* An asterisk indicates an incident described in Air Crashes and Miracle Landings.

~ A ~

ACARS, relaying info from AF447 and MH370   

ACARS (Aircraft Communications Addressing and Reporting System) started off as a basic system for transmitting information back to the airline, such as when the doors closed and opened, and when the aircraft wheels lifted off from and touched down again on the runway. This was mostly to prevent pilots and cabin crew, paid according to the time spent airborne, fiddling their time sheets, though it was also helpful for airlines to know in real time when their aircraft had departed.

 Subsequently it became very much more sophisticated, sending data such as the performance of the systems and engines back to the airline and engine maker to make maintenance more efficient. Incidentally, engine-makers are often paid according to amount of use and find it useful for keeping track of that as well as of any faults. Depending on the location of the aircraft the information would be transmitted cheaply via VHF radio or more expensively via satellite.

ACARS first really came to public notice in 1999 when Air France Flight 447 crashed in the South Atlantic en route from Rio de Janeiro to Paris*. Until the black boxes were miraculously recovered two years later, the only clue as to what had happened were a few ACARS messages showing anomalies.

ACARS came to even closer public attention in one of the greatest aviation mysteries ever. That was when Malaysian Airways 777 Flight MH370, flying from Kuala Lumpur to Beijing, disappeared on March 8, 2014*. Unlike in the case of Air France Flight 447, there was no wreckage found floating on the surface, perhaps because rescue aircraft had been searching in the wrong ocean!

Flight MH370’s ACARS, though switched off, was still making routine hourly handshakes with a satellite for some six hours, and these enabled experts at the satellite company Inmarsat to calculate that the aircraft had come down somewhere in the Indian Ocean off Perth, Australia, in the opposite direction of where it was meant to be going. The location was very imprecise and much of what wreckage might have been floating on the surface would have sunk or so dispersed in the notoriously rough seas there, when the search switched to there.

One year later a flaperon from the 777 was found washed up on the shore of La Réunion, off Africa. Though it is impossible to calculate exactly from where it might have drifted, the ocean currents are such that it could well have drifted the more than 2,500 miles from the search site off Australia.

ACARS in its normal role has three essential functions:

  1. Providing a data link.
    Exchanging operational information with the airline. United Airlines used this feature during 9/11 to warn Flight 93* (that came down in open country before reaching its target in Washington DC) about cockpit intrusions. Unfortunately, the ACARS warning did not stress that intrusions should be prevented whatever the threat, not that cockpit doors at the time could withstand aggressive forced entry.
  2. Maintenance data download.
    Previously, maintenance staff had had to access the quick-access data recorder on landing to find out how the engines and other equipment had been behaving and what special servicing was needed. Now, with ACARS they can be ready with the necessary staff and spare parts, and this is perhaps the reason why the system is so widely used. Data is also transmitted directly to the manufacturer of the engines.
  3. Ping messages
    If the ACARS connection has been silent for more than a given time the ground station can ping it to check it is still working correctly.

Provide Info for Air Accident Investigators?

Following the crash of Air France Flight 447 and the disappearance of Malaysian Airlines Flight MH370 there have been proposals for ACARS to constantly feed comprehensive data to ground stations that could help solve cases of air crashes where the black boxes may never be found. This is likely to be adopted sometime in the future, but currently the bandwidth and cost problems are too great.

Accelerate-Stop Distance (on runway)

Calculated overall distance required to accelerate from commencement of the takeoff run to takeoff decision speed (V1), abort the takeoff, and brake safely to a halt still on the runway.

Accident Models (Accidentology)

Academic work on the probability (inevitability), causes, and avoidance of accidents is very extensive, with great contributions by psychologists and academics working in such critical areas as the safety of nuclear power plants where the fate of millions of people has to be considered. Dozens, if not hundreds, of books can be found on the subject, which can seem so simple at some levels and so complex at others.   

