AM.II.H.S4

From the FAA library

• Instrument Flying Handbookp. 5-7
  Chapter 5: Flight Instruments › Pitot/Static Systems › Altimeter Enhancements (Encoding)

  determine the pressure altitude at which it is flying. A computer inside the encoding altimeter measures the pressure referenced from 29.92 "Hg and delivers this data to the transponder. When the pilot adjusts the barometric scale to the local altimeter ... ensure that all Mode C aircraft are transmitting data referenced to a common pressure level. ATC equipment adjusts the displayed altitudes to compensate for local pressure differences allowing display of targets at correct altitudes. 14 CFR part 91 requires…

• Pilot's Handbook of Aeronautical Knowledgep. 11-3
  Chapter 11: Aircraft Performance › Pressure Altitude

  terms must be defined. Pressure Altitude Pressure altitude is the height above the standard datum plane (SDP). The aircraft altimeter is essentially a sensitive barometer calibrated to indicate altitude in the standard atmosphere. If the altimeter ... aircraft operating at above 18,000 feet. The pressure altitude can be determined by any of the three following methods: 1. By setting the barometric scale of the altimeter to 29.92 "Hg and reading the indicated altitude, 2. By applying…

• 14 CFR Part 91 — General Operating and Flight RulesOpen
  14 CFR § 91.219 Altitude alerting system or device: Turbojet-powered civil airplanes.

  operated; (4) Be tested without special equipment to determine proper operation of the alerting signals; and (5) Accept necessary barometric pressure settings if the system or device operates on barometric pressure. However, for operation below 3,000 feet ... radio altimeter may be included to provide the signal if the operator has an approved procedure for its use to determine DA/DH or MDA, as appropriate…