitude was, as long as you were not climbing or descending then your CAS would stay the same.

[Rubber Dagger]

Thanks for all your replies,

It has helped me to,well, not really understand the question but to get me to the right answer!! which seems to me to be what 90% of the ATPL/CPL exams are all about, knowing which box to tick, lets face it who really understands half of it?? Lets do what we have to do to get through the piles of CR*P get the right results and move on! we can really start to learn when we get to the employer who will teach you and tell you what you NEED TO KNOW. He will not be interested whether or not you know which actuator moves which ram on the undercarraige, he would be very interested though in how you operate on the flightdeck!! its not as though you can whip out your toolkit and leap into the wheel bay and sort out the problem at FL350 over Jalalabad.

Anyway thats my gripe over, (Can't you tell iam up to hear with the "Books").

Again many thanks:

ok:

[High Wing Drifter]

Lets do what we have to do to get through the piles of CR*P get the right results and move on! we can really start to learn when we get to the employer who will teach you and tell you what you NEED TO KNOW.
Maybe I'm a bit of a nerd or something, but I find the ATPL syllabus fascinating. All this stuff I often wondered about and now I get to know (excl air law, comms and ops proc) :)

[Ojuka]

Keith Williams is correct. The only feasible answer provided in the multiple choice is Vmo. A barbers pole does not INDICATE temperature or altitude, it only INDICATES a speed limit. The question is worded stupidly. Had it asked what a barber's pole USES to provide its readings, then options A or B could possibly be considered (I don't know, I can't be bothered to dig out the books).

Are you absolutely sure this question is worded correctly in your post?

[peb]

In Spain the this question is worded like this:


The airspeed indicator of an aircraft is provided with a moving red and white hatched pointer. This pointer indicates the:

A) speed indicated on the autothrottle control box, versus temperature

B)speed indicated on the autothrottle control box versus altitude

C)maximum speed in VMO operation versus altitude

D)maximum speed in VMO operation, versus temperature


And the right answer C