Though not for the fainthearted, Wiegmann and Shappell’s A Human Error Approach to Aviation Accident Analysis, published by Ashgate, explains how various academics have treated the subject. See Swiss Cheese Model and Normal Accident.

Aces learnt from mistakes (Confidential Reporting)

In World War II, several German fighter aces claimed the experience of being shot down several times—and of course surviving—was what helped make them so good. The same was probably true for Battle of Britain pilots flying over home soil and likely to be picked up by their own people—in some cases shot down in the morning and up again in the afternoon.

The point is it shows one learns from mistakes and the value of schemes whereby pilots can report mistakes anonymously for the benefit of all. See ASRS (Aviation Safety Reporting System), a scheme run by NASA of the US for the confidential reporting of incidents. Like CHIRP (UK) or EUCARE.

Adam = A missing or abducted child

Codes enable organizations to make confidential announcements over public address systems. One such is Adam, meaning a child is missing, named after Adam, the six-year old son of the host of Fox TV’s America’s Most Wanted. The child was abducted from a Sears department store in Florida in 1981 and subsequently found dead. Of course, there are many others pertaining to the particular organization.

One code, Bravo, is intended to make people panic in the hope they will cower or even run enabling, say a terrorist, to be identified. Note that on a cruise liner, Bravo can mean a fire on board.

ADC: Air Data Computer

A computer handling data regarding the air with which the aircraft is in contact and, in particular,the static pressure, which is the actual pressure at that height, and the pitot pressure, which is the extra pressure produced when airflow enters a forward-facing port. By comparing the two, the ADC can calculate the aircraft’s airspeed.

The ADC also calculates barometric height (altitude), vertical speed (climb/sink rate), air temperature, and Mach number.

ADIRU: Air Data Inertial Reference Unit

A key unit (usually there are three for redundancy) supplying essential data to the pilots and systems as follows:

  1. Air Data: airspeed, angle of attack, and altitude.
  2. Inertial Reference: attitude, flight path vector, ground speed and positional data. [Before GPS, the position was determined by a combination of gyroscopes and accelerometers. As these drift with time, resets are made on flying over known beacons. Nowadays there is also input          from GPS.]

ADF: Automatic Direction Finder

Equipment that determines the direction of a radio transmitter.

Can be tuned to an NDB (nondirectional beacon) or even to an AM radio station, and therefore not limited to line-of-sight broadcasting stations, as would be the case with FM. Can be set in two modes:

  1. Needle pointing to transmitter relative to nose of aircraft.
  2. Needle pointing relative to magnetic north.

Administration or Agency (US Usage thereof)

Journalists not brought up in America should be aware that the A at the end of the acronyms for some well-known US organizations might well stand for administration and not agency.

Notable administrations:

NASA: National Aeronautics and Space Administration

TSA: Transportation Security Administration

FAA: Federal Aviation Administration

Notable agencies:

CIA: Central Intelligence Agency

NSA: National Security Agency

FEMA: Federal Emergency Management Agency

Note that Department is what is called a Ministry in other countries and TSA and FEMA are both part of the Department of Homeland Security (DHS), created in the aftermath of 9/11.

ADS-B, Game-changer in Air Traffic Control

Automatic Dependent Surveillance-Broadcast or ADS-B is technology expected to be at the heart of future, more efficient air traffic control systems. See NextGen.

It is a somewhat awkward to grasp name, but easier to understand if one knows ATC depends on all aircraft to tell it what is happening.

The principle is that aircraft automatically broadcast a whole range of information far exceeding that squawked by the traditional transponder, notably:

  1. Height, ground speed, and airspeed.
  2. Exact position (determined by GPS and cross-checked with other navigational instruments).
  3. Climb or sink rate, rate and direction of turn, or absence thereof.
  4. Latest control inputs indicating what the aircraft is about to do. For example, the throttle levers may have been advanced, but actual reaction as seen on traditional radar can take several seconds from idle detent and to register in the controller’s mind.
  5. Of course, info traditionally given out by airliner transponders, such as identity, squawk code, and so on, is included.

