PPL Questions help

[ewan2k_uk]

Hey folks,

Im struggling to get my head around these tyoe of questions, im kind of there but would be beneficial if someone could explain them. Im aware that 1mb roughly equates to 30ft at lower levels.

Q - You are setting your altimeter at an aerodrome, where the elevation is 18ft. The QHN you have been given is 1005mb. What would you expect the aerodrome QFE to be?
A) 1006mb B) 1003mb C)1004mb D)999mb


Q- You are flying at FL55, what is the altitude of your aircraft if the QNH is 1008mb?
A) 5200ft B) 5350ft C) 5650ft D) 5750ft


there was one more question that got me regarding the quadrantal rule and a mb setting but i cant find it, once i do il post that as well.

Ewan

[Tailwheels_r_us]

Hi Ewan

You have been given the QNH of 1005. If the airfield is at 18ft. above sea level, would you expect the pressure to be..

a) The same as at 0ft (QNH)
b) Lower than that at sea level
c) Higher than that at sea level

Flight levels are flown at standard pressure 1013.2

If you have selected 1008 (5.2 lower), would you expect your indicated altitude to be higher or lower than 5500?

How much higher/lower given that the differential is 5.2?

[Whopity]

Try doing a simple drawing, you know the relation of pressure to height so its only a case of applying the figure in the correct direction. QFE wil be lower than QNH unless the airfield is below sea level.

A Flight level is measured relative to a pressure datum of 1013 (forget the 0.2) If the QNH is 1013 the altitude will be the same as the flight level, if the QNH is lower, the altitude will be lower.

[Crash one]

I would go for C 1004 (1004.5), & B 5350ft.
Based on the fact that if the setting on the ALT is higher than the actual then you are lower.
If you are given QFE 1000 at an airfield 300ft elevation & you fly on the QNH 1010 you will find the trees a lot closer than they should be!
I think.:ok:

[keith williams]

Hopefully the previous posts have solved your problem. but if not the following may help.

Q - You are setting your altimeter at an aerodrome, where the elevation is 18ft. The QHN you have been given is 1005mb. What would you expect the aerodrome QFE to be?
A) 1006mb B) 1003mb C)1004mb D)999mb


QNH is the pressure at mean sea level.
QFE is the pressure at the surface of the airfield.
Pressure decreases at a rate of about 1 mb (or Hpa) per 30 feet altitude increases.
Elevation is measured from mean sea level.

In this question the airfield is 18 feet above mean sea level so the pressure at the field will be about (0.5mb less than that at mean sea level.

With QNH of 1005 mb the QFE (pressure at the airfield) must be about 0.5 mb lower, which is 1004.5 mb. So the best option is 1004 mb.


Q- You are flying at FL55, what is the altitude of your aircraft if the QNH is 1008mb?
A) 5200ft B) 5350ft C) 5650ft D) 5750ft


Flight levels are measured in hundreds of feet above the 1013 mb level. So FL55 is 5500 feet above the 1013 mb level.

QNH (the pressure at mean sea level) is stated to be 1008 mb, so mean sea level must be higher than the 1013 mb level.

Subtracting 1008 from 1013 gives a difference of 5 mb. Using the standard pressure lapse rate of 30 per mb this equates to 350 feet.

So mean sea level is 150 feet above the 1013 mb level. Subtracting 150 feet from 5500 feet give an altitude of 5350 feet above mean sea level.

[ewan2k_uk]

Thanks all, thats perfect.

Good explanations which has now made it clear :ok:

Ewan