(PART ONE of TWO)
One needs to read the whole report, however some highlights that I THINK relevant ;
AA:- From “Synopsis” ;
“2) Ice had formed within the fuel system, from water thatoccurred naturallyin the fuel, whilst the aircraft operated with low fuel flows over a long period ……..”
What does “naturally” mean ?
BB:- From “Synopsis” ;
“3)The FOHE, although compliant with the applicable certification requirements, was shown to be susceptible to restriction when presented with soft ice in a high concentration, with a fuel temperature that is below ‑10ｰC and a fuel flow above flight idle.”
Why just the “FOHE” here when they talk of the “fuel system” elsewhere ?
CC:- From “Synopsis” ;
“4) Certification requirements, with which the aircraft and engine fuel systems had to comply,did not take account of this phenomenon as the risk was unrecognised at that time.”
And how have thing changed now for ALL aircraft and engine combinations ?
DD:- From 1.11.2 ;
“….. As the DFDR record commenced, an active status message was recorded from the FQIS water detector located in the centre fuel tank (Figure 15). The status message remained active for five consecutive samples of that parameter: a total of five minutes and twenty seconds. After the pushback, the park brake was applied and the aircraft remained stationary for about three and a half minutes before taxiing. As the aircraft taxied, the levels of aircraft vibration increased. The water detector message remained active for a further 100 to 160 seconds before extinguishing; the exact time could not be confirmed due to the long period between successive samples of the parameter. There were no further indications from the centre tank water detector. There were no indications from the left and right fuel tank water detectors during the flight. ……..”
EE:- From 1.11.2;
“……Approach
As the flaps reached the 30° position the airspeed had reduced to the target approach speed of 135 kt and the autothrottle commanded additional thrust to stabilise the airspeed (Figure 20, Point A). In response to variations in the wind velocity and associated airspeed changes, there followed a series of four, almost cyclic, thrust commands by the autothrottle (Figure 20, Point B). It was during the fourth acceleration, and as additional thrust was being commanded, that the right engine, followed some seven seconds later by the left engine, experienced a reduction in fuel flow (Figure 20, Point C). The right engine fuel flow reduction occurred at a height of about 720 ft and the left engine at about 620 ft. Just prior to the reduction in right engine fuel flow, about 2.5 nm from the runway, the flight crew were visual with the runway and the co-pilot became pilot flying (Figure 21).
Of the four thrust commands it was the second that resulted in the highest delivery of fuel flow, reaching a peak of 12,288 pph for the left engine and 12,032 pph for the right (Figure 20, Point D). These peaks occurred about 26 seconds prior to the reduction in fuel flow to the right engine. Peak fuel flows during the first and third thrust commands were lower, at about 9,500 pph and 9,000 pph respectively.
During the fourth thrust increase, the right engine fuel flow had increased to 8,300 pph before gradually reducing. The recorded EPR then started to diverge from the commanded EPR and the right engine FMV opened fully (Figure 20, Point E). Some seven seconds later, the left engine fuel flow, which had increased to 11,056 pph, also started to reduce and the left engine FMV also moved to its fully open position (Figure 20, Point F). The QAR record ended shortly after. ……..”
FF:- From 1.12.4.2 ;
“……..Water Scavenge systems
The nozzles from all the water scavenge jet pumps were removed and examined on 2 February 2008 and the following discrepancies were noted.
When the nozzle from the right main scavenge jet pump was removed from its housing half a teaspoon of a ‘jelly‑like’ substance, later identified in a laboratory as “water”, was found in the housing. It is not known if this water was originally lodged in the nozzle and was pulled through the flap valve into the housing, or whether the water had been introduced into the housing during a previous maintenance activity. The water was tested for microbiological contamination and the quantity of contamination was assessed as negligible. ……..”
GG:- From 1.16.2.1 ;
“1.16.2.1 Fuel samples
Following the accident, 66 fuel samples were taken from the aircraft, and engines, and a number of these samples were tested by QinetiQ and another independent laboratory. ………………………………………………..”
