Your FTO is using an out-of-date worksheet. This is a question that used to be required for the old CAA Instruments exam, but has been deleted from the syllabus of the JAA (Flight Instruments) part of the combined Instrumentation paper. The JAA do expect you know ball park figures for how many feet to a hectopascal, but only by remembering - ie, about 30 feet/hectopascal at 5000 feet, about 110 at 40,000 feet, etc.
The old CAA syllabus required you to know the following formula:
No of feet to a millibar at any altitude = 96 x T/ pressure in millilbars at that pressure level
where T = OAT in degrees Absolute (or Kelvin, if you prefer).
So, to take your example:
No of feet to a miilibar at 25,000 feet = 96 x 233/375
which, my trusty calculator tells me, is 59.648 feet per millibar.
Well, I must say, common sense and experience tends to suggest 60 is about right, so the formula must work.
The FTO where I work stopped teaching this about 2 years ago, when we realised from the feedback that the JAA were no longer asking the question in this form and we have dropped it from our worksheets. Also, it's no longer in the Learning Objectives. The CAA actually specifically sent out a circular saying that it had been dropped.
Last edited by oxford blue; 23rd May 2002 at 20:01.