07:40 CET on Friday 27th March
Two important points today. First, investigators have detailed apparently-deliberate actions by the First Officer to initiate a descent and keep the Captain from reentering the cockpit. Colleagues with some experience have said that it is premature to rule out actions in course of experiencing a stroke (Schlaganfall in German). Second, the workings of the cockpit door locking mechanism, and the policies concerning a pilot leaving the cockpit have come into question. I explain the operation of the A320 cockpit locking below.
First, terminology. Everybody is writing “pilot”, “co-pilot”. The usual term is Captain (CAP) and First Officer (FO), referring to the command roles. The term “pilot” informally refers to the person flying the airplane at a given time, known as Pilot Flying (PF). The other cockpit-crew member is the Pilot Non-Flying (PNF). In this incident, the PF appears to have been the FO.
French investigators have said that the Captain left the cockpit, with the First Officer, the Pilot Flying, remaining at the controls, alone in the cockpit. That shortly afterwards a descent was initiated by – I am here interpolating with some knowledge of the A320 – dialing an “open descent” into the FCU (the autopilot control unit just under the glare shield). An A320-rated colleague says you can put, say “100ft” target altitude, and activate, and the aircraft will go into open descent with engines at flight idle at about 4,000 feet per minute right down to 100 ft altitude, i.e. the ground here. In other words, twist and pull one knob.
I would emphasise here that such autopilot systems are not unique to the Airbus A320 but are to be found on most commercial transport aircraft nowadays.
Now to the first major issue. Concerning stroke versus deliberation action, a colleague was present when someone 29 years old had a haemorrhagic stroke.
Inside 30 minutes he went from conversing like normal; to weirdly reticent and uncoordinated; to silently sitting on a bed, clutching an aspirin bottle like a crazy person, totally unresponsive to the world. And in that time he managed to open a laptop and hammer-out an email full of utter nonsense, all for reasons that are still totally lost on him.
During such an event, one may well continue “breathing normally”, as the French press conference is reported to had said the First Officer did.
So it seems to be possible that a confused FO in the course of experiencing a stroke dialed an open descent into the FCU, maybe imagining he has to land. I am a little surprised that medical experts have not yet pointed such phenomena clearly out. It does suggest that concluding murder-suicide is premature at this stage.
It has also been suggested that the FO secured the cockpit door against being opened from outside (that is, he activated the third function below). Evidence for this is that the emergency-entry thirty-second buzzer did not sound. Maybe. No one has yet said whether there is evidence that the Captain in fact tried to activate the emergency-entry function via PIN (the second function below). Apparently he knocked on the door and continued to knock; but nothing else has been said.
Second major issue: the cockpit door locking. The cockpit door on the A320 in normal flight is permanently locked. There are three technical functions. First function: on the central console between the pilots there is a toggle switch which opens the door when it is used: it must be held in “open” position and reverts to door-locked when released. I emphasise: a pilot must “hold the door switch open” for the door to open, and it locks again when heshe releases the switch. Second function: there is a keypad mounted in the cabin outside the cockpit by the cockpit door. Someone standing outside can “ring” (press a key) to activate a ringing tone in the cockpit. Or, of course, knock on the door. The Pilot Flying (or another person) can then use the first function to open the door, and the person outside can then enter. Suppose that does not happen for some reason. Then the person outside can enter a PIN code into the keypad (heshe must have knowledge of the PIN code). A warning sound activites in the cockpit for thirty seconds, at the end of which the door unlocks for 5 seconds, when the waiting person can enter, and then reverts to locked. This second function addresses the issue of the incapacitation of the pilot or occupation with other urgent tasks. The third function is a deactivation: by using a switch in the cockpit, the second function can be deactivated for a preselected period of time (the Operating Manual says between five and twenty minutes; colleagues understand that on the Germanwings aircraft it was five minutes). That means that for this period of time, even use of the PIN code outside does not unlock the cockpit door for entry. The cockpit door can still be unlocked during this time by using the first function, the “unlock” toggle switch. This third function addresses the possibility that a hostile person could physically threaten someone outside the cockpit with knowledge of the PIN code (say CAP or FO who went to the toilet in the cabin) in order to gain entry via the second function.
