British Young Pilots · Aircraft General Knowledge · Module AGK-24.6
The VSI (vertical speed indicator) answers one question: are we going up, going down, or staying level? Needle above the middle: climbing. Needle below the middle: descending. Needle resting in the middle on zero: flying level. The number tells you how many feet you gain or lose every minute.
Try it: hold one arm out flat as the needle. Arm up, you are climbing. Arm flat, level. Arm down, descending.
Source: Pooleys Vol 4 (10th ed.) p.342–343.
Picture a balloon sealed inside a biscuit tin, with one tiny pinhole in the lid. The balloon is joined to the outside air, so when you climb into thinner air it feels the change straight away. The tin only catches up slowly, one whisper of air at a time through the pinhole.
While the two are different, the balloon shrinks (climbing) or swells (descending), and that movement is what pushes the needle round. The faster you climb or descend, the bigger the difference, and the further the needle swings. Fly level and the two catch up with each other, so the needle drifts back to zero.
FOUNDATIONSReal VSI, real words: the balloon is a capsule, and the pinhole is called the metered leak.
Source: Pooleys Vol 4 (10th ed.) p.342–343.
Because the tin takes time to catch up, the VSI takes a few seconds to tell the truth after you move the nose. Pitch sharply and it can even twitch the wrong way for a moment.
So on the sim: look out and set the nose picture you want, wait, then check the VSI to confirm your descent rate. If the VSI sits on zero while the ground is clearly getting closer, the small air hole feeding it (the static vent) may be blocked, and it will just keep reading zero.
Where to find it: on a classic panel it is the dial in the bottom right corner.
Source: Pooleys Vol 4 (10th ed.) p.343–344.
You could estimate how fast you are changing altitude by timing the altimeter against a stopwatch, but that is tedious and distracting. The vertical speed indicator gives a direct read-out of the rate of change of altitude.
Like the altimeter, the VSI is a pressure instrument resting on the same fact: air pressure decreases with altitude. It converts a rate of change of static pressure into a rate of change of altitude, presented in hundreds or thousands of feet per minute (fpm, or ft/min). It is also known as the rate of climb indicator.
Source: Pooleys Vol 4 (10th ed.) p.342.
Static pressure is fed directly into a capsule. The case around the capsule receives the same static pressure, but only through a metered leak (a capillary, or "choke"), so the case pressure always lags behind.
Begin a descent and the new, higher pressure arrives inside the capsule at once while the case is still catching up through the choke. The capsule expands and a linkage drives the pointer round a scale graduated in fpm to show a rate of descent. In a climb the capsule pressure falls first, the capsule compresses, and the pointer shows a rate of climb. Mechanical stops limit the travel.
In level flight the pressures either side of the capsule equalise and the pointer rests on zero. A blocked or iced static vent produces the same equalisation, so the VSI reads zero even while the height is changing.
Source: Pooleys Vol 4 (10th ed.) p.343.
The practical habit: attitude first, VSI second. Set the pitch attitude, let the needle settle, then read the rate.
STRETCHWhy does a blocked static vent make the VSI read zero rather than freezing at its last reading? Think about what happens to the two pressures once nothing new can reach either of them.
Source: Pooleys Vol 4 (10th ed.) p.343–344.
On a conventional panel the VSI occupies the bottom right corner of the standard instrument layout. On an EFIS screen the vertical speed is placed along the right-hand side for exactly that reason: the glass panel keeps the geometry a pilot already knows.
EFIS designs vary, but most present vertical speed as a tape that shows a rate of climb, a rate of descent, or level flight, with zero mid-tape, climb above it and descent below.
Source: Pooleys Vol 4 (10th ed.) p.343–344.
Tap a card to flip it.
One situation or VSI reading at a time: is the aeroplane climbing, level or descending? Tap to sort it.