Ask Captain Lim

Dear Captain Lim,

I am a Hong Kong University student who is preparing for the stage two-interview/job knowledge tests of Cathay Pacific Cadet Pilot Program.

First of all, I would like to say thanks to you. I have been sticking to your website since I was invited to the initial test of the selection process. I was luckily enough to find your website before attending the test. I really appreciate your work on sharing your experience and the information related to aviation. They did help me a lot to get through the selection process, especially those about Boeing 777. I was so confident to say that it was my favorite airplane during the first interview, as from your website, you let me know how safe and hi-tech is this plane!

Cathay has given me an aviation knowledge booklet for preparing the job knowledge test. It covers a section called "pressure settings" in which "QFF", QNH" and "QNE" are introduced. But, I am not very clear about the differences between them. Are they the calculated values of the pressure at msl (mean sea level) from QFE? I know that QFF uses the actual temperature and QNH uses ISA value. But, how the calculation is done? Also, what is QNE?

It would be most grateful if you would explain the concepts.

I very much look forward to hearing from you.

Yours sincerely,

Leo Leung

Hi Leo,

Firstly, let me say that, out of the 4 "Q" Codes (QFF, QNH, QNE, QFE), only one is very important and used widely in all the International airports. It is the QNH ? the local altimeter setting; it changes all the time. For instance, if the QNH at Chap Lap Kok Airport in Hong Kong is 1000 millibars (as either given by the air traffic controller or automatically retrieved from the ATIS - Automatic Terminal Information Service) is set below the subscale on the altimeter, it will show 19 feet when an aircraft is parked at the terminal. That is the exact elevation of the airport - 19 feet above mean sea level.

There are two types of QNH - the airfield QNH and regional QNH. The airfield QNH, as given by the example above - 1000, when set, tells the pilot that the altimeter is indicating correctly -19 feet above mean sea level at a particular time.

The regional QNH is the lowest forecast QNH in an altimeter setting region (e.g. Hong Kong region, excluding the Shenzhen region, etc)

Let say, two hours later in an approaching typhoon, the pressure drops to 998 millibars, the altimeter reading would no longer be accurate as the elevation will be 60 feet out (one millibar equals 30 feet - accurate up to 5000 feet) Unless the pilot reset the latest QNH to 998, the altimeter becomes inaccurate.

QNE: This is the ISA (International Standard Atmosphere) Standard Pressure setting of 1013 millibars. All airplanes passing through the Transistion Altitude (varies from region to region) must set the QNE. In the Hong Kong region, all pilots set the QNE on passing 9,000 feet. This is important for all airplanes have similar setting in order to maintain an accurate lateral separation.

QFE: This is rarely used today except in certain region and elevation. It is the pressure setting at the aerodrome elevation datum point. If QFE is set on the altimeter pressure-setting scale while parked at, say Chap Lap Kok airport, the altimeter should read zero feet. (When set to QNH, it will show 19 feet above mean sea level)


QFF: Pilots do not commonly use this term but it is a value used mainly by the Meteorological Organizations. The QFF is similar to the QNH except that it uses the actual conditions to find the sea level pressure and not the ISA.

Here is a practical illustration: Lets say you are flying from Hong Kong to Kansai (on a Boeing 777). Control Tower gives you the QNH as 998 and the pilot sets it on his subscale. His altimeter will read 19 feet. After take off, the pilot changes the subscale to QNE (1013) at 9000 feet. When leveled at 35,000 feet, his separation with other airplanes will be safe as every plane is on QNE. On the descent to Kansai, if the pilot changes from QNE (1013) to QFE, say 999, the Boeing 777, when touches down on the runway at Kansai will read zero feet instead of 15 feet (the actual elevation of Kansai airport).

There are no calculations by the pilots where these codes are concerned. They are altimeter pressure readings given by the weather forecasters (meteorologists) to the air traffic controllers, who in turn, pass them to the pilots.

Hope that clears your doubts. (Pilots are not interested with QFF!)