NTSB Seeks the Black Boxes

July 1, 2000

NTSB seeks the Black Boxes!

Flight Data Recorders and Cockpit Voice Recorders

By Jim Sparks

July 2000

This is a headline that accompanies most major aviation accidents and everyone who follows the wire services knows the black boxes are technically known as the Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR).

In fact "Black Box" does not justly describe these devices. These metal cases are required by regulations to be either a bright orange or yellow with reflective tape.

History of data recorders

During the dawn of the Jet Age, world aviation authorities decided that certain flight conditions should always be recorded. Five parameters were originally chosen and include:

Compass heading

This data was imbedded on a roll of steel foil, which was continually moving at a rate of about 4 inches per hour. Unfortunately once recorded, this data was impossible to erase or to record over, so every 400 hours, the foil roll had to be replaced. It soon became apparent that five parameters were insufficient to assist in accurately determining the cause of an aircraft accident. The list was later increased to include:

• Control Column or Pitch Control Surface Position

• Engine Power Output

• Pitch and Roll Attitude

• Time of each radio transmission

A new means or recording this information also had to be developed as the steel foil could not accommodate the additional data. Magnetic tapes were used. Although these are more delicate than the steel foil, they could hold significantly more information - plus they could be written over - thus decreasing maintenance costs. As the magnetic tape is not as durable as steel foil, data recorder manufacturers had to invent new ways of making the tape survivable in the worst conditions.

This is only a test

This was accomplished by developing crash resistant housings that are subject to severe testing requirements. These include crash impact testing, which requires the unit withstand an immediate deceleration from 360 miles per hour to zero. This involves firing the data recorder from a cannon at an aluminum wall. Static load testing is accomplished by applying significant compression loads against the six sides of the case for five minutes. Puncture testing is accomplished by dropping a pointed steel rod on the recorder from a height of about 10 feet.

The device also needs to prove resilience against the effects of fire and heat. A furnace is used to initially heat the Recorder to 1,100 degrees C for one hour then 260 degrees C for 10 hours. Water resistance is the final check and involves subjecting the device to pressures equal to those at 20,000 feet below the water for 30 days.

More recent Digital Flight Data Recorders (DFDR) have the ability to monitor and store information from up to 700 sensors. And by using electronic data storage there is no further need of magnetic tape, nor are there any moving parts contained within.

Regulatory criteria

Different types of aircraft operations will abide by different criteria with regard to data recorders. United States Federal Air Regulation Part 91 appendix "E" provides a list of Flight Data Recorder Specifications. Even though this regulation does not mandate Flight Data Recorders for all, it does say that all turbine powered aircraft carrying 10 or more passengers with a crew of two as listed on the type certificate are candidates for an FDR installation. US Federal air regulations Part 135 and 121 governing aircraft for commercial use have somewhat different requirements.

Location is important

Location is an important factor when installing the Black Box. Many original installations placed the device in the aircraft avionics compartment. With earlier certification requiring the FDR to tolerate up to 100 Gs and the location frequently in the nose area most recorders did not survive sudden impact situations.

Most FDRs of today will withstand 10 times the force of earlier units, plus in most aircraft the data recorders are found in the tail section away from structures that could either puncture or crush the device. This location is, in most cases, away from fuel tanks and is considered less susceptible to the forces encountered from sudden impact situations. Some military aircraft even have the capability to jettison the data recorder in the event of impending crash.

Under control

A Flight Data Entry Panel (FDEP) is a component essential to data recorder operation. Control panels will include a means of setting the time as well as entering flight number. Most control panels have provisions for displaying operating status - that is, if the system is capable of working correctly, the warning lights will be off. The only real control the flight crew has is the system circuit breaker. There is no way from the flight deck to erase recorded data. Most FDRs in use today will store up to 25 hours of information with the oldest information being deleted while new information is being stored.

FDR activation

Activation criteria should always be considered prior to conducting maintenance on the aircraft as erroneous data may be induced.

Federal Aviation Regulations stipulate that the FDR must be operating from takeoff through landing. Aircraft manufacturers and STC holders of FDR installations have different means of activating this system. In some cases, as soon as aircraft power is selected "ON," the recorder will begin to operate. In other situations, recordings begin when the aircraft leaves the ground or even when the main cabin entry door is closed. Recordings are usually stopped by an action opposite to what initiated operation. There are also provisions for a G (gravity) force switch to terminate recorder power in the event of a three-G impact. These G switches are usually externally mounted in the same general area as the FDR. In most cases, these devices require a functional test. Fortunately, this does not require a three-G impact, but can generally be accomplished by using a jumper wire to activate the protection circuit. An operational test of this sensor can be accomplished on a rotating table where by spinning the sensor at a certain velocity and bringing it to an abrupt stop will simulate the force of gravity that causes the sensor to react. A G switch often resembles a large electrical fuse and installs in a clip type holder. It is very important to note the position of this sensor. There should always be a dot or some other means of identifying the forward facing end of the sensor.

