British Young Pilots · Aircraft General Knowledge · Module AGK-24.5
Air has weight. Down at the beach the whole sky is piled on top of you, like sleeping under six duvets. Climb a mountain and there is less sky above, so the squeeze is lighter: three duvets. That squeeze is called pressure, measured in hPa, and it drops by about 1 hPa for every 30 ft you climb.
Your altimeter cannot see the ground. It only feels the squeeze and turns it into a number of feet. Squeeze your own fist gently, then harder: that is all the altimeter ever feels.
Source: Pooleys Vol 4 (10th ed.) p.338.
The weather changes the squeeze every day, and the altimeter has no idea. So the pilot tells it. There is a tiny window on the dial with a knob beside it. Dial in today's sea-level pressure, the number called QNH, and the altimeter now shows your height above the sea.
Why the sea? Because every hill and radio mast on the map is printed as a height above the sea. Same ruler, same starting line, so you can see at once whether you clear them.
Say it together: QNH, height above the sea.
Source: Pooleys Vol 4 (10th ed.) p.339.
Now dial in the pressure sitting on the airfield itself. That number is QFE. Parked on the runway the altimeter reads zero, and in the air it reads how far you are above the airfield. Handy when you are flying circuits round and round it.
But every airfield has its own pressure, so QFE from one airfield means nothing at the next. Fly away and it stops being useful.
Say it together: QFE, zero on the field.
Source: Pooleys Vol 4 (10th ed.) p.340.
Here is the trick that fools everyone. Sit still on the ground and wind the knob up: the needle climbs, even though you have not moved an inch. Wind it down and the needle sinks. You are not changing your height, only where the altimeter thinks "zero" is.
There is one more number pilots can dial in: 1013, the "standard" pressure everyone in the world agrees on. It is the pressure of an average day at the seaside, and the altimeter is built around it.
Try it on the sim: dial QNH, read the height. Dial QFE, watch it drop to zero on the runway.
Source: Pooleys Vol 4 (10th ed.) p.338–340.
UK aviation switched from the millibar (mb) to the hectopascal (hPa) in 2011. The two are equivalent, so only the name changed; millibars still appear abroad and in older books. The USA uses inches of mercury and graduates its subscales in inches.
Real air varies from place to place and hour to hour, so aviation defines a fixed measuring stick, the ISA, and its main job is to calibrate altimeters:
Source: Pooleys Vol 4 (10th ed.) p.338.
The altimeter reads height above whatever pressure level is set in its subscale. The settings are called pressure datums, and the Q codes that name them are shorthand left over from the Morse era:
Pressure altitude is the height in the ISA above the 1013.2 hPa level at which the pressure equals the pressure at the aircraft. If the air round you has the pressure the ISA has at 6,800 ft, your pressure altitude is 6,800 ft, whatever your real height.
Source: Pooleys Vol 4 (10th ed.) p.339–340.
Terrain and radio masts are charted as heights above mean sea level, so QNH is the setting for checking clearance above them. QFE is only useful while you stay near the aerodrome it came from: every aerodrome has its own surface pressure, so its own QFE. It is often used for circuit operations. In the UK the regional pressure setting (sometimes wrongly called regional QNH) is the lowest QNH in a defined region; its use is covered in Volume 2.
When you land, the altimeter should read zero with QFE set and the aerodrome elevation with QNH set.
Source: Pooleys Vol 4 (10th ed.) p.339–341.
Hold a constant level and wind more hectopascals onto the subscale: the altimeter winds on more height, about 30 ft per hPa. Wind them off and the reading falls. The aeroplane has not moved; only the datum has. An invented example: level with QFE 996 set, the altimeter shows 2,100 ft above the airfield. Wind up to 1013 (17 hPa, about 510 ft) and it shows 2,610 ft pressure altitude. Wind on to a QNH of 1021 (another 8 hPa, 240 ft) and it shows 2,850 ft altitude.
Pre-flight checks on the apron: within 50 ft of the airfield elevation with QNH set; zero plus or minus 50 ft with QFE set; and the main scale must increase as the subscale increases, and vice versa.
STRETCHYou fly from high pressure towards low pressure without touching the subscale. As the flight goes on, does the altimeter over-read or under-read your real height? Reason it out from "wind off hectopascals, wind off height".
Source: Pooleys Vol 4 (10th ed.) p.340–341.
Density altitude is pressure altitude corrected for non-standard (non-ISA) temperature. When it is high, the aircraft performs as though it were taking off or cruising higher than it really is. The performance consequences are covered under Flight Performance and Planning.
True altitude is the actual vertical distance above mean sea level. In a non-ISA atmosphere it is calculated from indicated altitude on the CRP-1, and the first step is always the pressure altitude. Invented example: QNH 998 hPa, indicated 4,000 ft. Standard 1013 sits 15 hPa higher, about 450 ft, so the pressure altitude is 4,450 ft; the OAT then goes into the CRP-1 to finish the correction.
Source: Pooleys Vol 4 (10th ed.) p.341.
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