University of Bielefeld -  Faculty of technology
Networks and distributed Systems
Research group of Prof. Peter B. Ladkin, Ph.D.
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The B757 Air Data System

Excerpted from the B757 Operations Manual
by P. Ladkin

1 April 1996



The air data system consists of a pitot static system (see Pitot Static Schematic), one temperature and two angle of attack probes, two air data computers and electric flight instruments. Standby airspeed and altimeter indicators are also provided.



Sensed air data is provided to the air data computers. Processed pressure and temperature information from each computer provide input signals to certain flight instruments (Electric Mach/Airspeed Indicator, Electric Altimeter) and other using systems (IRS, AFDS, FMC, FDR, EEC, TMC, GPWS, ATC and others). The Captain's instruments use the L ADC and the F/O's instruments use the R ADC. The opposite ADC is available as an alternate source.

Unreliable air data signals cause warning flags to appear on the air data instruments. When a malfunction occurs in instruments with failure monitors warning flags appear.

Two test switches on the accessory panel are provided for ground air data computer testing. They are inoperative inflight. For further information on this test, see Supplementary Normal Procedures.

Mach/Airspeed Indicator

Each Mach/Airspeed Indicator displays airspeed, mach and Vmo from the selected air data computer. The command airspeed bug on each Mach/ Airspeed Indicator can be automatically positioned from the FMC CDU or manually from the MCP IAS/MACH Selector.

Standby Airspeed Indicator

A Standby Airspeed Indicator is installed on the center panel. The instrument is connected directly to the R AUX pitot and the alternate static ports.


Each pilot is provided with a Primary (electric) Altimeter that receives altitude signals from the selected air data computer. An Altimeter Altitude light and an Altitude Alert Light are provided for altitude alerting. The Automatic Flight chapter describes altitude alerting in greater detail.

Standby Altimeter

The Standby Altimeter (pneumatic) provides standby altitude information. Input for the indicator is from the alternate static ports.


An electrically driven Vertical Speed Indicator (VSI) is installed on each pilot panel. Vertical speed indications are in feet per minute. The Captain's VSI is connected to the left IRS and ADC, and the F/O's VSI is connected to the right IRS and ADC. The center IRS provides backup vertical speed data for either crewmember when ALTN is selected with the respective IRS switch. The opposite ADC provides backup vertical speed data for either crewmember when ALTN is selected with the respective ADC switch.

The VSI is inoperative until the associated IRS has completed alignment and entered the navigation mode.

Copyright © 1999 Peter B. Ladkin, 1999-02-08
Last modification on 1999-06-15
by Michael Blume