Differences Between Altitudes and Flight Levels

Differences Between Altitudes and Flight Levels

On a few occasions now I have had users report discrepancies between the reported altitude in the shift-Z information line and the indicated flight level on the instrument display. Some people have even suggested that this is due to weather engine problems which of course it is not. It is actually down to the usual differences between Altitudes and Flight Levels.

The shift-Z information line shows the actual Altitude (AMSL).

The aircraft instruments and auto-pilot displays will show Flight Levels when you have selected STD barometric pressure. The Flight Level is based on the International Standard Atmosphere (ISA) of 1 atmosphere which is1013.25 millibar or 29.92 inches of mercury.

You can use the Opus weather reports and Opus Surface QNH pressure weather map to check what the injected surface pressures are during your flight. These are the only injected weather parameters that can effect the difference between Altitudes and Flight Levels, and hence affect the difference seen between the shift-Z line and the instrument displays when setting STD barometric pressure.

When converting between altitudes and flight levels, the rule of thumb used by most pilots (especially when flying at lower altitudes or levels), sets 1mb equivalent to about 30 feet. This is true typically at altitudes of about 4000 feet. At much higher altitudes 1mb is equivalent to greater differences. For example, at 15000 feet the conversion factor rises to about 40 feet per millibar. At 25000 or 30000 feet the conversion rate would be even higher.

So in conclusion, you should expect to see larger differences between the Altitude and Flight Level when flying through either very high or very low pressure zones.

As an example, if you used the rule of thumb of 50 feet per millibar when at 30,000 feet altitude (just a guess to make the maths simple) then a surface pressure difference with the ISA (STD barometric setting) of just 100 millibars would result in a difference of 5000 feet between the 'actual' Altitude and the 'indicated' Flight Level on the instruments.

Considering even here in the UK we have had storms produced by low pressure zones with surface pressures lower than 890mb (more than 120mb difference from ISA or the STD pressure setting) you can see that it is quite possible to experience LARGE (>5000 feet) differences between actual Altitude and Flight Levels.

Please feel free to pass on this information to anyone suggesting such discrepancies is due to problems in any weather engine. I have even read some posts where people have stated that flight levels are used to provide terrain clearance, which of course is ridiculous as evident from the above.

Stephen :-)