Troubleshooting the FDR

Testing of the Flight Data Recorder and associated components is generally required once a year. Sensors are another important consideration, with most new FDRs being digital, aircraft systems that communicate with these recorders need to speak the same language. For most systems installed within the last 10 years, a Data Acquisition Unit is used to convert the aircraft analog, discrete and digital signals into a language that the Digital Flight Data Recorder can understand. As an example, the position of flight controls are monitored by potentiometers, thus providing a variable but linear voltage corresponding to a specific Aileron/ Elevator/Rudder position. Engine throttle position along with flaps and movable horizontal stabilizer position are often monitored in the same manner. Some examples of discrete inputs would be position of the landing gear. It is sensed as either locked or not locked based on a sensing switch condition. Entry door position and electrical system condition are other discrete inputs. Digital data comes from Air Data Computers, Inertial Reference Systems as well as auto flight and navigation devices. Many engines in current use are controlled and monitored by digital computers such as Full Authority Digital Controls (FADEC) and Digital Electronic Engine Controls (DEEC). These devices may also supply significant relative information to the Digital Flight Data Recorder. Thrust Reverser position could be sensed and supplied to the data recorder by an electronic engine control.


Flight Data Recorder

In many cases data recorders are fitted with "pingers." This is a device that is designed to produce a 37.5 kHz signal underwater that has the ability to travel up to 2 miles. Periodically, this device requires a functional test. To accomplish this task, a "pinglight" test set is placed in close proximity to the pinger being tested. Then, by installing a wire to short circuit the contact center with the body, the transmission is initiated. If the device is operating the light on the test set will flash once per second, which is the rate of signal transmission. This submarine transmitter is fitted with an internal battery, requiring periodic testing. By installing a 10 Meg ohm impedance voltmeter in the same manner as the shorting wire between the contact and the housing a small load is placed on the battery so a realistic voltage is observed. An identification plate should be installed with a battery identification code and a corresponding minimum voltage.

With the major impact or Solid State Digital Flight Data Recorders and their ability to monitor up to 700 different items it is now feasible to install video cameras at strategic locations such as on the aircraft's tail, under the belly or even in the flight deck. With these new capabilities crash investigation will be greatly enhanced. With the super computers of today, the stored data preceding a crash can be used to create a very exact profile of the fateful flight.

Sensor sense

In the event of maintenance being performed on systems monitored by the recorder, close attention should be paid to appropriate rigging of the sensor. Should the rudder system require a re-rig it is imperative to make sure the flight control position sensor is also taken into account. Most airframe manufacturers or STC holders will supply sensor rigging and testing instructions in their documentation. In fact in some cases functional and rigging checks of FDR sensors are part of the aircraft routine inspection program.

Cockpit Voice Recorders

Cockpit Voice Recorders (CVR) are also a product of the Jet Age and especially a result of magnetic tape. In fact by 1965, all commercial transport aircraft were required to have one. This device will record the voices of the flight crew as well as any other sound occurring in the flight deck. Most voice recorders have the ability to record up to four tracks. One input is from the pilot's microphone, headset mike ore oxygen mask microphone. A second recording is made from all the copilots' voice sensitive devices. A third channel is dedicated to a stand-alone cockpit microphone. This is installed in a central location within the flight deck so that it can pick up the conversation of the flight crew as well as any other sounds such as fire alarms, stall warnings, gear or flap alerts and ground proximity warnings.
A CVR has the ability to record for anywhere from 30 minutes up to three hours, after which it will begin to overwrite the earliest information with the latest. Like the FDR, a system of logic within the aircraft will start and stop the recording coinciding with the start of the engines or even the closing of the aircraft entry door. A control panel for the CVR is installed in the flight deck. This unit has a jack for installing a headset as well as a test switch and an erase selector. The erase mode is generally inhibited while the CVR is operating. Most devices are also equipped with a status indicator that is either digital or analog. This will provide the crew with information regarding the proper operation of the unit.

Most CVRs are equipped with a pinger that operates in exactly the same manner as the FDR. This means the functional check as well as the battery test should also be a planned part of the inspection program.

In an effort to save space and weight several manufacturers have developed combined Cockpit Voice and Flight Data Recorders in one unit. And, with solid state technology, the reliability of new equipment is very high. However, many countries around the globe have not yet entered the digital world, and it is still highly probable that someone somewhere is still operating an aircraft with a steel tape FDR.

Unfortunately there is often a negative stigma associated with data recorders just for the reason they are in the aircraft. This makes it all the more important that we in the aviation maintenance profession make sure these devices are 100 percent capable of doing their job. In the event of the worst case, the data they contain can be retrieved to make future operations all that much safer.