This information would be shown not only on ATC monitors but also on monitors on other aircraft within a radius of some 150 miles. Furthermore, traffic warning and collision avoidance systems (TCAS) would operate even more effectively since they would consider evasive action already being taken by the respective aircraft without waiting to see the response. Not only that, such a system would lessen the risk of any collision scenario developing in the first place.

Of course, the controllers would also have their traditional independent radar monitors, akin to military radar.

Purported advantages:

  1. Greater accuracy would allow aircraft to be closer together and mean aircraft would not have to be held back so much as now when there is congestion.
  2. Works in the absence of radar coverage, such as in isolated areas, or in the lees of mountains.
  3. Often possible to preclude potential conflicts.
  4. Functions also on the ground, even indicating when an aircraft is just moving off, or a runway incursion is about to occur. It would have prevented the worst-ever multi-aircraft aviation disaster, where two 747s collided on Tenerife in 1977.
  5. Other features, such as weather depiction and advisories, can be incorporated in the ADS-B data link.

Aircraft would save time and fuel by flying more direct routes rather than the traditional airways between radio beacons.

Objections voiced by opponents include the claim that terrorists might exploit the information an aircraft transmits regarding its position to destroy it, say by flying a model plane or GA aircraft into it. Advocates counter this by saying there are simpler ways, such as shoulder launched SAMs, to bring down an aircraft near an airport. See Aircraft-Centric.

Aerodynamics Index (NASA Glenn Research Center)

This is an amazingly comprehensive online site entitled Beginner’s Guide to Aerodynamics.  Although described as being for ‘beginners’, they mean dedicated students. Perhaps conscious of this, a means of accessing the site according to one’s knowledge level has now been incorporated.


Airfoils (aerofoils), turbine blades, and even bridges are not completely rigid and deform under the inertial, aerodynamic, and elastic forces imposed on them or produced in them due to resulting harmonic oscillations. While this phenomenon can have harmful effects, it can sometimes be used to advantage in that say an airfoil can be designed to change shape according to airspeed.

AFCS: Automatic Flight Control System

Also called IFCS: Integrated Flight Control System

Age of Pilot, How old is too old?

The maximum permitted age for commercial airline pilots varies from country to country. With older people who have looked after themselves—as airline pilots normally do—nowadays living longer, there has been a move to allow older pilots, even in the US, to work to sixty-five rather than sixty.

This is something of a contentious issue at the airlines, not so much on safety grounds—they should always be a paired with a younger copilot—but because of the differing financial situations of the pilots concerned. With so much at airlines based on seniority rather than ability, junior pilots do not want to see captains staying on as seat blockers. While senior pilots with advantageous pension plans are happy to retire early, those not so blessed want to continue. In France, the government has been thinking of forcing through legislation to raise the age limit from sixty to sixty-five in the face of the threat of strike action by some pilots as pilots there are so highly paid there they can afford to retire in comfort.

While the major airlines may be good at weeding out those whose faculties deteriorate with age, it is worth noting an article by Patrick R. Veillette, PhD, entitled Tombstone Mentality in Aviation Week (June 24, 2008). He consulted a hundred business aviation colleagues to learn their concerns about near misses with light aircraft, contaminated runways, and so on. As regards aging, he says the following:

The aging pilot population within certain segments of the business aviation industry gave serious concern to many. Colleagues who frequently fly with the over‑sixty group notice deterioration in important sensory, perceptual, cognitive, and motor skills that are important to piloting. They have noticed changes to an aging pilot’s ability to learn, memorize, tolerance to fatigue, sleeping habits, physical changes, and so forth.

He adds that the pairing of pilots to avoid two old pilots being together is not properly thought out—a mistake the major airlines would be unlikely to make.

agl (Above Ground Level)

Abbreviation used to indicate the height above ground level, as opposed to sea level, such as 5,000 ft agl.

