Excellent explanation from tech log.
To elaborate along the line of thought down which I believe Sanger was headed:
If the elevation of Sywell is shown as 424ft amsl, how do they know it will be 424ft when the pressure fluctuates each day?
Day 1.
The QNH is 1000mb. The aircraft is parked on the airfield. The altimeter sub-scale is set to 1000mb. The altimeter reads 424ft.
Day 2.
The QNH has changed overnight and it is now 1010mb. The same altimeter will now read 124ft. Has the elevation of Sywell changed? No, of course it hasn't. If you reset the altimeter sub-scale to 1010mb, the correct indication of 424ft will be shown.
Day 3.
Michael Fish is on duty and an intense low pressure system has moved across Northamptonshire. The altimeter from Day 2 now reads 724ft. What is the current QNH?
Last edited by eckhard; 6th Jan 2017 at 20:02.