This is the operation of the door locking/unlocking functions on the Airbus A320. We have not checked and compared with other aircraft.
I am told there is a rule in the USA that there must be two crew members in the cockpit at all times. So if CAP or FO leaves, a cabin-crew member must enter and stay until the cockpit crew member returns. This is not necessarily so in European commercial flying. As far as I know it is consistent with Germanwings operating rules that the PNF can leave the cockpit briefly under certain conditions, leaving just the PF within. (I omit discussion here of why’s and wherefores.)
It seems almost certain that there will be considerable technical discussion of whether these cockpit-door-locking procedures and rules are appropriate or need to be modified. I observe that the BBC has listed three apparent-murder-suicide events in commercial flight the last few decades (I do not know of more), and this might be a fourth (I emphasise again the word “might”). And in at least one of those incidents, the cockpit remained accessible to those outside. In contrast, on one day alone in 2001, four cockpit crews were overwhelmed by attackers from the cabin, and since the door-locking rules have been in force, none subsequently have been. And before that day in 2001, there were many instances of hostile takeover of an aircraft (“hijacking”). So arguments for and against particular methods and procedures for locking cockpit doors in flight are not trivial.
Finally, there seems to be a mistake in one of my observations below. The flight path corresponds more nearly to a 6° descent angle. This is steep, but within the normal range. London City airport has an approach glide path of 6°, and A320-series aircraft fly out of there (although, I believe, not the A320 itself). (Calculation, for the nerds like me: 1 nautical mile = about 6,000 ft so 1 nm/hr = about 100 feet per minute (fpm). So 400 knots airspeed = about 40,000 fpm. Flying at 400 kts and descending at 4,000 fpm is a slope of 1 in 10, which corresponds roughly to one-tenth of a radian which is about 6°.)
07:27 CET on Thursday 26th March
John Downer suggested the possibility of
- an inadvertent behavioral event that did not obviously fit into my classification below. He quotes a colleague on the regular occurrence of the highly unusual: “as Scott Sagan put it: stuff that’s never happened before happens all the time“.
Inadvertent behaviour would likely involve one pilot leaving the cockpit, and the other suffering a medical event. I could then see two ways to achieve the regular flight path: engaging descent mode in the FCU at 4,000 fpm or 3° descent profile (note: Friday 27th March – I think this should be 6°!) or retarding the throttles in speed-hold mode.
Since the throttles are forward at high cruise, I think that slumping on them would cause them to advance, if anything, not to retard. John informs me that, during a stroke, people can become very confused. Thereby manipulating the FCU or retarding the throttles does not seem out of the question. Many thanks to John for pointing out this possibility which didn’t fit into my classification below!
Karl Swarz made us aware of the NYT report Germanwings pilot was locked out of cockpit before crash by Nicola Clark and Dan Bilefsky. Karl had sent the note before my conversation with John, but I hadn’t yet read it. It seems this is a scoop – there is also a similar report today in The Guardian but it cites the NYT.
There is some preliminary unconfirmed information from the CVR read out. One pilot did leave the cockpit and could not reenter during the event. There is, as currently analysed, no indication of a reaction from the pilot flying. We may presume that the analysis will become much more precise. It seems the commentators cited by the NYT are ruling out cabin depressurisation; that eliminates one of the (now) six possibilities. It seems to me likely that many of the others will be quickly ruled out.
19:04 CET on Wednesday 25th March.
Update: there is no more information on the behavior of the flight than I reported yesterday (below).
There is discussion of possibilities, and whether my classification is right. It is appropriate and necessary that there should be such discussion. Here, in the next paragraph, is some.
