Broken Instruments, Can It Fly? A look at the regulations

A look at the regulations
By Jim Sparks

With the first 100 years of aviation rapidly coming to a close, technology continues its swift advance. Many aircraft developed in this decade use electronic devices to perform functions that used to be accomplished via mechanical components working in unison with barometric sensors and electromagnetic gizmos.

Most commercial aircraft of today have a much less cluttered instrument panel than their predecessors. In fact in some cases there are only four display units available to the crew. However technology makes fewer instruments capable of bringing the flight crews ever so much more information than their electromechanical counterparts.

What's required?
Per U.S. Federal Air Regulations Part 25, which governs the certification of Transport category aircraft, there are minimum required flight and navigation instruments. Instruments are defined as "Devices that are physically contained in one unit, or are composed of two or more physically separate units or components, connected together (such as a remote indicating gyroscopic indicator that includes a magnetic sensing element, a gyroscopic unit, an amplifier, and an indicator connected together)."

Required instruments in a transport aircraft include:
(a) Airspeed indicator. (For commuter aircraft, if the airspeed limitation varies with altitude, the airspeed indicator must show the variation of VMO with altitude.)
(b) Altimeter.
(c) Direction indicator (nonstabilized magnetic compass).
(d) For reciprocating engine-powered airplanes of more than 6,000 pounds maximum weight and turbine engine powered airplanes, a free air temperature indicator or an air-temperature indicator which provides indications that are convertible to free-air.
(e) A speed-warning device for turbine engine powered aircraft if the velocity maximum operating (VMO) or Mach maximum operating (MMO) is greater than 0.8 Mach in a dive. This device must give effective aural warning differing distinctively from aural warnings used for other purposes to the pilots whenever the speed exceeds VMO plus 6 knots or MMO+0.01.
(f) An attitude display including an aircraft symbol, which is nonadjustable by the crew other than to correct for parallax view.

Backup power systems
When passenger seating exceeds nine, excluding the pilot's seats and the aircraft is approved for IFR operations, a third attitude instrument must be provided that is powered from a source that is independent of the electrical generating system and activated without pilot input. If a backup power supply is used it should be able to operate the attitude indicator for at least 30 minutes after a failure of the normal electrical system. In addition this attitude display must have its own inertial sensor and not be dependent on the normal systems. The location on the instrument panel must also make the standby attitude display clearly visible to the pilot and include a lighting source.

In addition to dictating what instruments must be included the regulations also specify how these devices will function in other than normal conditions. Per FAR 25.1331 instruments using electrical power must have a visual means integral with the instrument to indicate when power adequate to sustain proper performance is not being supplied. The available power must be measured at or near the point where it enters the instruments and is considered to be adequate when the voltage is within approved manufacturer's limits.

Each required instrument must, in the event of the failure of one power source, be supplied by a backup supply. This may be accomplished automatically or manually and installation of secondary displays may fulfill this requirement.

Navigation instruments receiving data from remote sources where loss of information would render the flight deck data unreliable require a visual means to warn the crew. This annunciation must clearly show that the display should not be relied upon. Warning flags may have different meanings depending on the equipment manufacturer and should always be investigated and clearly understood.

Minimum equipment lists
Flight with inoperative instruments and equipment under certain conditions can be considered acceptable. This is providing an approved minimum equipment list exists for that aircraft or a Letter of Authorization (LOA) is written by the FAA Flight Standards district office having jurisdiction. The LOA may often be obtained by written request and along with the minimum equipment list constitute a supplemental type certificate for the aircraft.

If an approved minimum equipment list is used it must provide for the continued safe operation of the aircraft with the instruments and equipment in an inoperable condition. In addition aircraft records available to the pilot must include an entry describing the inoperable devices. The aircraft will then have to be operated under all applicable conditions and limitations contained in the minimum equipment list or the LOA. In some cases modified operational procedures will be required for the flight crew.