Flight Instruments

Table of Contents:-

Normal Operations (Refer to Chapter 4)

Alternate Operations:
  • Flight with unreliable airspeed/mach indications
  • Ground proximity alert
  • Below glideslope alert light illuminate/aural warning
  • Terrain avoidance
  • Upset recovery
  • Difference between altimeter indications
  • Maximum in-tolerance difference between altimeters
  • Reduced vertical serparation minimum (RVSM) altimeter cross check limits

Controls and Indicators:
  • Mach / Airspeed Indicator and Warning Test
  • Electronic Clocks
  • Altimeters
  • Flight Recorders
  • Standby Horizon Indicator
  • Vertical Speed Indicator
  • Altitude Alert
  • Ground Proximity Warning
  • True Airspeed, TAT/EPRL and Static Air Temperature Indicators

  • Pitot Static System

Supplementary Information:
  • Central Air Data Computer (CADC)
  • Flight Recorder (AIDS)
  • Ground Proximity Warning System (GPWS) (MK-II)
  • Enhanced Ground Proximity Warning System/Windshear (EGPWS/WS) (MK-VII)
  • Terrain Awareness Alerting and Display


Alternate Operations:-

Flight with unreliable airspeed/mach indications:

Upon recognition of eroneous or unreliable airspeed/Mach, check airplane attitude and check the probe hear ON by immediate recall action.

Eroneious or unreliable airspeed indications may be caused by blocked or frozen pitot-static systems, severely damaged or missing radomes or by other causes.

A blocked or frozen pitot/static and/or static system may affect the following primary airplane systems:
  • Airspeed/Mach Indicator
  • Vmo/Mmo Warning
  • Altimeter
  • Vertical Speed
  • True Airspeed
  • Static Air Temperature
  • Flap Load Relief System
  • Elevator Feel System
  • Rudder Ratio Changer
  • Stabilizer Trim Rate Controller
  • Autopilot
  • Autothrottle
  • Ground Proximity Warning System
  • Altitude Alert
  • Cabin Pressure
  • Flight Recorder
  • Transponder Altitude Reporting
  • Flight Director Altitude Select
  • Gust Response Suppression System

Early recognition of eroneous airspeed indications is essential to maintaining control of the airplane and avoiding unsafe attitude and airspeed conditions.
A loss of radome may normally be identified by noise and low frequency buffeting.

If eroneous or unreliable
airspeed/Mach indications are detected or suspected:

Airplane attitude - Check
Establish the airplane at the target attitude and thrust setting for the flight phase.
Probe heat - Check On
irspeed/Mach indicators - Cross Check
Cross check
airspeed/Mach indicators to determine, if possible, which indicator(s) and related systems are reliable.  Continue flight using reliable indicators if practicable.

If erroneous or unreliable
airspeed/Mach indications cannot be isolated to a single instrument or system:
Airplane Attitude and Engine Thrust - Monitor
Maintain desired flight profile and airspeed by maintaining appropriate attitude and thrust.  Refer to Chapter 23 for attitude and power settings for each phase of flight.
Gust Response Suppression System - Off
TE Flap Load Relief C/B (P12) - Pull
Deactivate flap load relief system to avoid unreliable retraction inputs.  Use normal landing flaps.

Ground proximity alert:

When a SINKRATE, TERRAIN, DON'T SINK, TOO LOW GEAR, TOO LOW FLAPS, TOO LOW TERRAIN or GLIDESLOPE aural alert sounds, or the BELOW G/S light illuminates with no associated aural signal, accomplish this procedure by immediate recall.
Correct flight path and/or airplane configuration to eliminate the cause of the aural alert.

Below glideslope alert light illuminate/aural warning:

Note:  The below glideslope alert mode does not alleviate pilot responsibility for normal monitoring and response to the glideslope pointers on the fliht instruments.
Upon recognition of visual or aural alert, accomplish the following by immediate recall:
Monitor flight instruments.
Follow approved airline operating procedures for below glideslope deviation.

Note: The below glideslope alert mode may be cancelled or inhibited for:
  • Localizer approach
  • Backcourse approach
  • Circling approach from the ILS
  • When conditions require deliberate approach below glideslope
  • Unreliable glideslope signal

Terrain avoidance:

Immediately accomplish the following by recall whenever the threat of inadvertent contact with the terrain exists.  Regard any of the following conditions as a potential for terrain contact:
  • Activation of the PULLUP warning
  • Other situations resulting in unacceptable flight toward terrain.

Upset recovery:

An upset can generally be defined as unintentionally exceeding the following conditions:
  • Pitch attitude >25 degrees nose up
  • Pitch attitude >10 degrees nose down
  • Bank angle > 45 degrees or
  • Within above parameters but flying at airspeeds inappropriate for the conditions.

