Airliner Instruments and Control

Baji J. Ram Rao
18:50 Sat. 10-Jun-2009

The man-machine interface in the passenger airliner has evolved
from manual mechanical cable controls and instruments with needles and painted graduations
to complete computer controls with CRT-monitor and LCD-monitor based displays (the glass cockpit).


The basic "T" holds four important instruments.

  1. Airspeed Indicator,
  2. Artificial Horizon,
  3. Altimeter
  4. Gyro-Compass
  5. and additionally the

  6. Turn-and-Bank indicator
  7. Vertical speed Indicator

The 1958-1979 Boeing 707 was the world's first commercially successful jet airliner.
Air-India had eleven of them.
Six with the Rolls-Royce Conway low-bypass engines and five with Pratt & Whitney JT3D turbofans.

707 Basic "T" panel

Boeing 707 basic flight panel.

707 cockpit

Boeing 707 cockpit.

The overhead panel above the pilots, contains ignition and engine start switches, fire extinguisher switches, HF radios, alternate flap control, emergency exit lights, rain repellent, and exterior lights, compass and vertical gyro transfer functions, Mach Trim test, Fuel filter heaters, Winshield wipers, engine de-icing, hydraulic pumps, Heating switches for pitots and alpha vanes and Airconditioning & Gasper fan switches for Gyro motor and caging for gyrocompass and vertical gyro for bank angle and pitch attitude.

The 1974-2007 Airbus A300 and 1983-2007 A310, had CRT monitors for altitude and navigation info display.
However they still used traditional mechanical gauges with needles and painted graduations, for airspeed, altitude and vertical speed.

A300 cockpit

Lufthansa Airbus A300 cockpit.

With the 1988-onwards Airbus A320, and following that the 1993-onwards Airbus 340, 1994-onwards Airbus 330 and 2007-onwards Airbus 380 they all have full "glass cockpits".

A330 cockpit

Full glass cockpit of Airbus A330.
Note sidesticks and absence of control yokes.

All mechanical gauges and warning lights are replaced by interchangeable general purpose CRT/LCD computer screens
that display whatever the EFIS (Electronic Flight Instrument System computer) wants to display via a graphics card.


Autothrottle on the Airbus is a feedback control system that can be in fixed SPEED mode or fixed THRUST mode.

In SPEED mode, the computer tries to maintain the speed to the pilot-ordered or autopilot-ordered speed, but still within the safety envelope of the aircraft.

In THRUST mode, Autothrottle maintains a fixed power setting (what we car people call BHP, for a jet engine it's static thrust measured in kiloNewtons or pounds).

The throttles have six detent positions specified according to flight phase:

  1. TOGA (Take-Off/Go Around) power,
  2. FLX/MCT (Flex/Max. Continuous Thrust) power,
  3. CLB (Climb) power,
  4. IDLE power,[for flaring when landing]
  5. REV IDLE (reverse idle) power,
  6. MAX REV (max. reverse thrust) power

While the throttle is in the CLB detent and Auto thrust is active, the Airbus's FADEC (Full Authority Digital Engine Ctrl) can control the thrust from FLX/MCT to IDLE.
The throttles are linked to the FADEC by electronic signals, not by cables unlike in Boeings.
This reduces weight (similar to fly-by-wire controls).
Airbus also has brake by wire.

You order the CLB airspeed, and the autothrottle adjusts to maintain it.
Pilot workload greatly decreases, while fuel efficiency and performance improve because of the computerized precision power management.

However to work, Autothrottle needs precise speed measurement.
If the pressure pitots are choked and send questionable speed info, everything can quickly go awry.

2009 Atlantic Ocean Airbus crash

Remember the Air-France Airbus A330 crash in June 2009?

On the night between Sun. 31st May 2009 and 1st June 2009, an Airbus A330-203 registered F-GZCP,
flying Air France flight 447 from Rio de Janeiro to Paris, crashed into the Atlantic Ocean, killing all 216 passengers and 12 aircrew.

