Return to Human Space Flight home page

Air Data System

Two air data probes are located on the left and right sides of the orbiter's forward lower fuselage. During the ascent, on-orbit, deorbit and initial entry heat load environment phases, the probes are stowed inside the forward lower fuselage. The air data probe (except for the probe itself) is covered by thermal protection system tiles while in the stowed position. At approximately Mach 3, the air data probes are deployed.

The air data system provides information on the movement of the orbiter in the air mass (flight environment).

The air data system senses air pressures related to spacecraft movement through the atmosphere to update navigation state vector in altitude; provide guidance in calculating steering and speed brake commands; and update flight control law computations and provide display data for the commander's and pilot's alpha Mach indicators, altitude/vertical velocity indicators and CRTs. The AMIs display essential flight parameters relative to the spacecraft's travel in the air mass, such as angle of attack (alpha), acceleration, Mach/velocity and knots equivalent airspeed. The altitude/vertical velocity indicators display such essential flight parameters as radar altitude, barometric altitude, altitude rate and altitude acceleration.

Each probe is independently deployed by an actuator consisting of two ac motors connected to rotary electromechanical actuator and limit switches. Each probe is controlled by its air data probe switch on panel C3. To deploy the air data probes, the left and right switches are positioned to deploy. The redundant motors for each probe drive the probe to the deployed position. When the probe is fully deployed, limit switches remove electrical power from the motors. Deployment time is 15 seconds for two-motor operation and 30 seconds for single-motor operation. The deploy position deploys the probe without electrical heaters. The deploy/heat position also deploys the air data probes with heaters powered.

The air data probe stow left and right switches on panel C3 are used during ground turnaround operations to stow the respective probe. Positioning the respective switch to enable and positioning the corresponding air data probe switch to stow stows the corresponding air data probe. The air data probe stow inhibit position opens the ac motor circuits, disables the stow and protects microswitches.

Each air data probe has four pressure-port sensors and two temperature sensors. The pressures sensed are static pressure, total pressure, angle-of-attack upper pressure and angle-of-attack lower pressure. The four pressures are sensed at ports on each probe: static pressure at the side, total pressure at the front and angle-of-attack lower near the bottom front. The probe-sensed pressures are connected by a set of pneumatic lines to two air data transducer assemblies. The two temperature sensors are installed on each probe and are wired to an ADTA. The pressures sensed by the left probe are connected by pneumatic tubing to ADTAs 1 and 3. Those sensed by the right probe are connected to ADTAs 2 and 4. Temperatures and sensed pressure from the probes are sent to the same ADTAs.

Within each ADTA, the pressure signals are directed to four transducers, and the temperature signal is directed to a bridge. The pressure transducer analogs are converted to digital data by digital-processor-controlled counters. The temperature signal is converted by an analog-to-digital converter. The digital processor corrects errors, linearizes the pressure data and converts the temperature bridge data to temperatures in degrees centigrade. These data are sent to the digital output device, which converts the signals into serial digital format, and then to the onboard computers to update the navigation state vector. The data are also sent to the commander's and pilot's altitude/vertical velocity indicators, alpha Mach indicators and CRT.

The ADTA SOP uses ADTA data to compute angle of attack, Mach number (M), equivalent airspeed (EAS), true airspeed (TAS), dynamic pressure (q), barometric altitude (h) and altitude rate ( . h ).

The altitude/vertical velocity indicators and alpha Mach indicators are located on panels F6 and F8 for the commander and pilot, respectively. The information to be displayed on the commander's AVVI and AMI is controlled by the commander's air data switch on panel F6, and the pilot's AVVI and AMI data are controlled by the pilot's air data switch on panel F8. When the commander's or pilot's switch is positioned to nav , the AVVIs and AMIs receive information from the navigation attitude processor. When the air data probes are deployed, the commander's and pilot's switches can be positioned to left or right to receive information from the corresponding air data probe.

The AVVIs display altitude deceleration ( alt accel ) in feet per second squared, rate in feet per second, navigation/air data system (nav/ADS) in feet and radar ( rdr ) altitude in feet. The AVVI's altitude acceleration indicator remains on the navigation attitude processor from the IMUs. In addition, the radar altimeters on the AVVIs will not receive information until the orbiter reaches an altitude of 5,000 feet.

The AMIs display angle of attack ( alpha ) in degrees, acceleration (accel) in feet per second squared, Mach number or velocity (M/vel) in feet per second and equivalent airspeed ( EAS ) in knots. The AMI's acceleration indicator remains on the navigation attitude processor from the IMUs.

All but the alpha indicators (a moving drum) and the altitude acceleration indicators (a moving pointer displayed against a fixed line) are moving tapes behind fixed lines. The AMI's angle-of-attack indicator reads from minus 18 to plus 60 degrees, the acceleration indicator from minus 50 to plus 100 feet per second squared, the Mach/velocity indicator from Mach zero to 4 and 4,000 to 27,000 feet per second, and equivalent airspeed from zero to 500 knots. The AVVIs read altitude acceleration from minus 13.3 to plus 13.3 feet per second squared, altitude rate from minus 2,940 to plus 2,940 feet per second, altitude from minus 1,100 to plus 400,000 feet and then changes scale to plus 40 to plus 165 nautical miles (barometric altitude), and radar altitude from zero to plus 9,000 feet.

Failure warning flags are provided for all four scales on the AVVIs and AMIs. The flags appear in the event of a malfunction in the indicator or in received data. In the event of power failure, all four flags appear.

The four computers compare the pressure readings from the four ADTAs for error. If all the pressure readings compare within a specified value, one set of pressure readings from each probe is summed, averaged and sent to the software. If one or more pressure signals of a set of probe pressure readings fail, the failed set's data flow from that ADTA to the averager is interrupted, and the software will receive data from the other ADTA of that probe. If both probe sets fail, the software operates on data from the two ADTAs connected to the other probe. The best total temperature from all four ADTAs is sent to the software. A fault detection will illuminate the air data red caution and warning light on panel F7, the backup caution and warning alarm light, and the master alarm and will also sound the audible tone and generate a fault message on the CRT. A communication fault will illuminate the SM alert light.

The four ADTAs are located in the orbiter crew compartment middeck forward avionics bays and are convection cooled. Each is 4.87 inches high, 21.25 inches long and 4.37 inches wide and weighs 19.2 pounds.

The air data probe sensor contractor is Rosemount Inc., Eden Prairie, Minn. The air data transducer assembly contractor is AirResearch Manufacturing Co., Garrett Corp., Torrance, Calif. The contractor for the air data probe deploy system is Ellanef, Corona, N.Y.

Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 04/07/2002
Web Accessibility and Policy Notices