British Young Pilots · Aircraft General Knowledge · Module AGK-24.1
Hold your hand out flat and keep it perfectly still. Feel anything? No, but the air is pushing on it from every side right now. That push is called static pressure. It comes from the weight of all the air stacked up above you, pressing down. Fly up high and there is less air on top of you, so the push gets weaker.
An aeroplane feels this push through a small flush hole in its side called the static vent. Nothing blows through it; it just senses the push outside.
Source: Pooleys Vol 4 (10th ed.) p.323.
Now imagine your hand out of the window of a moving car, palm facing forward. Whoosh: an extra push. That extra push only exists while you are moving through the air. It is called dynamic pressure. Faster car, bigger push. Thin air high up, smaller push.
Try it: blow on your palm gently, then hard. Feel the difference?
The aeroplane catches this push with a small tube at the front pointing straight into the wind: the pitot tube. The wind rams into it, so the tube feels the ordinary push and the extra moving push added together.
Source: Pooleys Vol 4 (10th ed.) p.324–325.
Pipes carry the air from the two holes to three round dials. The airspeed indicator (ASI) needs both pipes: it compares them, and the difference is how fast you are flying. The altimeter needs only the static pipe: weaker push means you are higher. The vertical speed indicator (VSI) needs only the static pipe too: it watches how quickly that push is changing, up or down.
Source: Pooleys Vol 4 (10th ed.) p.326–327.
Both holes are tiny, so tiny things can block them: a bug, a drop of water, a bit of ice. A blocked hole means a dial telling lies, so on the walk-round before every flight the pilot checks the pitot tube and static vent are clean and undamaged.
Three rules: take the pitot cover off before flight (it keeps water and insects out on the ground), never blow into the tube (the instruments on the end are very delicate), and remember there is a little electric heater in the pitot tube, switched on from the cockpit, to melt ice.
Source: Pooleys Vol 4 (10th ed.) p.326.
Flight instruments fall into three families: pressure instruments (variations in air pressure), gyroscopic instruments (gyroscopic inertia) and magnetic instruments (the Earth's magnetic field). The three basic flight instruments, the ASI, the altimeter and the VSI, are all pressure instruments, and they work with two kinds of air pressure.
Static pressure is the pressure of the atmosphere at a point: the weight of all the air above that point. It acts equally in every direction, needs no movement of the air, and falls as you climb. On the aeroplane it is sensed by a static vent on the airframe surface.
Source: Pooleys Vol 4 (10th ed.) p.323.
Dynamic pressure is the extra pressure a body feels when it moves relative to the air (or the air moves past it), on top of the static pressure that is always present. It depends on two things: speed relative to the air (more molecules strike per second) and the density of the air (denser air, more molecules; in outer space there would be none at any speed).
Total pressure is static plus dynamic. It is also called pitot, ram or impact pressure, and it is measured by a pitot tube facing into the airflow. Subtract the static pressure from it and what is left is the dynamic pressure, which is exactly what the ASI shows, on a scale marked in knots.
STRETCHAt the same airspeed, why does the hand out of the window feel a smaller push at 10,000 ft than at sea level?
Source: Pooleys Vol 4 (10th ed.) p.324–325.
Two arrangements are common: a combined pitot-static head, or a separate pitot tube (often on the wing) with one or two static vents on the side of the fuselage. The pitot tube must sit where the airflow is not greatly disturbed, often ahead of or beneath the outer wing; a poor position gives significant airspeed errors. Two static vents, one each side, average out the reading when the aeroplane is slipping or skidding.
Source: Pooleys Vol 4 (10th ed.) p.326–327.
Damage or a blockage in the pitot tube or a static vent gives false readings that can degrade safety, so both are checked carefully on the pre-flight external inspection. The pitot cover (which keeps out water and insects) comes off for the inspection and must be off before flight. Never blow into a pitot tube: very sensitive instruments sit on the end of it.
Pitot heat, an electrical element in the tube switched from the cockpit, guards against ice. Test it briefly on the pre-flight, then switch it off: without cooling airflow the element burns out easily, so it stays off whenever the aeroplane is not flying.
An alternate static source in the cabin covers a blocked external vent. Cabin pressure in flight is usually slightly below outside pressure, so readings on the alternate source are slightly in error.
STRETCHIf the cabin pressure is slightly lower than outside, which way would the altimeter be wrong on the alternate source, and why?
Source: Pooleys Vol 4 (10th ed.) p.326–327.
Dynamic pressure = ½ ρ V², where ρ (rho) is air density, which falls with altitude, and V is the speed of the body relative to the air. It makes no difference whether the body moves through the air, the air moves past the body, or both: only the relative speed counts. Because the pressure varies with V², doubling the speed gives four times the dynamic pressure, one of the square laws common in nature.
Bernoulli's relationship: static pressure (from the static line) plus dynamic pressure (½ ρ V²) equals total pressure (from the pitot tube). Inside the ASI, total pressure is fed to one side of a flexible diaphragm while static pressure from the vent surrounds it, so the mechanism responds to total minus static, which is the dynamic pressure, and the scale is calibrated in knots.
Source: Pooleys Vol 4 (10th ed.) p.325. The ASI mechanism in full is module AGK-24.2.
Tap a card to flip it.
Tap a name, then tap the box its pointer line touches. Which dial needs both pipes?