The answer is at the end of this short, but true story.
Training for my IMC Rating at Booker c 1978.
During the pre-brief the instructor asked me this question. I rationalise about the primary and secondary data you can get from the AI, T&S, DI, ASI etc. Instructor promptly clips me round the back of my head and tells me I know nothing - and he will teach me in the air.
Lined up on Booker’s runway 24, and I put the Hood on (Before takeoff) - and did the takeoff fully under the hood using the DI, (as this instructor insisted on).
We spend 30 minutes flying around under the hood, with the instructor giving me “ATC style” instructions whilst gradually covering up all the instruments in turn.
I’m sitting there thinking “this is easy, I’ve got this sussed” when the instructor tells me I can take to hood off - and then tell him which instrument is the most important.
When I looked up, I found that we were at approx 50ft agl, flying up a valley that closed in a few hundred yards ahead. (Near Chinoor and the Stokenchurch mast)
I simultaneously applied full power, pulled back into a steep climb and shouted .........
“Altimeter George - Altimeter !”
A lesson hard learned, is one well learned. Since then I’ve always been a big fan of Altimeters and knowing the SSA.
Perhaps quite relevant after last week’s tragedy at Brecon Hill.
So Graham B was the only one to guess the right answer. (Isn’t he supposed to be at work at this time of the day)?