Definition

For the purpose of this article, it is assumed that the flight instrument display is the Electronic Flight Instrument System (EFIS) design found on modern airliners. The article seeks to provide generalized guidance for pilots. Nothing herein should be taken as a substitute for aircraft-specific guidance found in the Quick Reference Handbook (QRH) or other relevant manuals.

Among the simplest types of degraded flight instrument display is a screen failure. EFIS systems usually provide automatic reversion in the event of screen failure. For example, if a pilot's Primary Flight Display (PFD) fails, the adjacent Multifunction Display (MFD) will normally switch automatically to display PFD data. If not, QRH procedures can direct pilots to use reversionary switches to change screen displays.

What is more problematic, more dangerous, and sometimes more difficult to detect quickly is unreliable airspeed and/or altitude data.

In a 2020 study, researchers from the U.S. National Aeronautics and Space Administration (NASA) said, "Loss of control inflight continues to be the leading contributor to airline accidents worldwide and unreliable airspeed has been a contributing factor in many of these accidents." The report went on to say, "Unreliable airspeed can lead to systemic failures that are difficult to understand, often manifesting themselves with both overspeed and stall warnings simultaneously, leaving no simple, effective troubleshooting method." According to the NASA report, over a 10-year period from 1996 to 2006, incorrect instrument indications played a role in 300 accidents. 

Description

Flight instrument degradation can result from a number of causes. The most common involve unreliable airspeed and/or altitude indications resulting from blockage of pitot tubes and static ports caused by ice, dust, ash, or other contaminants. In addition, damage or malfunction involving air data computers and other avionics modules can cause instrument failure.

Depending on the manufacturer, the instrument failure may be accompanied by an Engine Indicating and Crew Alerting System (EICAS) message or a warning flag noting a miscompare between the captain's and first officer's PFDs. The malfunction may present itself gradually and in ways that can be misinterpreted. An erroneous indication of increasing airspeed might first be interpreted as an autothrottle commanding a higher power setting or an autopilot commanding a lower pitch attitude.

Signs of the malfunction may include:

• Continuous or intermittent stick shaker
• Stick shaker on one side only
• Pitch limit indicator on one side only
• Overspeed warning when aircraft attitude and power setting do not correlate to high speed
• Aural airspeed alerts when aircraft attitude and power setting do not correlate to high or low speed
• Blank or intermittent airspeed or altitude indication
• Discrepancy between captain's and first officer's airspeed indications or altitude indications
• Speed or altitude miscompare or disagree alerts
• Speed or altitude failure flags
• Angle of attack failure or miscompare flags

Typically, initial QRH guidance will direct the pilots to compare indications on their PFDs with data displayed on a standby or backup instrument. For example, if the captain's airspeed indications correlate with standby indications and the first officer's do not, then the first officer's data is assumed to be faulty. QRH guidance might then direct the first officer to use a reversionary switch to select an alternate air data system. Alternately, if data on one pilot's displays is faulty and the problem cannot be corrected, the QRH may direct that the other pilot shall take on the Pilot Flying duties.

Emergency Procedures

If unreliable indications are not available on either pilot's displays, QRH guidance will vary by aircraft type, but it generally follows certain broad guidelines. First steps typically include disengaging the autopilot, autothrottles, and flight director, and establishing an initial pitch and power setting. For most aircraft, an initial pitch and power setting will be listed for a clean configuration and for a flaps-down configuration. If the aircraft is at a safe altitude, the procedure will normally call for leveling the aircraft and holding a steady attitude. These first steps may or may not be designated as memory items, depending on Standard Operating Procedure (SOP) guidance. If the first steps for unreliable airspeed are not required memory items, it is recommended that pilots at least become very familiar with the procedure. This will mitigate the startle factor and other sources of confusion when this malfunction first presents itself. 

After the initial steps, QRH guidance usually directs pilots to a set of tables for pitch and power settings. Normally, tables will be included for various aircraft weights and altitudes and for different phases of flight, such as climb, cruise, descent, and approach. Again, applying this information during an emergency can be challenging if the procedure is unfamiliar, so it is recommended that pilots periodically review the various pitch and power settings, and take note of typical pitch and power settings during normal flight.

QRH and SOP guidance will often remind pilots of alternate sources of data if flight instrumentation fails. For example, GPS data available through the Flight Management System (FMS) will usually show altitude and groundspeed. Air traffic controllers can also provide that information.

Additionally, on most EFIS systems, the flight path angle or flight path vector indications are still reliable, because they receive inputs from the aircraft's inertial reference system and not from the air data system or pitot-static sources.

Requesting a block altitude while troubleshooting this malfunction can be a good idea. Also, if the malfunction occurs in instrument meteorological conditions (IMC), it is recommended to exit IMC if possible and remain in visual conditions. As workload permits, declare an emergency and squawk 7700 on the transponder.

Defences

• A thorough preflight inspection can prevent instrumentation problems. If pitot tubes and static ports are obstructed by snow, ice, or dirt, or if covers are left in place, degraded flight data becomes likely. 
• Pilots should be aware of conditions that can cause degraded instrumentation. If flying in icing conditions and airspeed data appears faulty, pitot tubes and/or static ports may have iced over.
• Solid hand-flying skills will make this malfunction much easier to handle. Pilots should routinely practice flying approaches without the aid of the autopilot and autothrottles.