Question

Summarize how a pressure altimeter works to determine and display aircraft altitude and explain the meaning of the saying in aviation, “high to low, or hot to cold, look out below.” Give an example of an aircraft incident related to pressure altimeter error.

Answer 1

Altitude determination can be done with the help of measurement of atomspheric pressure. The lower the altitude, the higher will be the pressure. The device used to measure Altitude using atmospheric pressure is called altimeter.

WORKING OF AN ALTIMETER ( In an aircraft ) -

The static wents of the altimeter detect statis pressure. The detected static pressure is sent to a static container/cage. which contains a partially evacuated aneroid capsule and a leaf spring mechanism connected to the index pointer, which moves as the capsule expands/contracts. Now as the prssure reduces, the capsule starts expanding and this moment is made on the pointer dial. To measure the changes in the pressure a pressure datum is selected on the dial to take the pressure. Height above ground level is detected, if set to airfield pressure; height above the mean sea level.

The meanng of the saying "high to low, or hot to cold, look out below", When you fly from a high pressure to a low pressure area, the altimeter will read higher because of the lower pressure. If you don't adjust the altimeter setting, and you continue to fly the same indicated altitude, you will be lower than you think, so "look out below."  

An Example Would Be-

An altimeter failure on a PILATUS Eagle aircraft caused a 2500 ft discrepancy between the displayed altitudes on the aircraft’s two altimeters. This contributed to a very serious airprox which identified a number of safety issues of interest to the aviation community.

On initial contact, ATC had correctly verified the SSR Mode C of the PC12 in accordance with ICAO Doc 4444, PANS ATM § 8.5.5. The displayed level corresponded to the aircraft’s assigned flight level and the altitude shown on both altimeters. At this point the controller and pilot believed that everything was normal. However, when the pilot noticed the discrepancy in altitudes (the No1 altimeter was showing FL270 and the No2 FL290), he informed ATC and asked the controller to check the displayed level on radar. The controller decided to obtain an independent verification of aircraft’s Mode C from a third-party agency; he subsequently received confirmation that the displayed altitude on radar corresponded to that shown on No1 altimeter and the aircraft’s assigned flight level and he informed the pilot accordingly. It transpired however, that due to a faulty altimeter, the displayed altitude information was incorrect - the Mode C and No1 altimeter were both showing an altitude 2000 feet below the PC12’s actual level.