…………………An explanation of the testing conducted on these samples and the results are detailed in Appendix C.
The fuel samples from G‑YMMM complied fully with the specifications for Jet A-1. The sampled fuel had a fuel freeze temperature of -57°C and water content of between 35 and 40 parts per million (ppm). …………”
HH:- From 1.16.2.2 ;
“…………….. It was reported in the AAIB Interim Report, dated 4 September 2008, that 71,401 kg of No 3 Jet Fuel (People’s Republic of China) had been loaded onto G‑YMMM at Beijing prior to the start of the accident flight. Since receiving this initial information the AAIB was provided with further documentation indicating that the fuel was Jet A-1, which had originated in South Korea and was shipped to Tianjin, China, before being transferred to the airport bulk fuel storage facility at Beijing.
The investigation was provided with a number of documents including the refinery test certificates, airport storage tank records, hydrant records and refuelling vehicle records. The Korean test certificate indicated that the fuel was compliant with Check List Issue 22, which ensures that the fuel meets the requirements of Defence Standard 91-91 and ASTM D1655. Quality assurance checks undertaken at various points on its journey to the aircraft showed no evidence of significant contamination of the fuel. Moreover, the properties of the fuel recorded in the refuelling receipt and quality assurance certificates were consistent with the test results for the fuel samples taken from G‑YMMM after the accident. ………..”
N.B. I would note that both here, and earlier in the Synopsis, opportunities for mentioning water levels and/or explaing likely water levels in the actual flight fuel are not taken.
II:- From 1.16.5.4. ;
A large section which again generally carefully avoids comments re. actual/perceived flight fuel water content, except for ;
“….. The repeatability of the test was such that with a water concentration of 100 ppm there was a 95% chance that the test would show a result between 76 and 124 ppm. ……”
“………. A target water concentration of 90 ppm (as defined in ARP 1401) was selected for all the tests. ……”
“………… using fuel conditioned with approximately 90 ppm of water and maintained at a temperature of -20°C, which was near the fuel temperature at which the rollbacks occurred on G‑YMMM and N862DA (refer to 1.18.2.1). ……..”
(But not the water level of the flight fuel)
“………… The only occasion when it appeared that there might be a sufficient quantity of ice to block a pipe occurred when approx 6 litres of water had been injected into the boost pump inlet over a period of 7 hours. During this period the flow rate was maintained at 6,000 pph and the water content varied between 100 and 150 ppm. ………”
“…….. -As little as 25 ml of water, when introduced into the fuel at an extremely high concentration17, can form sufficient ice to restrict the fuel flow through the FOHE.
17 The water concentrations in the fuel, when the water was introduced in the manner used in these tests, were of the order of 100 times the concentration levels specified in the certification requirements. …………”
JJ:- From 1.16.5.4
“………….Refuelling at Beijing
A test was run to simulate refuelling the aircraft at Beijing when fuel at a temperature of 5°C was added to fuel tanks containing fuel at a temperature of -20°C.
During the test a fresh batch of fuel at 10°C was added to fuel at a temperature of -22°C in the environmental tank at a ratio of 1/1. The boost pump was run for 25 minutes to provide pump cooling and motive flow for the water scavenge pump. Whilst a few ice crystals were observed floating in the fuel, there was no build up of ice either in the tank or on the boost pump inlet screen or inlet pipe. …….”
Note :- no mention of water levels
KK:- 1.16.5.4 ;
“…….. Water concentration
During the environmental tests the amount of water sprayed into the fuel was closely monitored to try and maintain the concentration at 90 ppm. Frequent fuel samples were taken throughout the tests and the water concentration was established by running at least two Karl Fischer tests on each sample. The results indicated that the water concentration in the fuel flowing through the test section of the rig varied between 45 to 150 ppm. The discrepancy between the metered and measured water content might be explained by ice collecting, and being released from the supply tank and the pipes being tested. However, it was also observed, from the results of several Karl Fischer tests carried out on the same sample of fuel, that the measured water concentration varied by up to 60 ppm. ……….”