A colleague has suggested that the crew could have been overcome by carbon monoxide in the bleed-air from the engines (which is used to pressurise the aircraft). It has happened before that crew has been overcome by something. In each case, the flight has continued as configured until fuel is exhausted, and then come down. So if this happened here, why did the flight not continue at FL380 until the fuel was exhausted? Another colleague has suggested that the descent rate almost exactly corresponds to a descent profile of 3°, which is normal descent profile for (say) an ILS approach. OK, but why would a crew in cruise flight, continuing cruise enroute to Düsseldorf, change the autopilot setting to a descent profile?
Somebody said on Twitter this morning, in response to my interview with a radio station in Hessen, that enumerating possibilities is speculation and one should just let the investigators do their job (and presumably deliver results).
First, this misunderstands how things are investigated. Speculation is a major component of investigation – one supposes certain things, and tries to rule them out or keep them as active possibilities. And one carries on doing this until possibilities are reduced as far as possible, ideally down to one.
Second, each technology is constrained in behavior. Airplanes can’t suddenly turn left and crash into a lane separator. Cars can’t suddenly ascend at 4,000 feet per minute. Bicyles can’t stop responding to input and show you the blue screen of death. How each artefact can behave in given circumstances is constrained. And even further when there is a given partial behavioral profile. Why not attempt to write that down? If it’s wrong, someone will say so and it can be corrected.
Third, such a process obviously works most efficiently when experts with significant domain knowledge attempt to write it down and other such experts correct. And most inefficiently when people with little domain knowledge write down what they are dreaming, and attempt to argue with those who suggest their dreams are unrealistic. It’s a social process, which works better or worse, but I see no reason why it should generally be deemed inappropriate. Speculation is a necessary component of explanation.
18:42 CET on Tuesday 24th March.
Here is what I think I know at this point.
Germanwings Flight 4U 9525 has crashed against an almost vertical cliff in the Alps. The Flight was enroute from Barcelona to Düsseldorf and took the route which had been flown the day before. At about 0931Z (=10:31CET) he was at FL380 in level flight and started a descent at a rate of about 4,000 feet per minute, which continued more or less constant until about 7,000 ft altitude, when he levelled off. The descent lasted until 0941Z (=10:41CET).
He continued level for either 1 minute or 11 minutes. Contact was reported to have been lost at 0953Z. Such basic facts are often unclear in the first 24 hours, even though they appear to come from reliable sources.
I see five possible contributing events, not all mutually exclusive:
- Loss of cabin pressure. A crew should react by starting a descent at about this rate, but the descent should have stopped before 7,000 ft altitude;
- Fire. The crew would wish to descend and land as soon as possible. Emergency descents in excess of 4,000 feet per minute are possible, especially at higher altitudes, and a crew in a hurry to land, as in a case of fire on board, could have been expected to do so;
- Dual engine problems, maybe flameout. Descent at best-glide speed, though, I have been informed is somewhere between 2,000 and 3,000 feet per minute. One would not wish to come down faster, since the more time one has to troubleshoot, and then to try to restart, the better
- An air data problem affecting the handling of the aircraft. Recent air data problems with these aircraft, as well as with A330 and A340 aircraft that have almost-identical air data sensorics, during cruise and other phases of flight have occurred since 2008 and there have been a series of Airworthiness Directives from EASA and the FAA in this period, including recent Emergency Airworthiness Directives within the last few months. However, one would expect aircraft behavior associated with such a problem not to last nine minutes at constant, moderate rate of descent
- Hostile – and criminal – human action on board
I’ve already given a TV interview in which I only mentioned four of these five. Such is life. Are there more?
In a number of these cases, one would expect a crew to turn towards a nearby adequate airport for landing, such as Marseille. One would certainly not expect them to continue flying towards high mountains! In particular, towars the Alps at 7,000 ft. So the question is raised whether the crew was or became incapacitated during the event.
I’ll update when I know more.
Categories : Aircraft Accidents, Aviation