British Airways flight BA -038 and water/Ice beheaviour in Jet Fuel & cause of accident part 8

British Airways flight BA -038 and water/Ice beheaviour in Jet Fuel & cause of accident  part 8


BritishAirways flight BA-038 accident and Aircraft Jet A-1 fuel System - part 5 of earlier blog describe various components of Boeing 777 on accident flight. The aircraft fuel tanks, fuel boost pumps, water scavenge pumps, Fuel Oil Heat Exchanger (FOHE) etc are shown in detail.


This article gives Water behaviour in Aviation turbine fuel related to the accident flight. Results of laboratory testing  and experiments conducted as a part of Investigation  and cause of accident - are as follow:
Aviation Turbine Fuel-Water Properties
Aviation turbine fuel consist of a mixture of different hydrocarbons. Depending on the hydrocarbon mix, each property such as specific heat and freezing point can vary from batch to batch.. Aviation turbine fuel is manufactured at the refinery to a specification that defines 30 properties that the fuel must meet. Fuel is manufactured to have better properties than the specifications, so it is typical that the freezing point may be lower than the -47C  freezing point specification. Fuel on board BA-038 was tested after the accident and had a  freeze point of -57C.
Fuel freezing point definition is the temperature at which hyderocarbon wax crystals form in the fuel when it is cooled, and completely disappear when warmed. This is different than the fuel becoming cloudy upon cooling, which is typically due to the freezing of entrained water.
Water is normally always present in any fuel and aircraft fuel systems. When fuel temperatures approach zero deg C, "entrained" water will freeze and form ice crystals which will turn the fuel cloudy. Water can also be introduced from refuelled fuel, or through the fuel tank vent system when descending in humid air.
Water in Aviation turbine fuel can have 3 forms:
1. Dissolved state Water
Water molecules are already attached to the fuel hydrocarbons and can be compared to humidity. As fuel cools, the dissolved water is released and takes the form of either free wate or rentrained  depending on quantity. This form of water will not form ice  since it is chemically bonded to the hydrocarbon fuel as part of the molecule.
2. Suspended  Entrained Water
Water suspended in fuel as droplets, and may not be visible to the naked eye in concentrations below 30 ppm. At higher concentrations, entrained water will appear cloudy. As a general rule, under static conditions, entrained water will settle down, however is unlikely that on an in-service aircraft all of the entrained water will settle out.
suspended water in fuel will freeze and form snow-like ice crystals at fuel temperatures around  -1C to -3 C and will turn the fuel cloudy. This formed ice flakes travel through the aircraft fuel system and begin to stick to their surroundings  and each other, at temperatures between -5C and -20C which is considered the "sticky range" defined in Society of Automotive Engineers (SAE).
As part of Investigation, cold fuel laboratory testing conducted, using high concentrations of water and ice entrained in the fuel, demonstrated clearly that a slushy ice formed within the fuel feed system pipes of the fuel system.
3. Free water
Free Water is which is neither dissolved nor entrained. Free water takes the form of droplets or puddles at the lowest point in the fuel tank. The aircraft design has water scavenge system as well as the sump drain to remove this water. Periodical water sumping from fuel tank is done after landing.

Airplane Fuel System Ice Accumulation

Fuel samples taken from the accident airplane, did not show any evidence of excessive water or other contaminants. Fuel had a freeze point of -57C.   This testing further confirmed during investigation that extreme cold fuel temperatures were not a factor in the fuel system restriction.
Considering the temperatures during refueling, investigators estimated that about 3.0 liters of dissolved water (40 ppm) were uploaded onto the airplane, and a maximum of 2 liters (30 ppm) of entrained water. Additionally, it wasestimated that as much as 0.14 liters of water could have been drawn through the fuel tank vent system during its journey up to Heathrow. These quantities of water were considered normal for aviation turbine fuel.
Analysis of the flight data recorder and fuel system hardware, investigators concluded that ice accumulated in the aircraft fuel system (fuel lines, piping) over a long period of operation at low fuel flows and low fuel temperatures. Fuel flow was restricted at the fuel-oil heat exchanger, as ice  released from the insides of pipes and tubes, causing an “avalanche” effect, after a commanded fuel flow increase on approach.
The investigation conclusion based on the following evidence that the fuel flow reduction had been the result of an ice flushing event:
1.  Both engines experienced restrictions in the fuel delivery system as shown by the fuel metering valve.
2. The right and left high pressure pumps had signs of fresh cavitation, indicating that the restriction was upstream of these HP pumps.
3. The aircraft boost pumps had no low pressure indications.
4. The spar-fuel shutoff valves were fully opened throughout the flight and there was no indication of uncommanded movement.
5. The flight data recorder (FDR) showed that the fuel flow restrictions on both engines occurred seconds after a commanded fuel flow increase.
6. Laboratory testing on FOHE duplicated the scenario, showing that the increase in fuel flow flushes the accumulated ice and creates an “avalanche” effect in the fuel feed system.

Investigation findings confirmed that fuel flow was restricted at the fuel oilheating exchanger, and resulted from the release of accumulated ice in the airplane fuel feed system. After accident laboratory testing demonstrated that restrictions occurred at the inlet to the FOHE.

Water/Ice/fuel Laboratory Testing

As part of the investigation, a fuel icing test unit was constructed to simulate the BA-777 fuel feed system. Laboratory testing showed that ice forms in fuel feed lines with normal concentrations of dissolved and suspended water present. The testing was performed to establish, over long periods of time, how much ice accumulates in the fuel feed system . The test conditions were typical of long range flights operating in the critical temperature ranges. The fuel contained water concentrations at  appropriate levels intended to simulate the most critical condition for fuel system icing.
During this testing, observations were made of the formation and mobility of ice produced by a number of different methods. It was seen that ice would more readily stick to other ice crystals and surrounding surfaces when the fuel temperature range was between -5 and -20 degrees C. Below -20 deg C, ice crystals appeared to have a more crystalline appearance and did not easily adhere to their surroundings or other ice crystals.
Testing results revealed:
1. Ice formed in the fuel feed system when fuel temperatures are between -5C and -20C, and when warm fuel (5C) flows through cold pipes.
2. Ice does not form at fuel temperatures  is less then -35C.
3. The accumulated ice is generally soft and mobile.
4. Fuel flow increase can cause accumulated ice to release.
5. Ice 
accumulation and generation in fuel systems is often random.

Fuel Oil Heat Exchanger Testing

Lab tests conducted by the investigation also demonstrated that at fuel temperatures below -15C, using as little as 50 ml of water at high concentrations, an “avalanche” type event can result in a fuel flow restriction at the Fuel Oil Heat Exchanger (FOHE) inlet.
Based on above the investigation reported cause of accident as follow:

The probable cause of the accident was a fuel flow obstruction, resulting in a considerable thrust loss during final approach to Heathrow. The fuel flow restriction was concluded to have been the result of an accumulation of ice in the fuel piping which released in a short time period, which collected on, and restricted fuel flow through the Fuel-Oil Heat Exchanger. The associated thrust loss resulted in a reduction in aircraft performance, and deterioration in the flight path such that landing on the runway was not possible.

-Report on the accident to Boeing 777-236ER, G-YMMM, at London Heathrow Airport on 17 January 2008

keywords-
Jet A-1, Aviation Turbine Fuel,  Freezing Point,  dissolved water, free water, FOHE, fuel system, fuel temperature,  aircraft accident, British Airways, Heathrow, SAE, FAA

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