WHAT YOU NEED TO KNOW:
Pitot-Static Systems
Three basic pressure-operated instruments are found in most aircraft instrument panels. These are the sensitive altimeter, airspeed indicator (ASI), and vertical speed indicator (VSI). All three receive the pressures they measure from the aircraft
piton-static system.
Flight instruments depend upon accurate sampling of the ambient atmospheric pressure to determine the height and speed of movement of the aircraft through the air, both horizontally and vertically. This pressure is sampled at two or more locations outside the aircraft by the piton-static system. These ports are located on the sides of the fuselage.
The pressure of the static, or still air, is measured at these flush ports where the air is not disturbed.
This dual location prevents lateral movement of the aircraft from giving
erroneous static pressure indications.
Pitot pressure, or impact air pressure, is taken in through an open-end tube pointed directly into the relative wind flowing around the aircraft. It is located on the bottom of the left wing and is heated. The pitot tube connects to the airspeed indicator, and the static ports deliver their pressure to the airspeed indicator, altimeter, and VSI. If the static ports should ice over, or in any other way become obstructed, the pilot is able to open a static-system alternate source valve to provide a static air pressure source from inside the aircraft. This value is normally located on the bottom of the instrument panel just to the left of the control column. The pitot-static system must be inspected every 24 months.
Sensitive Altimeter
A sensitive altimeter is an aneroid barometer that measures the absolute pressure of the ambient air and displays it in terms of feet above a selected pressure level.
Principle of Operation: The sensitive element in an altimeter is a stack of evacuated, corrugated bronze aneroid capsules. The air pressure acting on these aneroids tries to compress them against their natural springiness, which tries to expand
them. The result is that their thickness changes as the air pressure changes. Stacking several aneroids increases the dimension change as the pressure varies over the usable range of the instrument.
A sensitive altimeter is one with an adjustable barometric scale that allows you to set the reference pressure from which the altitude is measured. This scale is visible in a small window, called the Kollsman window.
Rotating the knob changes both the barometric scale and the altimeter pointers in such a way that a change in the barometric scale of 1@ Hg changes the pointer indication by 1,000 feet. This is the standard pressure lapse rate below 5,000 feet. When the barometric scale is adjusted to 29.92@ Hg, or 1,013.2 millibars, the pointers indicate the pressure altitude. The altimeter must be inspected every 24 months.
Altimeter Errors: A sensitive altimeter is designed to indicate standard changes from standard conditions, but most flying involves errors caused by nonstandard conditions, and you must be able to modify the indications to correct for these errors. There are two types or errors: mechanical and inherent.
A preflight check to determine the condition of an altimeter consists of setting the barometric scale to the altimeter setting transmitted by the local automated
flight service station (AFSS). The altimeter pointers should indicate the surveyed
elevation of the airport. If the indication is off more than 75 feet from the surveyed elevation, the instrument should be referred to a certificated instrument repair station for recalibration. When the aircraft is flying in air that is warmer than standard, the air is less dense and the pressure levels are farther apart. When the aircraft is flying at an indicated altitude of 5,000 feet, the pressure level for that altitude is higher than it would be in air at standard temperature, and the aircraft will be higher than it would be if the air were cooler.
If the air is colder than standard, it is denser, and the pressure levels are closer together. When the aircraft is flying at an indicated altitude of 5,000 feet, its true altitude is lower than it would be if the air were warmer.
Temperature also has an effect on the accuracy of altimeters and your altitude. The crucial values to consider are standard temperature versus the ambient temperature. It is this Adifference@ that causes the error in indicated altitude. When the air is warmer than standard, you are higher than our altimeter indicates. Subsequently, when the air is colder than standard you are lower than indicated. It is the extreme Acold@ difference that normally would be of concern to the pilot. Also, when flying in cold conditions over mountainous country, the pilot should exercise caution in flight planning both in regard to route and altitude to ensure adequate en route and terminal area terrain clearance. Extreme differences between ambient and standard temperature must be taken into consideration to prevent controlled flight into terrain (CFIT).
The fact that the altitude indication is not always true lends itself to the memory aid, AWhen flying from hot to cold, or from a high to a low, look out below..........Sky wings