AIDS: Aircraft Integrated Data System

AIM (Aeronautical Information Manual [FAA])

Official Guide to Basic Flight Information and ATC Procedures, issued by the FAA. Covers all aspects, and very comprehensive.

Air China (Mainland China)

Air China (CA) is China’s flag carrier, based in Beijing. Not to be confused with Taiwan’s China Airlines (CI). The IATA codes are confusing, since AC for the former, like AF for Air France, would be more logical.

Air Crashes have survivors if….

 Sometimes the most horrendous-seeming crash with the aircraft breaking up into sections has more survivors than one that appears benign with the aircraft seeming intact.

For example, when a British European Airways triple-engine BEA Trident* stalled and belly-flopped onto a field on taking off from London’s Heathrow Airport its fuselage appeared intact but only a single survivor who soon expired. This is because with little under the cabin floor to crumple there was little to absorb the shock.

Contrast that with the famous case* of the DC-10 which crash landed at Sioux City Airport with the pilots miraculously maneuvering it by varying engine power alone. It broke up into five pieces but had 185 survivors!  

Flying is so much safer these days thanks to the lessons learned and advances in technology.

For details of air crashes that taught us so much see our book Air Crashes and Miracle Landings describing 85 of the most notable.

Air Cushion Seats

Some premium cabin seats now use air cushions, which have advantages over foam in that they can be rendered soft when the seat is upright and hard when horizontal, besides saving a considerable amount of weight.

Air Force One

An emotive term for Americans, partly because of the dramas, including movies, built around it, and partly because of the awe once associated with the presidency.

Even though the call sign Air Force One was first used in the 1950s for the aircraft carrying the president of the United States, the Boeing 707 used by President Kennedy was the first particular aircraft to be so designated by the public—or rather, the media. In fact, there are two Air Force Ones to ensure one is always serviceable.

Special features on these modified Boeing 747-200s included a shower for the president, accommodation for him or her and his or her family, a conference room, and quarters for the press, officials, and Secret Service. Naturally, high-tech defensive mechanisms and secure communication facilities are important features.

The new Air Force Ones are going to be modified Boeing 747‑8s, with the work taking a considerable time—they even must be able to be refueled in the air.

While the US president might have to stay airborne in a crisis, there is another aircraft, the so‑called Doomsday Plane, or National Airborne Operations Center (NAOC), that similarly must be able to remain airborne in a crisis so that commanders can liaise with the president, secretary of defense, and commanders on the ground, at sea, and in submarines under the sea. Its communications facilities are far more comprehensive than those on Air Force One and include a very-low-frequency antenna that can be trailed five miles behind for communicating with submarines, and a hump on top of the fuselage for super-high-frequency and Milstar communications. There are four such aircraft based at Offutt Air Force Base just south of Omaha, with one always ready to fly at an instant’s notice.

September 11, 2001 (9/11) is the only occasion the Doomsday Plane has played its true role.

Air Marshals

Although many countries, and notably El Al, do have security people on board their aircraft, the US has taken the idea of having incognito armed marshals on board the most seriously. The exact figure is classified but following the events of 9/11 the number of air marshals apparently jumped from thirty-three to between three thousand and four thousand. Following that there were no further hirings, though they will likely restart, as attrition depletes their numbers.

On July 16, 2015 the Homeland Security Subcommittee on Transportation questioned the incoming head of the Transportation Security Administration, Rod Allison, about the air marshal program, saying that though they supported it and the marshals, there were complaints about air marshals, including passengers being angry at being bumped for the benefit of marshals with no explanation. Republican, Earl “Buddy” Carter, described a flight where six marshals sat in first class, and then more marshals from a canceled flight joined them.

The committee thought that with reinforced cockpit doors, there was less need for marshals, and more focus ought to be put on flights representing the greatest risk.

The presence of air marshals may have dissuaded some people with bad intentions, but there do not seem to be many, if any, cases where they have saved the day. In the US there are often tough military people on a flight, and somewhat like the free service doctors happening to be on board provide, they can overpower crazed passengers, with no need for an air marshal with his gun.

