Pitot and Static Sources — Page 24, Lesson 29

Let’s pick up with the alternate static source, because that’s the first thing this passage deals with. On light aircraft, you normally get an alternate static source in case the static head or the static vents become blocked. The selector switch for it lives in the cabin. Now, that alternate static source can be plumbed either to the outside of the aircraft, or it can draw from inside the cabin — but only in unpressurized aircraft. You can’t use cabin air as a static source in a pressurized aircraft, because the cabin pressure is held above ambient, and that would wreck your static instruments. Here’s the key accuracy point. The alternate static pressure is less accurate than the primary static vent or head, because the primary one was mounted in the optimum position. The alternate static pressure you sense is likely to be lower than ambient, and that’s because of aerodynamic suction. And this is generally true whether the alternate source is external or from inside an unpressurized cabin. Why does that matter? Because your instruments that use static — the altimeter, the vertical speed indicator, the airspeed indicator — are calibrated to take account of the pressure error, the position error, at their normal supply. So when you switch to the alternate, standby pressure system, the correction values for the affected instruments can be found in the Operating Data Manual for the aircraft. That’s your source for the correction. Now let’s move to pitot covers and static vent plugs. Pitot and static openings are highly sensitive. Any dirt, dust, or sand in them can distort their measurements, often drastically. And here’s a neat detail — because they tend to be warmer at night than the rest of the aircraft, insects are attracted to them as resting places. So it’s necessary to cover them when the aircraft is not in use. Pitot covers are canvas or rubber closed tubes that fit over the outside of the pitot probe. Static plugs are usually rubber, shaped like small corks. And it is clearly essential that they are not left in place in flight, so removing them is part of the preflight external checks. They normally have a very conspicuous streamer or ribbon — up to a metre long — attached, so they’re not overlooked. Then we have pitot and static heaters. All aircraft have pitot heaters, and on combined pitot/static probes, the static opening will consequently receive some heating as well. Modern systems have an alert warning if the heaters are not switched on in flight. It’s essential to test the pitot heater before flight. You can do this by switching it on for about 30 seconds and carefully feeling the pitot probe to check it’s warm. On other aircraft, you might check that switching it on causes a rise in the ammeter current, or that the magnetic compass is deflected. Then you switch it off again so it doesn’t burn out. And make sure the pitot covers and static plugs are removed before turning the heaters on to check them — otherwise you may burn or melt them. It’s normally part of the pre-take-off checks to ensure the heaters are switched on again, and part of the after-landing checks to switch them off. Finally, the preflight checks of the pitot/static system. Three items. All covers and plugs removed and stowed. All tubes, holes, and slots free of obstructions. And the pitot head heater operating. That figure shows the emergency static source arrangement. So the whole picture is: protect the openings on the ground, verify the heater works before flight, and know where to find your corrections if you ever have to fall back on the alternate static source.