Nav help. Hi Team. Any assistance for this one?

[FOD Hazard]

Hi Team, I have been trying to get through the nav stuff.The ones Im having difficulty with don't seem to have any good worked examples in teh AFT notes. Can i please get some advice on the question below?

Q: You are cruising at A070 directly above a mountain of elev 6420ft. The QNH is 1001 hpa with an OAT of -14 deg C. The radio/radar altimeter would indicate:

a) 188ft
b) 972ft
c) 390ft
d) 580ft

Apparently it is a). I was happy to remove b) and d) based on the temperature being colder than ISA (hence the altimeter overreading & the true alt being below the indicated)...but I can't work it out.

Some of the ILS above or below ones are brain breakers too...can't get the exact answers...which makes me doubt my method.

Any help on the above question would be greatly appreciated. There is a beer in it for someone in Darwin.

[gunnaboy]

Hi FOD,

I had the same problem with this area too. Thankfully I didn't get a question in the exam asking about this topic.

I had emailed NH on this topic and below is an explanation I received back. I hope this helps you in any way.

Cheers. :)


The error your altimeter is going to experience in this case depends on the QNH source.

If you have local QNH, you are only exposed to the error based on the height of the altimeter above the local QNH field elevation, ie, the height AGL of the OM.

If on the other hand you have Area instead of Local QNH, then the altimeter is going to be exposed to the total error AMSL.

The magnitude of the error is about .1% per degree of ISA deviation (4% per 10 degrees of ISA deviation in other words). This will be .1% of the height AGL of the OM above the aerodrome per degree of ISA deviation on local QNH or .1% of the total altitude AMSL per degree of ISA deviation on Area QNH.

If you use the whiz wheel instead of the .1% per degree rule then you must also differentiate your QNH source. With the OAT against the Pht in the True Alt window, read indicated HEIGHT on the inside against true HEIGHT on the outside main scales with local QNH. With Area QNH, read indicated ALTITUDE on the inside against true ALTITUDE on the outside main scales.

[FOD Hazard]

Thanks GB,

I can calculate an answer of 180 at least now, maybe that's as close I will get. Thanks for the reply.

[Checkboard]

Altimeter is indicating 7000' above the 1001 hpa level
ISA at 7000'' = 15-(7*2)=1ºC, OAT is -14ºC thus you are at ISA-15ºC
Temp correction at 4' per 1º per 1000 feet = 4*15*7 = 420 feet, so true alt above QNH level is 6580 feet
Mountain top is at 6420 feet, difference is 160 feet
Area QNH can be plus or minus 5 hpa in the area (from memory).
a is closest.