Understanding how high you are off the ground and above obstacles can be quite useful in an aircraft, especially where it comes to avoiding hitting things. Altimeters can do the job quite well, although they can only tell you the difference in height between two points, namely the one you set on the sub scale and where you are. The altimeter does not know the sea level or aerodrome pressures. You set these yourself. Nor does it know where the top of a hill, the ground or the sea is. That is up to you to know. You need know how high to fly to avoid an obstacle, and you need to know how high you are above the sea in order to avoid it. A clear understanding of the altimeter settings is essential for this. Altimetery is an important subject for all pilots.
‘Q’ codes were originally created by the British government in the early 1900’s and were a list of abbreviations licensed by the Postmaster General for the use on ships over the radio. They were either a question or an answer (advice). It was a way of abbreviating the morse code. The list is extensive, even the original had 45 codes, and is still widely used by amateur radio enthusiasts.
QNH and QFE are both altimeter settings. When QNH is set on the sub scale, the altimeter will tell you how high you are above sea level (your altitude, or Nautical Height), and when QFE is set, it will tell you your height above the ground (or Field Elevation). It might be an obvious thing to say at this stage, but sea level remains constant as far as aviators are concerned.
QNH, when set on your altimeter, will give your height above sea level. So if you are on the ground at an aerodrome and you set the QNH (the pressure at sea level) the altimeter will read the airfield elevation. Your height above sea level is called ‘altitude’, so QNH set on the sub scale gives your altitude. If you are at Elstree your altitude at ground level is about 330 feet, at Biggin Hill it is about 600 feet, and Southend about 60 feet. Remember the QNH is a measurement or calculation of air pressure at sea level. Pressure changes constantly, whereas the height of an airfield doesn’t, so the difference between QHN and QFE is constant. At Elstree the QFE is always 11hPa less than the QNH, Biggin Hill will always be 20hPa less, and Southend 2hPa. When you set the QFE on the ground at any airfield your altimeter should read zero feet. Most circuits are then flown as a height, usually 1000 feet above ground level (AGL).
When you are flying cross country and you are in the vicinity or below the controlled airspace of a large airport like Heathrow or Birmingham, you would use the QNH for that airfield. The UK is also divided into several “altimeter setting regions”, and if you are not flying in the vicinity of a large aerodrome you could use one of the regional settings. It is defined as the lowest forecast setting for that hour. They can be obtained from any ATCSU (air traffic control service unit).
Finally there is ‘Pressure Altitude’ and ‘Flight Levels’. I doubt as a PPL you will use these often but you should know about them. Pressure Altitude is simply the altitude the altimeter gives you when you set 1013hPa on it. If you have a transponder in your aircraft with a digital readout, you might have noticed that the height given on the display is a pressure altitude (transponders have no barometric components). Sometimes flying around Elstree on a day with particularly low pressure, our transponder reads 2700 feet, even though we are 2300 feet above sea level (on the QNH). But the London TMA is at 2500 feet!! Don’t worry, the local radar stations know we are not infringing and it is because of low the pressure.
Flight Levels generally used when you are high enough that a large drop in sea level pressure would not result in the aircraft descending enough to put the plane in danger from obstacles or aircraft flying on the QNH. When you fly a Flight Level on the pressure setting 1013hPa, you call your altitude ‘Flight Level’. Flight Levels are stated in increments of 500 feet, you knock off the last two zeros. So reading 5000 feet on your altimeter with 1013 set on it becomes ‘Flight Level Five Zero’, or FL50. Imagine all the planes flying across the Atlantic having to change altimeter settings as pressure increases or decreases. Even airline pilots aren’t payed enough for that, so they use 1013hPa. If the pressure on route drops by 20 hPa, and the aircraft descends 600 feet, as long as all the other planes flying that route have the same pressure setting, there would be no problem that the actual altitude is changing as there are no obstacles. Things become slightly more complex when looking at the altitude at which you can change from using an actual pressure setting (QNH) and 1013. This is known as the ‘Transition Altitude’. As long as the actual QNH is above 1013hPa, then flying on the 1013hPa pressure setting as a flight level is safe. When the pressure is above 1013hPa, there will be no conflict with aircraft on QNH below the transition altitude. If however the sea level pressure is below 1013hPa, then if you were to fly at the first flight level above the transition altitude you might be in conflict with an aircraft using altitude.
There are lots of books and apps that deal with altimetry, and as I mentioned earlier, it is a really important subject to understand. For example, it you are doing a standard overhead join to an airfield below a TMA (eg Elstree), and have the wrong altimeter setting, you could, at 2200 feet indicated (on QFE) be in the London TMA, which starts at 2500 altitude. Make sure this is a topic you understand both mathematically and practically. Your instructor will help you with the practical side of altimetry. If you have any questions, please email me.