Pitot and Static Sources — Page 24, Lesson 29

Right, let's pick this up with the alternate static source. For light aircraft, if the primary static head or vents become blocked, you normally have an alternate static source available, and the selector switch for it lives in the cabin. Now, that alternate source can be plumbed to the outside of the aircraft, or it can draw from inside the cabin — but only in unpressurized aircraft. Here's the key point: the alternate static pressure is less accurate than the primary static vent, because the primary was positioned in the optimum location. The pressure you sense from the alternate source is likely to be lower than ambient, due to aerodynamic suction. And that's generally true whether the alternate source is external or from within an unpressurized cabin. Why does that matter? Because your instruments that use static pressure are calibrated to account for the pressure, or position, error at their normal supply. So when you switch to the alternate, standby pressure system, you introduce a different error. The correction values for the affected instruments can be found in the Operating Data Manual for the aircraft. So the takeaway is: alternate static works, but it's a degraded, less accurate source, and you need the manual's correction values to interpret it properly. Now let's move to pitot covers and static vent plugs. The pitot and static openings are highly sensitive. Dirt, dust, and sand inside them can distort their measurements, often drastically. Also, at night they tend to be warmer than the rest of the aircraft, so insects are attracted to them as resting places. That's why you must 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. It's 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. Next, 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 for a rise in the ammeter current when you switch on, or that the magnetic compass is deflected. Then switch it off again so it doesn't burn out. And a critical caution: make sure the pitot covers and static plugs are removed before turning the heaters on to check them, or 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.