+27C on the ground. +26C at 4000ft. +23C at 5000ft.
QNH was correctly set at 1011 and the altimeter reading was 5060ft
yet the real altitude was around 5380ft
This sort of thing is very common at say FL150 where the true altitude is often 300-500ft higher simply due to nonstandard temperature (say ISA+15) but it is unusual to see such a difference low down.
This means that flying at say 5400ft below the 5500ft LTMA base, one would be significantly busting it, but not in any detectable way.
The rule of thumb is 7% per 20 degC off ISA. SO at ISA + 20 degC, 5000 ft indicated = 5350 ft true. Pretty close.
Peter wrote:
This means that flying at say 5400ft below the 5500ft LTMA base, one would be significantly busting it, but not in any detectable way.
The base is defined by the barometric level, not the geometric level.
one would be significantly busting it, but not in any detectable way.
One wouldn’t be busting it at all. Altitudes (as in airspace boundaries) are defined as “on QNH” (whether it is precise on true altitude or not). True altitude has no significance there.
I knew that 
The use of “busting” is not right because one is not actually busting it.
I have just realised the 7%/ISA+/-20 rule is straight out of the absolute temperature (Kelvin) scale i.e. 20/273 is 7.3%. The gas expansion law, whatever it is called.
I got to know this rule as 4% per 10° ISA deviation which is easier to handle …
It just needed the "" around busting and no one would have been confused
The previous day I had a 35 knot headwind at only 3,500 feet in the morning, 8 knots on the ground, almost nothing coming back 5 hours later, reciprocal track. Seemed an odd combination at the time.
I got to know this rule as 4% per 10° ISA deviation which is easier to handle …
Nah, you have to multiply by 2 today! Much harder.
+4° at FL100 today over most of Scandinavia today – density altitude almost 11000. My poor PA28-181 was struggling to get there. (Service ceiling is 12000.)
I had an Engine Temp warning in the climb with a Centurion 2.0s engine tonight.
OAT was still +35°C when passing 2000ft. Coolant Temp was in the red.
I reduced the power and increased speed and it got slowly back in the green.
Same with the Rotax 914 on Wednesday – it was a shallow climb at MTOM to FL 120
