The Pitot-Static System and Altimeters
Modern jet transports usually have three pitot-static systems. There are separate systems for the captain's and copilot's instruments, plus an auxiliary system that provides a backup for either of the two primary systems. See Figure 11-1.
The instruments that require static pressure input are airspeed, Mach, altitude and vertical speed indicators. In addition, the airspeed and Mach indicators need a source of pitot pressure.
Besides the flight instruments, static pressure input is required for the Mach warning, autopilot, flight director, flight recorder and cabin differential pressure. Pitot input is required for all those systems except for cabin differential pressure. The usual source for these non-instruments is the auxiliary pitot-static system.
Figure 11-1. Typical pitot-static system
Altimeters compare the sea level pressure setting in their window with the outside air pressure sensed through the static systems. The difference is displayed as the altitude above sea level. Part of the preflight check is to verify the accuracy of the altimeters. An altimeter should be considered questionable if the indicated altitude varies by more the 75 feet from a known field elevation. See Figure 11-2.
A common reason for altimeter errors is mis-setting the altimeter. If the setting in the altimeter is higher than the actual sea level pressure, the altimeter will read higher than the actual altitude. If the setting is too low, the altimeter will read lower than it really is. As a rule of thumb, the magnitude of the error is about 1,000 feet for each 1" that the altimeter is off. Example: if the altimeter is set to 29.82", but the real sea level pressure is 28.82", the altimeter will read about 1,000 feet higher than the actual airplane's altitude (29.82 - 28.92 = 1.0" = 1,000 feet). In this example, the airplane would be 1,000 feet lower than the indicated altitude.
In the United States, all altimeters are set to 29.92" when climbing through 18,000 feet MSL. The local altimeter setting is used when flying below FL180 and the altimeter is 31.00" or less. Special procedures apply when the local pressure is more than 31.00", because most altimeters cannot be set higher than that. Outside the United States, the transition altitude is often something other than FL180.
Figure 11-2. Altimeter
If a pitot tube becomes blocked, the airspeed and Mach indicators will read inaccurately. If pressure is trapped in the line, the airspeed will read inaccurately high as the aircraft climbs, low as it descends, and will be unresponsive to changes in airspeed.
If the static line becomes blocked, all the pitot-static instruments will be affected. The altimeter will "freeze." The airspeed will read low at altitudes above the one where the malfunction occurred and fast at altitudes below that point.
If a static line breaks inside a pressurized part of the aircraft, the altimeter and airspeed indicator will both read inaccurately low.
Unlike vacuum-driven instruments, electrically-powered instruments must have some type of failure indicator to show loss of power.
OAT (Outside Air Temperature) or SAT (Static Air Temperature)—the free air or ambient air temperature.
TAT (Total Air Temperature)—the OAT plus any ram rise due to friction and compression caused by the aircraft's movement through the air. A rough rule of thumb is that there will be about 1°C of ram rise for every 10 knots of indicated airspeed. TAT will always be warmer than OAT.
RAT (Ram Air Temperature)—OAT plus a portion (around 80%) of the ram rise. Some temperature probes cannot register all the ram rise and thus indicate RAT rather than TAT. RAT will always be warmer than OAT but colder than TAT.
All three of these temperatures are the same value when the aircraft is stationary because there is no temperature rise due to ram effect.
All airline aircraft have Mode C radar transponders. A Mode C transponder replies to radar interrogations with uncorrected barometric altitude information as well as the code set by the flight crew. This altitude information is unaffected by the aircraft's altimeter setting. A local altimeter correction is applied to ATC's display by his/her ground-based computer.
Hand signals are used for controlling the movement of the aircraft on the ground. See FAA Figure 1.