The majority of the wreckage and black-boxes could not be located for nearly two years after the accident.
When finally located, experts determined that:
The computer autopilot system was confused by inconsistent airspeed measurements from ice-choked pressure pitot tubes.
The computer system abandoned the aircraft in distress to a human crew, handicapped of sensing equipment.
The aircraft entered an irrecoverable aerodynamic stall and crashed into the ocean killing all 228 on board.
This was the second and deadliest fatal accident of an Airbus A330.

1990 Bangalore Airbus crash

One remembers the Bangalore Airbus crash on Wed. 14-Feb-1990. It was an Airbus A320-231 twin-jet on final approach to Bangalore.
The aircraft was registered: VT-EPN, construction no:079. It was flying Indian Airlines flight 605 from Bombay to Bangalore. The aircraft was barely a year old.

At seven miles from the airport, with the autopilot still connected, the flightcrew activated Autopilot approach mode. Activating the approach is unique to Airbus aircraft.
The Airbus flight computer implements a set of control functions. Airbus calls them control laws. examples: Normal law, Alternate law, etc.
It is a Flight Management Guidance System (FMGS - software autopilot) feature which automatically commands slower aircraft airspeeds, if in Managed Speed Mode, which is normal, during an approach.

During visual approach, with the crew manually flying the airplane, both the left and right flight directors remained unnecessarily engaged.
A320 procedures manuals do not prohibit this -- a mistake.

The copilot dialed the wrong (Altitude) knob (thinking that he dialed the Vertical Speed knob).
This resulted in selection of a lower altitude than the altitude of the runway.
The pilot’s words regarding vertical speed, influenced the copilot’s action and thus he selected the altitude of 700 feet without realising that he selected the wrong altitude.
My italics: Poor User Interface design by French Airbus designers with poor MMI design skills.

Defective Flight Control Unit and Flight Director user-interface settings caused both engines to remain in IDLE/OPEN DESCENT state too long. Airspeed dropped. Flight control law software automatically commanded thrust increase, toward go-around power. Simultaneously, the flightcrew, at 106 knots, also advanced the throttles to go-around power. But the computer had already taken-over charge.
The pilot pulled back on the sidestick to lift the nose, but the software change of state to ALPHA-FLOOR state took too long.
There was not enough time for the engines to spin up to TOGA power.

The aircraft under computer control flew into the ground, while the pilots couldn’t do anything.
It crashed on the Karnataka Golf Course, 2,800 feet short of the runway, just outside the airport boundary wall, and burst into flames.
4 of 7 crew including both pilots died; and 88 of the 139 passengers died.

And to think that the old Boeing 707s had mechanical-hydraulic, non-computerized functions. With the pilots always fully in control.

1994 Nagoya Airbus crash

This is about the Airbus A300B4-622R twin-jet crash at Nagoya, Japan, at 20:16, Tue., 26th April 1994.
The aircraft, construction no:580 registered: B-1816, was flying China Airlines flight 140 from Taipei, Taiwan to Nagoya, Japan.
The aircraft was 3½ years old.

For final approach, the copilot had initiated ILS-approach to runway 34 under manual control.
He inadvertently activated the GO lever at 1,070 ft, 145 knots and 5 degrees pitch. The Flight Director Software switched to GO AROUND mode, causing a engine power increase.
As auto throttles were engaged, the aircraft climbed above its normal glide path.

The pilots applied down elevator. The computer system fought back with pitch-up stabilizer. The THS (Horizontal Stabilizer) moved to full nose-up position causing an abnormal out-of-trim situation with an 18 degree pitch up.
This caused the aircraft to climb. The computer software system activated ALPHA FLOOR state due to excessive angle-of-attack. That triggered maximum thrust, which increased the nose-up attitude to 52.6 degrees.
Then the captain disengaged ALPHA-FLOOR by retarding engine thrust.
The computer system prevented the crew from overriding the flight director's throttle and pitch control movements.
Speed dropped and the aircraft stalled at 1,800 feet, hit the ground tail-first 300 ft. right of the runway and burst into flames. 249 of 256 passengers and all 15 crew were killed, totalling 264 fatalities out of 271 on board.