British Young Pilots · Aircraft General Knowledge · Module AGK-24.4
The altimeter is the aeroplane's height dial. It reads in feet, and it works by feeling the air. Air gets thinner the higher you go, so it squeezes less and less on everything around it.
You have seen this trick already: take a sealed crisp packet up a mountain and it puffs up like a balloon. Inside the altimeter is a small sealed metal can that does exactly the same thing. As the aeroplane climbs, the can swells; as it comes back down, the air squeezes it small again. The can is joined to the pointer, so swell and shrink become "up" and "down" on the dial.
Source: Pooleys Vol 4 (10th ed.) p.335.
A pilot needs to know the height of the aeroplane above the ground and above mean sea level (MSL) for three reasons: terrain clearance (not hitting the ground or fixed obstacles), traffic separation (aircraft cruising at different altitudes are kept safely apart vertically) and performance calculations (working out what the aeroplane can do, and flying it safely and efficiently).
The pressure altimeter is simply a barometer, an instrument that measures atmospheric pressure. It relies on one fact: the higher you are in the atmosphere, the lower the static air pressure. Near the surface the pressure falls by roughly 1 hPa for every 30 ft of climb. The most compact and rugged barometer for an aircraft is the aneroid type, the same family as the one on a living-room wall.
Source: Pooleys Vol 4 (10th ed.) p.335.
The instrument case is fed with static pressure from a vent on the outside of the aircraft. Inside sits the aneroid: a sealed metal capsule that can expand and compress, containing a fixed amount of air. As the aeroplane climbs, the pressure around the capsule drops and the capsule expands; a linkage and gearing turn that movement into a pointer sweeping round the scale. As the aeroplane descends the rising pressure compresses the capsule and the pointer winds back to a lower reading.
The scale is graduated in feet, never in pressure units (millibars, hectopascals or inches of mercury), even though pressure is what the capsule actually senses.
Source: Pooleys Vol 4 (10th ed.) p.335–336.
The dial has numbers 0 to 9 and pointers like a clock. The long pointer counts hundreds of feet: on the 2, that is 200 ft. The short fat pointer counts thousands: just past the 5, that is 5,000 and a bit. Put them together and this dial reads 5,200 ft. A third, very thin pointer counts ten-thousands, so it hardly moves in a small aeroplane.
Tip: read the short pointer first for the thousands, then add the long one.
Source: Pooleys Vol 4 (10th ed.) p.336.
Short fat pointer first (thousands), then the long pointer (hundreds). Shout out each answer before your instructor reveals it.
A: short pointer just short of 2, long pointer on 8: 1,800 ft. B: short pointer just past 0, long pointer on 7: 700 ft.
STRETCHThe thin third pointer sits between 1 and 2, the short one just past 2, the long one on 3. What height is that? (Answer: 12,300 ft.)
Source: Pooleys Vol 4 (10th ed.) p.336.
The conventional altimeter carries three pointers over a 0 to 9 scale: a long 100 ft pointer, a short broad 1,000 ft pointer and a thin 10,000 ft pointer with a small marker at its tip. A striped sector shows on the face whenever the aircraft is below 10,000 ft, as a warning that the thin pointer has not yet completed a lap. The pressure window (the subscale) is set by the subscale knob: the pilot dials in a pressure datum there, and which pressure to set is the subject of the next module.
On an EFIS display the altitude is an altitude tape down the right-hand side of the screen, mirroring the altimeter's top-right place in the "Standard T" panel layout, with a digital pressure setting alongside it that is entered with knobs or buttons.
Source: Pooleys Vol 4 (10th ed.) p.336–337.
The air reaches the altimeter through a small hole in the side of the aeroplane called the static vent. That is why pilots check it before every flight: if ice or an insect blocks the hole, the air trapped inside cannot change, the sealed can cannot swell or shrink, and the height dial freezes on the last reading even while the aeroplane keeps climbing or descending.
One more thing on the face: the little window. The pilot turns the knob to set a pressure number in it before flying. What number, and why, is the next lesson.
Source: Pooleys Vol 4 (10th ed.) p.336–337.
Follow the blockage through. Ice the vent during a climb-out and the altimeter stays at the altitude where it froze: it under-reads. Block it in a descent and it holds a reading higher than you really are: it over-reads, which is dangerous where terrain clearance matters.
STRETCHA blocked static vent also upsets the airspeed indicator. Which reading would you trust more in a climb, and why?
Source: Pooleys Vol 4 (10th ed.) p.337.
Sensitivity. With pressure falling about 1 hPa per 30 ft near the surface, a 300 ft height change is only about 10 hPa of pressure change at the capsule. The gearing has to magnify a very small movement into a full pointer sweep, which is why build quality (instrument error) and response time (lag) matter.
Blockage arithmetic. A sealed static line holds the pressure of the moment it blocked. Climb a further 600 ft and the capsule should have seen roughly 20 hPa less pressure; it sees none of it, so the reading is about 600 ft low. Descend instead and the same trapped pressure leaves the reading high by the height lost since the blockage.
Position error is a property of where the vent sits on the airframe, not of the instrument, so a perfect altimeter can still be fed the wrong pressure.
Source: Pooleys Vol 4 (10th ed.) p.335–337 (the 600 ft and 20 hPa figures are illustrative arithmetic from the 1 hPa per 30 ft rate).
Tap a card to flip it.
Tap a name, then tap the box its pointer line leads to. This dial is reading 6,500 ft.