Difference between altimeter indications:

At sea level:
  • Max difference between Capt and FO:  50 feet
  • Max difference between Capt or FO and Field Elevation:  75 feet

Maximum in-tolerance difference between altimeters:  See 14.20.06

Individual servo-driven altimeters are subject to undetected malfunctions (no warning flags).
An abnormal altitude difference (increasing with altitude) between altimeters is usually an indication of such a malfunction.

Reduced vertical serparation minimum (RVSM) altimeter cross check limits:

At sea level:
  • Max difference between Capt and FO:  50 feet
  • Max difference between Capt or FO and Field Elevation:  75 feet

Controls and Indicators:

Mach / Airspeed Indicator and Warning Test:

Maximum Operating Airspeed Pointer:
Indicates maximum operating airspeed.

Indicated Airspeed Pointer:
Indicates airspeed derived pneumatically from pitot-static pressures.

Command Airspeed Bug:
Set by autothrottle (A/T) speed indicator.

Mach Number Indicator:
Displays computed mach number from central air data computer.
Yellow flag covers digits when CADC or Mach indicator fails.

Indicated Airspeed Scale:
Linear from 60 to 250 knots only.

Electronic Clocks:

Chronograph Switch:
Controls start, stop and reset of CHR display and second hand with successive push operations.  Displays CHR time in ET/CHR display.  Overrides existing ET display.  ET will continue ET switch in RUN, but will not be displayed until CHR is reset.

GMT Indicator:
Displays time in 24 hour format.

GMT Selector:
TEST - Both digital displays indicate 88:88
RUN - Starts GMT indicator counting in hours and minutes.
HLD (Hold) - Stops GMT indicator time and sets seconds to zero.
SS (Slow Slew) - Advances GMT minutes.
FS (Fast Slew) - Advances GMT hours.

On Pilot's Auxiliary Panel (Captain's illustrated).

Clock Switch:

Performs same function as CHR switch.


Standby Flag:
In view when there is loss of power to instrument, instrument servo failure, or CADC valid signal lost.  In each case, the indicator automatically reverts to standby (pneumatic) mode of operation.
100 Foot Pointer:
Displays altitude in 100 foot increments (smallest divistion equals 20 feet).  One full rotation of pointer equals 1000 feet.

Barometric Setting Indicator in both Millibars and Inches of Mercury.

Mode Switch:
CADC - Raw altitude information from the static system is corrected for static source position error by the CADC.
STBY - Raw altitude information is displayed.

Altitude Numerical Counter:
Displays altitude in thousands and hundreds of feet.

Barometric Setting Control:
Rotation of the control adjusts the barometric setting on the millibar (MB) and inches of mercury (HG) indicators.

Flight Engineer's Panel

Barometric Setting Control:
Rotation of the control adjusts the barometric setting on the millibar (MB) and inches of mercury (HG) indicators.

Flight Recorders:

Standby Horizon Indicator:

Warning Flag:
In view when electrical power is not available to indicator.

Caging Control / Pitch Trim Control:
- Pull out for caging
- When control is in rotation adjusts fixed airplane reference.

Powered from the Battery Bus

Vertical Speed Indicator:


Altitude Alert:

Altitude Selector:
Rotation of selector sets the desired altitude in the adjacent indicator.
After altitude is set, the altitude alert system provides light and aural signals when approaching the altitude or when deviating from the altitude.

Altitude Alert Light (Amber):
- Illuminates steady and 2 second aural tone sounds when approaching 900 feet above or below selected altitude; remains illuminated until 300 feet above or below altitude.
- Extinguishes when 300 feet above or below selected altitude and remains extinguished while within that 300 feet above or below range.
- Illuminates flashing and 2 second aural tone sounds when deviating 300 feet above or below selected altitude; light continues to flash until 900 feet above or below, at which time the light extinguishes and the system is automatically reset for subsequent altitude alerting.
- Deviation from altitude alerting is inoperative when landing gear is extended except during altitude alert test.  See Airplane Manual Chapter 4 for test procedure.

Ground Proximity Warning:

Inhibit Below G/S Switch:
PRESS - Inhibits or cancels glideslope alerting if pressed while in the soft alerting area.

Below G/S Alert Light (Amber):
Illuminated (flashing) - Airplane more than 1.3 dots below the glideslope.

Pull Up Warning Light (Red):
- Excessive descent rate
- Excessive closure rate
- Altitude loss after takeoff or go-around
- Non landing configuration descent.

