Fully-Automatic Execution of Critical Manoeuvres in Airline Flying

David Learmount’s semi-annual review of commercial air accidents has just appeared in Flight International (3-9 August, p34). There were three accidents to high-performance large commercial passenger jets: (1) a Ethiopian Airways Boeing 737-800 took off from Beirut over the sea at night and ended up in the ocean (25 January); (2) an Afriqiyah Airways Airbus A330-200 impacted the ground violently on approach to Tripoli’s RWY 9 (12 May); (3) an Air India Express Boeing 737-800 overran the runway at Mangalore (22 May). Recently, not included in David’s survey, (4) an Airblue Airbus A321 impacted high terrain while on approach to Islamabad (28 July); (5) an AIRES Colombia Boeing 737 landed and broke up on RWY 6 of San Andres Island (16 August); (6) an Embraer 190 of Henan Airlines impacted short of the runway and broke up on approach to Yichun (28 August).

Taking a random six months of accidents is not a sample conducive to pointing to trends using statistical methods; it is well-known amongst students of commercial air accidents that there are “fashions”, common features which cluster at a certain time, but which then reduce, without anybody necessarily doing anything much different. However, let us start here with the question that is the theme of this note:

Which of these accidents would likely have been avoided had the aircraft been fully automatically controlled?

Unmanned aircraft such as the military Global Hawk reconnaisance aircraft routinely fly complete missions under automatic control, from full stop to full stop. Other unmanned aircraft, such as the Predator «drones» used by the US Military in Afghanistan, and for US southern-border patrol, are remotely piloted, but have had control problems with the remote-piloting regime, as for example in this analysis of a US southern-border accident by Johnson and Shea. I want to emphasise here that we are indeed in the era in which fully automated long-distance flights are routinely flown (if only at present by the US Military, and, soon, other NATO allies with the Euro Hawk).

(1)Ethiopian had taken off into a «black hole» over the ocean at night, in other words into an environment in which there were no outside visual references whatsoever. The aircraft was performing a climbing turn, when it started to descend and disappeared from radar. There were electrical storms in the vicinity. The causes are not yet known, but certain factors have been proposed as hypotheses. The accident is almost certainly loss of control (LOC): no one presumes that the pilots committed suicide/murder. First, spatial disorientation of the pilots. This is a historical factor in the records of accidents in night takeoffs and landings in «black holes», such as over oceans. Second, a weather-related upset, say windshear of some kind causing loss of control (LOC). Such phenomena are also known historical factors. It is understood that no technical defects have been yet identified, but I also understand that the investigation is not yet complete.

If spatial disorientation of the pilots had been a causal factor, this would have been avoided by full automatic control of the takeoff and after-takeoff manoeuvring

(2)Afriqiyah was approaching RWY 9 at Tripoli, in clear weather but with reported «low, hazy visibility» (Learmount, op. cit.). «Information from the FDR and CVR indicates that there were no technical faults on the aircraft and fuel starvation was not an issue» (Learmount, op. cit.). Aviation Herald confirms this in its report, see in particular the update from the investigator’s information on 14 August. It impacted the ground heavily (even violently), some vertical distance below the approach path, indicating a high rate of descent. The impact was about 900m from the runway, according to Aviation Safety net’s report. The ground in the area of the airport is more or less flat. Although the VOR was NOTAMed unreliable, there is an NDB approach to RWY 9. The aircraft is capable through GPS equipment and NDB reference of constructing a «Continuous Descent Approach» (CDA) path, which gives a more-or-less constant rate or angle of descent to the point of touchdown, constructed by the Flight Management Systems using the exterior navigation aids, and it would have been able to do that at this airport at this time, as far as is now known. If the aircraft had been on a CDA, it would have been at about 200 feet altitude at this point (the arithmetic: assuming 3° approach path, about one-in-twenty, and a touchdown point 300m from the runway threshold, the aircraft impacted about 1200m from touchdown point, at which point it should have been at 60m above touchdown zone elevation (TDZE)).

Automatics are capable of controlling the airplane within a tens of feet of a given path, and routinely do so (indeed, they must do so in certain flight phases, such as cruise in european RVSM airspace). Given that there were no technical issues identified with the aircraft by the investigation, and violent weather was not a factor, a fully-automated CDA would have landed the aircraft on the runway; at least ensured it was not 300 ft below where it should have been assuming a normal 3° continuous-descent approach path.

(3)Apparently the Air India Express Boeing 737 «landed on RWY 24 just beyond the touchdown zone, in fair weather with no rain. It overran the runway end and plunged into a ravine (Learmount, op. cit.). According to the report by Aviation Herald, the runway has an ILS, required landing distance was 7500 ft and the runway length was 8100ft. There is no word yet, to my knowledge, on possible causal factors.

This seems to have been a routine landing, with no compromising weather. Such landings are routinely accomplished fully automatically, by the Hawk UAVs.

