Opinion on Sample Question on Altimetry

[Bjarnum]

An aircraft flies from aerodrome "A", where the QNH is 1020 mb, to aerodrome "B" where the QNH is 999 mb. Aerodrome "A" is 800 feet above mean sea-level and Aerodrome "B" is 500 feet above mean-sea level. If the altimeter sub-scale is not changed from 1020 mb, what will be the altimeter indication on landing at Aerodrome "B"?

a. 1430 ft
b. 130 ft
c. 1130 ft
d. -130 ft

This question is straight out of the PPL Series Book #4 on Meteorology. In Chapter 6: Altimetry Questions #6 on page 84.

My answer is D. -130 feet. The answer key in the book says C. 1130 feet.
I am having trouble understanding this and would like to hear your thoughts on this question.

Thank You

[Sir Herbert Gussett]

Difference in pressure = 1020mb - 999mb = 21mb

21mb x 30 (for feet) = 630 feet, roughly

630ft + 500(amsl of aerodrome "B") = 1130ft

[Whirlygig]

Sometimes it helps to draw a picture with mean sea level and aircraft altitudes to decide whether to add on the difference or to take it away.

Similarly, if you think about the aircraft you fly and the altimeter sub-scale knob, the altimeter needle will move in the same direction as you turn the knob. Have a look next time you fly!

Cheers

Whirls

[Bjarnum]

Thank you for the help!

[Obs cop]

The easy route here is to know what happens when you change the pressure setting datum on the instrument.

If you "wind the pressure setting up the indicated altitude goes up".

Let's assume you are stationary at an airfield 500' above mean sea level. The airfield's QNH is 1000mb. With 1000mb set on the altimeter your indicated altitude will be 500'.

If you wind up the QNH setting to 1010, then your indicated altitude will also increase. The amount is generally considered to be 30' change per 1mb difference in pressure setting for altitudes below 5000'. Hence winding up our QNH by 10mb in this example will wind up our indicated altitude by 300' (10 x 30) giving an indication of 800' now, even though we havn't actually moved.

Returning to your example and how I would work it out.

Sat at airfield B, with the correct QNH of 999mb set will give the correct altitude of that airfield of 500'. Now we actually have 1020 set, so if we imagine winding up our altimeter from 999 to 1020, we increase our QNH by 21mb and following the winding up theory, we wind up our indicated altitude by 630' (21mb x 30' per mb). Adding this increase in altitude to our airfield B elevation we get an indicated altitude of 1130'.

The elevation of airfield A is irrelevant in the question.

Hope this helps,
Obs

[Mike Parsons]

"High to low" (press)..."down you go" (actual height less than altimeter height)

[Hakeem]

Its " High to Low" ----> "Look below"

Means when you are flying from High to Low the Altimeter will over read...

It will over read by 630 ft as per "Sir Herbert Gussett" said....

This is the way i remember....


"High to Low look below"