Difference between QFE, QNH and QFF

Difference between QFE, QNH and QFF 

QFE is the pressure at the station (or aerodrome) level. 

QNH is the mean sea level pressure, derived by applying ICAO’s standard atmosphere corrections to QFE. 

QFF is the mean sea level pressure, derived by taking into account the actual temperature conditions.

 

e.g. If an airfield is 270 ft above sea level and the pressure here is 1000 mb then 1000 is the QFE. 

Sea level is below the airfield so sea level pressure will be more than the airfield pressure. 

Sea level pressure can be calculated by using the standard lapse rate of 1 mb per 27 feet. 

270 / 27 = 10 mb 

Thus seal level pressure will be 1000 + 10 = 1010 (10 more than 1000). This is the QNH. 

In cold and dense air, pressure changes more rapidly. 

Considering the above example, in case of colder than standard temerature, the pressure change will be more than 10 mb (as calculated above). 

Assuming that the change in pressure is 20 mb, the pressure will now be 1000 + 20 = 1020 mb. This is QFF. 

Thus in colder than standard temperature QFF is more than QNH. 

In warm and less dense air, pressure changes less rapidly. 

If by applying the standard lapse rate, the pressure change was 10 mb, then for warmer than standard temperature it will be less than 10. Assume it to be 5 mb. 

So if QNH is 1000 + 10 = 1010 by applying the standard lapse rate, then QFF will be 1000 + 5 = 1005 when temperature is warmer than standard. 

That means QFF is less than QNH in warmer than standard temperature. 

However for an airfield below mean sea level things are opposite. 

e.g. If an airfield is 270 ft below sea level and the pressure here is 1000 mb then 1000 is the QFE. 

Sea level is above the airfield so sea level pressure will be less than the airfield pressure. 

Sea level pressure can be calculated by using the standard lapse rate of 1 mb per 27 feet. 

270 / 27 = 10 mb 

Thus seal level pressure will be 1000 – 10 = 990 mb (10 less than 1000). This is the QNH. 

In cold and dense air, pressure changes more rapidly. 

Thus in case of colder than standard temerature, the pressure change will be more than 10 mb (as calculated above). 

Assuming that the change in pressure is 20 mb, the pressure will now be 1000 – 20 = 980 mb. This is QFF. 

Thus in colder than standard temperature QFF is less than QNH. 

In warm and less dense air, pressure changes less rapidly. 

If by applying the standard lapse rate, the pressure change was 10 mb, then for warmer than standard temperature it will be less than 10. Assume it to be 5 mb. 

So if QNH is 1000 – 10 = 990 by applying the standard lapse rate, then QFF will be 1000 – 5 = 995 when temperature is warmer than standard. 

That means QFF is more than QNH in warmer than standard temperature. 

To summarize: 

If the temp = ISA conditions, then QNH = QFF. 

1) For above MSL case: 

If the temp > ISA then QNH > QFF 

If the temp < ISA then QNH < QFF 

2) For below MSL case: 

If the temp > ISA then QNH < QFF 

If the temp < ISA then QNH > QFF 

For exams, the only thing that needs to be memorized is the above MSL case in warmer than ISA conditions i.e. 

If temp > ISA then QNH > QFF (arrows point in the same direction). 

The rest you can reproduce yourself i.e. 

If temp < ISA then reverse the sign (QNH < QFF). 

The below MSL case is then opposite of the above MSL case.