Why Must The Critical Variable Be Discovered By Accident?

When TWA flight 800 blew up in 1996 from a fuel tank explosion, the word around the aviation industry is that not much was known about basic properties of the fuel loaded aboard the airplane. Needless to say, the accident spawned a whole lot of research into fuel properties and flammability limits, not to mention wholesale fuel system design reviews that led to costly efforts to mitigate ignition sources and explosive vapors.

The Federal Aviation Administration (FAA), and the airline industry, has a habit of closing the barn doors after the horses have stampeded out, causing catastrophic loss of life.

Now the UK’s Air Accidents Investigation Branch (AAIB) is investigating the January 2008 crash of a British Airways B777 at London’s Heathrow Airport. The airplane experienced a loss of thrust at the same time increased engine power was needed to compensate for the deployment of flaps and extension of the landing gear. With the increased drag, and without the added thrust to compensate, the airplane smacked into the ground short of the runway. Fortunately, everyone aboard escaped. But the airplane was deemed damaged beyond economical repair. (See Aviation Safety & Security Digest, ‘Ice Blocked The Heat Exchanger, Leading To Crash,’ home page.)

The AAIB issued an interim report on 4 September 2008 into the crash, indicating that ice in the fuel is the leading suspect in the engines’ failure to produce more power at the critical moment it was demanded.

The AAIB said that it compared the fuel loaded aboard the accident airplane (registration G-YMMM) to other samples of the same Jet A-1 fuel:

“The fuel sampled from G-YMMM was compared with 1,245 batches of Jet A-1 tested in the UK during 2007. With regard to the distillation range, which is the boiling range of the fuel, the fuel from G-YMMM was approximately in the middle of the sampled range. The freezing point of the fuel sampled from G-YMMM was -57ºC (-71ºF), which was slightly below the average freezing point but within the normal range for Jet A-1.”

So far so good; the samples complied fully with the specifications for Jet A-1. But elsewhere in the interim report, we read:

“As the fuel temperature is further reduced, it reaches the Critical Icing Temperature, which is the temperature at which the ice crystals will start to stick to their surroundings. When the fuel temperature reduces to approximately -18ºC (0ºF), the ice crystals adhere to each other and become larger. Below this temperature little is known about the properties of ice crystals in fuel and further research may be required to enable the aviation industry to more fully understand this behavior.”

Unbelievable. More research is needed on a basic fuel property. Let’s recall that on its flight from Beijing to London the accident airplane flew through ambient temperatures as low as -76ºC (-105ºF).

It is becoming obvious that water, even at the parts per million level and well within the fuel’s specification, can be a problem. By the way, polar flights like this one were authorized a few years ago, apparently without consideration of freezing temperatures on fuel.

With TWA flight 800, it was fuel tanks blowing up. Fuel-tanks freezing up seems to be a risk of equal magnitude at the opposite end of the spectrum.

The question is, why does it take an accident to uncover the critical variables?