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

Nov. 1, 2003
A look at the regulationsBy Jim Sparks
Standby display air data module.

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.

Standby instrument backup batteries.

Per FAR Part 25 the following instruments and equipment may not be included in a minimum equipment list:
(1) Instruments and equipment that are either specifically required by the airworthiness requirements under which the aircraft is certified and which are essential for safe operations under all operating conditions.
(2) Instruments and equipment required by an airworthiness directive to be in operable condition.
(3) Instruments and equipment required for specific operations.

It may be possible to dispatch an aircraft in some cases with inoperative instruments and equipment without an approved minimum equipment list providing: the flight is conducted in a Rotorcraft, nonturbine-powered airplane, small aircraft, glider, or lighter-than-air aircraft for which a master minimum equipment list has not been developed. Inoperative instruments and equipment can not be part of the VFR-day type certification package.

Inoperative instruments
The aircraft's basic equipment list or operations equipment list for the kind of flight operation being conducted may also provide information on operations with inoperative indications. In fact for general aviation operators FAR 91-205 provides guidance specific to different kinds of flight operations.

In some cases the inoperative instruments and equipment are removed from the aircraft, the flight deck controls are placarded, and the maintenance recorded in accordance with FAR 43.9. The removal of equipment may also require an adjustment to the aircraft weight and balance. Deactivated instruments are placarded "Inoperative." If deactivation of the inoperative instrument or equipment involves maintenance, the rules dictate it must be accomplished and a record made.

In all cases a determination has to be made by a pilot certified on the aircraft or by a person appropriately rated to perform maintenance, that the inoperative instrument or equipment does not constitute a hazard to safety of flight.

Under the above conditions an aircraft with inoperative instruments or equipment may be considered to be in a properly altered condition acceptable to the Administrator.

Top: Standby electronic display. Note: Red on the sides shows air data information not valid.
Bottom: Display showing fault flags - invalid information.

Additional requirements
If the flight is to take place with an extended time period over water there are usually some additional requirements. Flight over water more than 30 minutes or 100 nautical miles from the nearest shore requires the following operable equipment:
Radio communication equipment appropriate to the facilities to be used and able to transmit to, and receive from, any place on the route, at least one surface facility and includes two transmitters, two microphones, two headsets or one headset and one speaker, and two independent receivers. Also appropriate is electronic navigational equipment consisting of at least two independent electronic navigation units. However, a receiver that can receive both communications and navigational signals may be used in place of a separate communications and navigational signal receivers.

In certain cases, an aircraft may be flown with no passengers to a maintenance facility where repairs can be made. This is providing not more than one of each of the radio communication and navigational equipment malfunctions or becomes inoperative. Local airworthiness authorities should always be consulted anytime there is doubt as to dispatchability.

VOR systems and checks
Aircraft operating in conditions where instrument flight rules (IFR) apply and use of the very high frequency omni directional range (VOR) system requires the equipment be maintained, checked, and inspected under an approved procedure. An alternative is to perform an operational check within the preceding 30 days. And the check must verify the system to be within the limits of the permissible indicated bearing error.

Any person conducting a VOR check should use an approved test signal. Appropriately rated radio repair stations also have the authority to check the VOR equipment and should check that the maximum permissible indicated bearing error is plus or minus 4 degrees. If neither a test signal nor authorized repair agent is available on the ground, use of an airborne checkpoint may be acceptable with a maximum permissible bearing error of plus or minus 6 degrees.

Even if no check signal or point is available, while in flight an option may be for a pilot to select a VOR radial that lies along the centerline of an established VOR airway. Then locate a prominent ground point along the selected radial preferably more than 20 nautical miles from the VOR ground facility. The aircraft can then be maneuvered directly over the point at a reasonably low altitude; and the VOR bearing indicated by the receiver can be noted when over the ground point with a maximum permissible variation between the published radial and the indicated bearing of 6 degrees. If dual VOR systems (units independent of each other except for the antenna) are installed in the aircraft, the person checking the equipment may check one system against the other in place of the check procedures specified above. Both systems shall be tuned to the same VOR ground facility and note the indicated bearings to that station. The maximum permissible variation between the two indicated bearings is 4 degrees.

Once the VOR check has been accomplished the date, place, and bearing error must be entered in the aircraft log.

Standby instruments
Standby instruments also require consideration when it comes to dispatching aircraft as well as consideration during normal maintenance. In the event of a malfunctioning standby attitude indicator some aircraft minimum equipment lists will still allow flight but only in daylight visual conditions. Backup instruments commonly used today include an electronic display capable of showing not only attitude information but also airspeed, altitude, and even navigation data. Devices like this may include their own air data module which receives pitot and static pressure and then converts it to an electric signal which is sent to the flight deck display. Even though it is considered a standby device regulations still apply which govern altimeter tests along with leak checks of the system any time integrity is jeopardized. In addition the backup batteries which power many backup displays are often armed by the aircraft ground flight systems. This may impact aircraft jacking as when the standby devices come to life and are operating using power from a backup battery; their operation may only last a short time, which will unfortunately result in a depleted battery and possibly a missed or delayed departure for the aircraft.

It is anybody's guess what Orville and Wilbur's vision was for our industry. The technological advancements in only the past couple of decades are truly amazing. What does it mean? That the vision is now ours and the next 100 years are going to be one heck of a ride.