In addition, the US Air Marshalls trains a few pilots and aircrew to handle guns and deal with hijackers with them having permission to keep the weapon in the cockpit.

Air Rage

The term air rage conjures up images of drunken individuals causing disruption. However, there was a case in India where a group of passengers acted in unison. They were so frustrated that the aircraft was being kept for ages in a holding pattern that they all rushed toward the cockpit to confront the pilots, almost fatally compromising the control of the aircraft, due to the shift in center of gravity.

In the entry on air marshals we mentioned burly military types on hand in the US ready to help subdue an unruly passenger. Subjugation must not get so out of hand that the individual succumbs—say due to strangulation—as indeed happened in one case.

While a panicking passenger trying to open a door to get out may cause much concern, fellow passengers should not worry for unless the aircraft is extremely low the pressure differential forcing the plug doors shut means they are impossible to open.

A crazed passenger trying to take over control of the aircraft is another matter. On a BA flight* to Nairobi in December 2000, a crazed man wanting to save the aircraft entered the cockpit and tried to take over the controls. The force he exerted on the control column was enough for the autopilot to disengage, and the aircraft dived so precipitously it was in danger of breaking apart even before hitting the ground. Fortunately, he was overpowered seconds before the situation became irretrievable.

The reinforced and locked cockpit doors installed after 9/11 to keep out hijackers have virtually eliminated the possibility of such a passenger gaining entry. Unfortunately, they once prevented a Germanwings captain on a toilet break regaining access to the cockpit when his suicidal copilot was intentionally crashing the aircraft into mountains*.

Air Traffic Clearance

Permission from ATC notably, but not exclusively, to taxi, take off, land, climb, descend, or enter controlled airspace. The clearance includes instructions on what to do and of course must be repeated by the pilot to confirm he has heard and understood correctly. In the worst air disaster ever*, where two 747s collided in fog on the runway at Tenerife, the first officer was reading back the clearance as the captain began his disastrous takeoff roll.

Air Traffic Control (ATC)

To avoid aircraft colliding, an air traffic control system was established whereby the aircraft are given instructions by radio regarding routes, height, and so on.

Different sets of controllers handle the various stages of a flight. In the US they would be referred to as follows:

  1. Ground control, handling taxiing at the airport.
  2. Tower, handling takeoffs and landings.
  3. Departure control, handling period between control by the tower and handover to the air route traffic control, called a center in the US.
  4. Center, handling traffic using air routes in the region. For instance, there will be a Chicago Center, New York Center, and so on. The aircraft is handed from center to center until it wants to land, at which point it descends and is handed over to
  5. Approach, which handles the aircraft until handover to the tower for the actual landing.

Air traffic control   in many countries, including the US, is conceptually antiquated and having difficulty in coping with increasing traffic. New technologies (See ADS-B) should allow aircraft to fly closer together and on more direct routes compared with the traditional airways between radio beacons.

Air Traffic Flow Management (ATFM)

A largely computer-based system adjusting departure and arrival times so aircraft arrive at their destination at a time when they can land immediately, rather than arriving simultaneously and having to burn up fuel while holding.

Air Transport Association (ATA) [US]

Washington-based lobbying group for the airline industry.

Allegedly, the ATA recently tried to divert passenger outrage at long delays by blaming corporate jets. In fact, though corporate jets have equal rights to the airlines as regards taking off and so on, on a first-come, first-served basis, they usually use different airports.

Underlying this dispute is a larger one, namely that according to the FAA’s own estimates, private (GA) planes, which include both corporate jets and weekend flyers, account for 16 percent of the air traffic control system’s overhead but contribute only 3 percent of the fees earmarked to run it. At major US airports, corporate jets pay next to nothing compared with an airliner.

Air Transport World: ATW

Trade magazine for the aviation industry, with daily news on website and via email. Based in Maryland, USA, with offices around the world. Even has a section in Chinese.