Sunday, March 09, 2014

As Airplane Malfunctions: What to Expect

[Pictures can be enlarged by clicking on them. References are supplied in the embedded URLs]

Now that MH370 is suspected to have crashed into the sea, one cannot but hope that there will be survivors. People are praying all over the world, and we all wish for miracles. The thought of an airplane malfunctions and crashes is not new; it happens time and again, often out of the blue (which led statisticians to conclude that driving a car puts you at a much higher risk than taking an airplane). Air France Flight 447 crashed into the Atlantic Ocean near Brazil in year 2009, in fact not long ago. The thought that this happened to a Malaysian-owned plane leaves me shuddering. This incident tells me that I am not an outsider but am within the reach of such risk. As this broods on my mind, I then ask, what should I expect when a plane malfunctions? What should I expect, e.g. how long will it take for it to crash into the land / ocean? What if there is a structural disintegration, will I have a chance to survive?

Pray for MH370 (source: MH370 News Update Facebook page; see bottom for link)
To put this into perspective, a flight experience can be separated into the following stages: 1) taxiing in airport waiting for takeoff call; 2) takeoff; 3) achieving airlift; 4) rapid altitude climb; 5) gradual altitude climb and 6) cruising at high altitude (usually at 35000 feet or 10668 meter). The reverse is true for landing. In contrast to the fear of flying, in fact most of the airplane accidents happened in airport. Air Traffic Control (ATC) is often overloaded with information and requires a huge amount of efforts to communicate radar information to one another, so if one landing plane is overlooked, chances are two planes might cross each other. Same is true for takeoffs. So the risk of dying as a result of airplane crash is the highest when you are on a plane taxiing, about to take off or about to land. So when flight attendants and pilots tell you to buckle up your seatbelt during taxiing, take off or landing, you better heed it and behave. At least when your plane is hit, you are not thrown out of your seat and snap your neck or break a rib if nothing serious happens. I always admire people who are always in such a hurry to unbuckle their seatbelts and stand up to retrieve their cabin luggage while the plane has just come to a taxi after landing.
[see the SCMP report on the near miss of a Cathay Pacific and Dragonair jet on 18 September 2011 as they descended into the Hong Kong airport]

Aircraft check (source: AirTeamImages)
Now if a plane is well-maintained, one can be assured that great safety is what you can expect for stages like (2), (3), (4), (5) and (6). Especially (6), since auto-pilot can take over manual maneuvering. Indeed there is no such thing as total manual maneuvering anymore, since there will always be a network of motion sensors calibrating the stability of the pitch, yaw and roll. Now if something goes wrong during any of the stages when the plane has achieved airlift, it will go very wrong. This is because stability control is crucial to safety. Redundancy is built into the system to buy time for the pilot to execute emergency landing or ditching. I need compiled data to draw a statistics on how often is this desired outcome achieved. For now, my interest is on the worst case scenarios. Let us go through them one by one.

Pitch, yaw, and roll of an aircraft relative to the direction of gravity and the plane body [source: Wikipedia]

1) Electrical Power Loss

Multiple redundancies are built to ensure the scenario of power loss is next to zero. A typical Boeing 747 has 4 engine driven generators, 2 APU generators and batteries for last back-up. 2 generators can run the airplane good for the whole flight. If all else fails the batteries will supply emergency power for about 30 minutes. This 30 minutes is designed to be used in extreme condition when a lightning strike knocks out all engine generators. So there you go. At least emergency ditching can be achieved in time.

2) Human Errors (most of the time, in addition / in response to machine errors)

Yes you read it correctly. When engineers design the aircraft, they make sure humans have the last say if there are errors in the sensors or machines. But if you have operated a machine before, you will know that it is the human "error" that often leads to the fatal blow. Machines and sensors are honest stuff. They show you everything. You have to judge in time what goes wrong and how to react to them. I will go back to the availability of reaction time later, but more often than not, the time available is short (typically less than a minute; extend that to 5 minutes for an early detection of fishy machine / sensor errors). An experienced, calm and decisive pilot is very, very crucial to the passengers' survival. The rest is luck. I will give the example of Air France Flight 447 crash in year 2009 again:

The vertical stabilizer recovered (source: Wikipedia)
The final report states that the temporary inconsistency between the speeds measured caused the autopilot to disconnect. The pilots, who failed to understand the situation completely in time, made inappropriate control inputs that destabilized the flight path. Aircraft went into stall and the pilots again failed to make inputs that would have made it possible to recover. 

All this happened in a really short time. Think about the emotionally charged situation, initial 'small' error will then lead to an emotionally-laden quick decision to correct the error, which you and I (if you are an alien / intelligent animal reading this, please disregard the 'you and I' reference) are so familiar of, often than not escalates the initial error. This is very sad indeed, but we are all humans. That's why Google is serious about self-driving cars. That way we can reduce accidents on the road if all cars self-drive.

3) Structural Disintegration

Although I speak of the assumption of a 'well-maintained' airplane, how well-maintained is well-maintained? That means how often you routinely check the plane and service it. Well, that's what most people would say. But little do they know that how you check it is actually much more important than how often you check it. You can check an airplane twenty times per day with eye inspection and might end up with a malfunctioning aircraft in flight. 