(4)The Airblue A321 had completed an ILS approach to RWY 30 at Islamabad, had turned right at low altitude and then left, to fly parallel to the runway. The crew is supposed at this point by many (with whom I currently concur, given the information available) to have been attempting a circle-to-land (CTL) manoeuvre, likely to land on RWY 12 (the reciprocal of the approach runway). CTL is a routine instrument flight rules manoeuvre, permitted from the ILS approach to RWY 30 as shown in this snippet from an approach plate, posted by «aterpster» in the PPRuNe discussion forum. In a CTL manoeuvre, the pilot, upon «obtaining a visual with» (i.e., seeing) essential parts of the runway or its environment, manoeuvres to land the airplane, provided the visual contact is continually maintained. If visual contact is lost, a routine «missed approach» manoeuvre must be immediately initiated. During the manoeuvre, the airplane must be flown within a given radius, just over 5 nautical miles, of a specified point on the airport. A diagram of this circling radius, overlaid on a plan of the airport and environment, appears in this post by «aterpster» in the PPRuNe discussion forum. A first approximation to the crash sight by, overlaid on a map with some of the navigation detail, including the CTL radius from a post by «aterpster» may be seen in this post by «PJ2», who updated his estimate of the approximate crash location some time later in this post. The crash site is reported by Aviation Herald to be about 10 nautical miles away, and in this early article in FlightGlobal, the WWW site of Flight International, to be 9.66 nm. The print version of the article (Flight International, 3-9 August 2010, p7) says 9.7 nm. There were reported to have been «no technical problems» in a later article in Flightglobal. So the impact site was at about twice the allowable CTL radius. The CTL radius encloses only flat land; the aircraft impacted «rising terrain», in other words a hill/mountain range nearby, but not so nearby as to constitute any danger to normal IFR operations.

There is a question, currently unanswered, as to why the EGWPS terrain-warning equipment did not enable the crew to manoeuvre to avoid the terrain.

Unlike a (presumed-)straightforward approach as at Tripoli, current commercial-aircraft automatics do not assist CTL manoeuvring in any reliable manner; the procedure should be hand-flown. However, it is a straightforward manoeuvre well within the capabilities of automatic control systems such as those on the Global Hawk to follow an ILS, and circle to land on the reciprocal runway, within the given limits. Automatics could have accomplished this manoeuvre within going outside the given CTL radius and therefore without a danger of impacting high terrain.

Furthermore, systems currently in test for the USAF, and shortly to become operational, perform automatic terrain-avoidance manoeuvres, even – expecially – during the kind of low-level manoeuvring performed by military pilots. The system is called Auto-GCAS and was extensively reported and flight-tested recently by Aviation Week (August 2, 2010, pp50-57). Here is a short blog on it by Stephen Trimble of FlightGlobal from last year.

Some proponents of EGWPS have suggested that avoidance manoeuvres in commercial air operations be automatically initiated and flown. This is well within current capability, as shown by Auto-GCAS.

(I have mentioned anonymous writers above. Here is what I know of them. “PJ2” is someone I know, and with whom I have discussed accidents for a decade. He is a recently-retired captain for a major airline, where he was deeply involved in setting up the airline’s FOQA program. He is expert in aviation safety matters and I value his advice considerably. I do not know “aterpster”, but have read many public contributions by him. He self-indentifies as a former airline pilot who has been officially involved in accident investigations as a designated representative of pilots’ organisations.)

(5)Initial reports of the AIRES accident suggest that the aircraft landed short, for example this report in Aviation Herald. Weather is reported by FlightGlobal to have included thunderstorms in the vicinity. Some commentators on the PPRuNe thread have suggested that the main gear was torn off upon reaching the runway hard surface, which is elevated slightly above the surrounding terrain (one imagines the wheels sinking into software ground before the runway, and then impacting the hard runway construction).

It is not possible at this point to estimate the causal influence of the weather – one notes in the above references that the aircraft was reported to have sustained a lightning strike on final approach. But a landing of this sort to the TDZ is routine, even in stormy weather, for digital flight control systems. Providing, of course, they are sufficiently well insulated from the effects of a lightning strike.

(6)The Henan incident was also a landing-short, in reportedly benign weather – see for example the report in Aviation Herald – on a non-precision approach (NPA). The weather was reported as «foggy», but of course fog is incompatible with the kinds of atmospheric disturbances which might lead to control problems, and is not an issue for automatic control. A fully automatic landing was possible in these conditions, but not necessarily in the E190 accident airplane.

At this point, there is no public information about any technical problems with the flight. NPAs have been known for decades to be more accident-prone than precision approaches (ILS), but modern automation such as on the Embraer 190 can routinely perform CDAs, as discussed above with respect to the Afriqiyah accident.

None of the final reports are out, or expected yet, for any of these accidents. As things stand at present, the Ethiopian and ARIES accidents could have had the causal involvement of atmospheric disturbance, we don’t know. But other potential causal factors would have been mitigated if the manoeuvres had been performed fully automatically. In the case of the other four accidents, it seems quite reasonable to assert that, had the manoeuvres been performed fully automatically, outside the current capabilities of commercial-aircraft avionics but certainly within the routine capabilities demonstrated by Global Hawk UAVs, and the USAFs Auto-GCAS.

There are of course substantial safety issues with fully-automatic flight in civil airspace. It is correct to say that at this point it is not operationally feasible. For a recent review of some issues, see the forthcoming paper Computational Concerns About Integration….. by Johnson, to be read in two weeks at the SAFECOMP conference in Vienna.

So no one is yet suggesting, even for the medium term, pervasive fully-automatic commercial air transportation. But in light of the observations above concerning the six 2010 fatal accidents to large commercial jet aircraft, it does look as if it would be worthwhile to research whether standard approach and landing manoeuvres could be transitioned to routine fully-automatic execution.

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