10
$\begingroup$

I was watching an Air crash investigation video where the cause of the plane crash was determined to be due to blockage of fuel pipes by ice crystals (from the water in the jet fuel) which would have formed while the plane was cruising over Siberia where temperatures are as low as −74 °C. Now the engines were working fine until the fuel temp. was −30 °C. But as soon as it started getting warmer near London, Heathrow and the fuel temp hit −20 °C the ice crystals coalesced to form a mesh that completely blocked the fuel pipes and caused the engines to lose power and end up crashing.

My Question is: What caused the ice to not stick at a colder temp (−30 °C) when I believe the molecules would be vibrating slower than those at −20 °C? It seemed from the investigation's experiment that below −20 °C big ice crystals don't form at all, if they do they're tiny. So why does the increase in temperature help ice sticking?

$\endgroup$
3
$\begingroup$

I think I remember that episode. Basically, crystals of water in kerosene were not a problem during the whole flight (at minus thirty something) but crystals were too sticky when the temperature raised to -20°C and prevented the fuel to go to the engines just a few millimeters before entering the "warming cell" which was supposed to prevent any such blockage.

Crystals at low temperature (let's say -35°C) will be quite hard and quite big, even if some molecules of the fuel will be trapped in the crystals of ice (because there is much more fuel than water). If the temperature goes up to a certain level (let's say -20°C), crystals will break where there was molecules from the fuel. As a consequence, the big crystals will break into very small crystals. Ice crystals do not like their environment (i.e. the fuel) because they cannot find any stabilizing effect with it, but they love other ice crystals. Moreover, the overall surface greatly rises when the size of the crystals diminishes. As a consequence, they will try to stay together and form a "slush".

In that accident, it was not the only factor. The other two problems were that acceleration increased the flux of kerosene (so more small crystals of ice were available to create a slush), and there was a misconception in the heat exchanger which was aimed at avoiding water crystals.

$\endgroup$
  • $\begingroup$ so basically your explanation is that the increase in surface area allowed for better stickiness? Wouldn't the bigger ice crystals at (-35 C) be more efficient in blocking the fuel since it's bigger? But according to the video I think those crystals aren't big enough below -30C $\endgroup$ – Weezy Aug 14 '16 at 8:33
  • $\begingroup$ Big crystals could allow some kerosene to go through but small crystals (especially if they are sticky) would not. Imagine water going through sand. The quite large and solid particles leave some space between them and water can go through rather easily. With flour however, the tiny particles leave much less space (it is more compact) and water hardly runs though. $\endgroup$ – SteffX Aug 14 '16 at 18:14
  • $\begingroup$ But is surface area the only reason to account for the extra stickiness? $\endgroup$ – Weezy Aug 14 '16 at 18:16
  • $\begingroup$ No but it makes stickiness more problematic in that case. Maybe my answer was a bit misleading. $\endgroup$ – SteffX Aug 14 '16 at 18:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.