I do not wish to go in depth into NDT (Non-Destructive Testing) techniques, but I attended a state-of-the-art seminar last December and came across a shocking revelation that modern airplane NDT technique cannot reveal some of the fatigue cracks in the fuselage. I am not saying that the current techniques are not working. What I am saying is that, there are rooms for improvement. Fatigue cracks are common in structural elements that construct the building you live in, and also the airplane you are riding on. Fatigue, as you might have guessed, refers to forces coming on and off the element and over time, cracks will form. If fatigue crack happens in a bolt, it will be hard to detect! Don't worry, engineers turn to reinforced + redundant welding to make sure that does not happen. The interesting thing is, once cracks are formed, no matter how small they are, they will propagate once the element is loaded again. We do not wish to let them propagate during flight. But these tiny creatures are still a nuisance. They are very hard to detect. Next time don't complain so much when your flight is delayed again and again. Let those people make damn sure there are no fatigue cracks!

Fatigue cracks emanating from at least 42 of the 58 rivet holes in the fuselage skin, a test results on the investigation of Southwest Airlines Flight 812 accident on April 1, 2011 [source: National Transportation Safety Board]
However, if so unfortunate that it happens that fatigue should have occurred in one of the crucial structural elements of the plane, then you will risk structural disintegration of the plane. Upon this point, you will be left with almost no option but to prepare for the worst and hope for the best.

A) How Much Time to Crash?

Here I calculate possible altitudes at cruising / altitude climb at which structural disintegration begins and the plane takes a dive. Say the inner compartment stays intact, how much time do you have?

Now, nature dictates that the easiest way is the best way. So when a plane loses stability, it will fall along its long axis because nature dictates that it falls in the easiest manner, that is in the least air drag configuration. In this case I think I can safely assume there is next to zero air drag. High school physics teach us that the time taken is simply the square root of [ ( 2 * distance to fall ) / gravity constant] if you regard the initial falling velocity to be zero. Similarly, the terminal velocity is calculated with the square root of [ 2 * gravity constant * distance to fall]. Results are plotted as follows:

Time to crash in seconds vs. Altitude in feet
Terminal velocity at crash in km/hr vs. Altitude in feet
As the typical cruising height is at about 35000 feet, it takes about 46.6 seconds for the plane to crash into the sea or the ground. You have approximately a few seconds to panick, then to put on your life vest and brace for impact. That is to say, if you can survive a 1647 km/hour dip at the point of impact without snapping your neck and lucky enough to avoid flying objects coming your way. The story of the calculation lies within the short reaction time. If the pilots lose control of the aircraft (most likely when structural disintegration happens), you have less than a minute to prepare, often than not less than half a minute. It's near impossible to do that unless you are well informed of where the life vest is and how much time you have. And now you know you don't have much time to waste. Also it is very important to listen carefully when aircraft safety instructions are given and to grope for the location of the hidden life vest before you go about your own activity. The rest is luck.

B) What If Cabin is Exposed to Air?

The cabin is pressurized during flight in order to make sure the passengers do not suffer from hypoxia and go into hyperventilation. Hypoxia happens because the lower partial pressure of oxygen at high altitude reduces the alveolar oxygen tension in the lungs and subsequently in the brain. It is a matter of minutes or seconds until you lose consciousness due to the conditions of hypoxia. So let's assume you buckle up your seatbelts. As the cabin is exposed to air, it loses pressure and you are not sucked out because you have your seatbelts on. However, when this happens, you are exposed to lower pressure of the outside atmosphere, and here is a table of the time of useful consciousness you can maintain, depending on how high you are. See to it that at 35000 feet, you still have a chance of staying conscious to brace yourself against the impact for the 46.6 seconds fall. But if the plane starts falling at a higher altitude, you probably need oxygen to remain conscious. That's why there are oxygen masks popping down from above in these dire moments. It's for you to maintain conscious so you can help yourself to the life vests and brace for impact. Again, safety instructions on how to wear the mask is extremely important. And again, the rest is luck.
[Be reminded that the table is plotted for tests done for people at rest. Any exercise will reduce the time considerably]

Average time of useful consciousness at various altitudes [source: Wikipedia]


The MH370 accident is a huge shock that jostles me out from my ignorance of the risk I face if anything happens during a flight accident. As I put together the pieces, I hope I would be mentally prepared in such an event if it should happen (immediately touch wood). I share with you the knowledge so you listen carefully to the safety instructions given to you by the cabin crew and be reminded that, in face of such a misfortune, you do not have much time left. Time is of the essence to prepare yourself so that if luck happens to cross your path, you have a conscious brain to wear a vest that floats you on the waters and a whistle to blow to the direction of the search party.

For more information on the latest news update, follow:

[Disclaimer: I summarize the contents in a go, so there must be errors. For in depth studies, one is advised to refer to the scientific literature and reliable sources instead of jumping to conclusions by basing your judgment on this article. This article is written by the author to express his views instead of stating the facts.]

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