True Airspeed, TAT/EPRL and Static Air Temperature Indicators:

True Airspeed Indicator:
Provides visual indication of True Airspeed computed by CADC.
With system or indicator failure, a yellow flag covers the readout.

TAT / EPRL Indicator:
Provides total air temperature for the TAT/EPRL computer as input from the left total air temperature probe.
Indicates from -60 to +60.
With instrument or electrical failure, the display will show dashes or be blanked out, depending on source of malfunction.

Static Air Temperature Indicator:
Provides static air temperature computed by CADC number 2.
The right two counters display positive temperatures.
The counters not being used are covered by the sign.
With indicator or electrical failure, a yellow flag covers the readout counters.

Pitot Static System

Supplementary Information:

Central Air Data Computer (CADC):

The two central air data computers receive pitot and static pressure inputs from the pitot-static system and electrical temperature signals from the total air temperature probes.
The inputs and signals generate altitude, airspeed and temperature information to calculate pressure altitude, vertical speed, mach number, true airspeed, computed (calibrated) airspeed and static air temperature.
Some of these computed outputs display on the pilot's instruments and several air data computer outputs go to other systems.

Data valid signals from the computers go to each instrument display.
Loss of a data valid signal causes a warning flag to appear on the applicable instruments.
Internal failure monitors in each instrument also cause the warning flags to appear when a malfunction is detected.

Flight Recorder (AIDS):

The Aircraft Integrated Data and flight recorder System consists of acquistion and recording units located within the pressurised sections of the airplane.
The flight recorder is in the aft section of the airplane and incorporates an underwater locator beacon.
The system receives inputs and records data from safety of flight items, ie: fire and smoke warnings, positions of the thrust levers and flight control surfaces.
It also records data from the autopilots, CADC's, engine parameters, and move during all phases of flight.

The system operates automatically when auxiliary power is removed and the essential bus is powered.
The only requirement of the flight crew is to manually enter the identification data prior to flight and to monitor the system during flight.

Ground Proximity Warning System (GPWS) (MK-II):

The GPWS alerts the flight crew when one of the following thresholds are exceeded between 50 and 2,450 feet radio altitude:
Mode 1 - Excessive Descent rate
Mode 2 - Excessive Terrain Closure rate
Mode 3 - Altitude Loss After Takeoff or Go-Around
Mode 4A - UnsafeTerrain Clearance with Landing Gear Not Down
Mode 4B - Unsafe Terrain Clearance with Flaps Not in Landing Position
Mode 5 - Below Glide Slope Deviation Alert
Mode 6 - Below Decision Height Alert

Enhanced Ground Proximity Warning System/Windshear (EGPWS/WS) (MK-VII):

The EGPWS/W alerts the flight crew when one of the following thresholds is exceeded between 10 and 1,500 feet radio altitude:
Mode 1 - Excessive Descent rate
Mode 2 - Excessive Terrain Closure rate
Mode 3 - Altitude Loss After Takeoff or Go-Around
Mode 4 - UnsafeTerrain Clearance during high speed flight or while not in the landing configuration.
Mode 5 -
Below Glide Slope Deviation Alert
Mode 6 - Altitude callouts and Bank Angle alert
Mode 7 - Windshear conditions.

Terrain Awareness Alerting and Display:

A new feature of the EGPWS is the incorporation of the terrain and obstacle awareness alerting and display functions.
These functions use aircraft geographic postion, aircraft altitude and a terrain and obstacle database to predict potential conflicts between the aircraft flight path and the terrain and to provide graphic displays of the conflicting terrain or obstacle.

The EGPWS internal database consists of four sub-sets:
  • A worldwide terrain database of varying degrees of resolution
  • An obstacles database containing catalogued obstacles 100 feet or greater in height located within N America and Caribbean
  • A worldwide airport database containing information on hard surface runways of 3500 feet or more
  • An envelope Modulation database to support the Envelope Modulation Feature
With the use of accurate GPS, the EGPWS is provided present position, track and ground speed.
With this information the ECPWS is able to present a graphical plan view of the aircraft relative to the terrain and advise the flight crew of a potential conflict with the terrain or obstacle.
Conflicts are recognized and alerts provided when terrain violates specific computed envelope boundaries on the projected flight path of the aircraft.
Alerts are provided in the form of visual light annunciation of a caution or warning, audio annunciation based on the type of conflict and colour enhanced visual display of the terrain or obstacle relative to the forward look of the aircraft.

Due to terrain features at or near certain specific airports around the world, normal operations have resulted in nuisance or missed alerts at these locations in the past.
With the introduction of accurate position information and a terrain and airport database it is possible to identify these areas and adjust the normal alerting process to compensate.