(But as mentioned in “II” above the “repeatability” of these tests is questionable.)
LL:- 1.16.6 ;
“…… 1.16.6 Ice formation in B777 fuel tanks
Following the defueling and draining of G‑YMMM’s main fuel tanks, some fluid remained trapped between the stringers adjacent to Rib 8. Approximately 0.6 litre of fluid was removed, by syringe, from each main fuel tank and stored in a clean glass jar. The fluid settled into two distinct layers and was analysed by QinetiQ who confirmed that the 0.6 litre sample from the left wing included 0.2 to 0.25 litre of water and the sample from the right wing 0.1 litre of water.
To establish if the quantities of free water found in the dead space in the fuel tanks on G‑YMMM were normal, two of the operator’s other Boeing 777s were inspected after arriving from Beijing on 21 February 2008 and 17 March 2008. Access was gained to the main fuel tanks within 3 hours of the aircraft landing and while the fuel temperature was still below 0°C. On both occasions small amounts of ice were found adjacent to Rib 8, around the edge of the stringers in front of the forward boost pump inlet. The largest piece of ice, on both aircraft, had built up around the inboard of the tank hatch and measured approximately 14 cm x 11 cm x 3.5 cm. This ice was firmly attached to the bottom of the fuel tank. There was no evidence of ice or slush in the rear part of the tank; however there were small pockets of water in a number of locations along Rib 8. It is estimated that the total amount of ice and water was about 0.5 litres. From the distribution of the ice and water, it would appear that water collects and ice forms mainly forward of the front boost pump inlet where there are no water drain holes in the stringers. ……………….
MM:- From 1.18.1.2 to 1.18.1.4 ;
1.18.1.2 Water in aviation turbine fuel
Water is always present, to some extent, in aircraft fuel systems and may be introduced during refuelling or by condensation from moist air which has entered the fuel tanks through the tank vent system. The latter effect is greatest when a cold soaked aircraft descends into a warm moist air mass. The water in the fuel can take one of three forms: dissolved, entrained (suspended) or free water.
Dissolved water: Dissolved water occurs when a molecule of water attaches itself to a hydrocarbon molecule; the amount of water dissolved in fuel is a function of humidity, temperature and the chemical constitution of the fuel. As a general guide the dissolved water content of aviation turbine fuel in parts per million (ppm) is approximately numerically equal to the temperature of the fuel in degrees Fahrenheit. When warm fuel is cooled the dissolved water is released and takes the form of either entrained or free water.
Entrained (suspended) water: Entrained water is water that is suspended in the fuel as tiny droplets and may not be visible to the naked eye in concentrations below 30 ppm. At higher concentrations entrained water will give the fuel a cloudy or hazy appearance, depending upon the size and number of water droplets. Entrained water can be formed by the release of dissolved water as the fuel cools, by violently agitating water and fuel together, or if there is a surfactant in the fuel.
A surfactant acts to stabilise small water droplets so that they do not form large water droplets that would settle out on the bottom of the tank. The maximum amount of surfactant allowed in aviation turbine fuel is not directly specified in the fuel specification but is controlled by water separation testing as part of fuel delivery requirements.
Agitation can occur during refuelling, mixing of the water scavenge outlet with the bulk fuel, or as the fuel and water pass through the aircraft fuel pumps. Entrained water will settle out of the fuel, but the rate is dependent on the droplet size, the density of the fuel and theamount of fuel agitation. As a general rule, under static conditions, entrained water is considered to settle at a rate of about one foot per hour; however it is unlikely that on an in-service aircraft all the entrained water would have the opportunity to settle out of the fuel.
Free water: Free water is the water which is neither dissolved nor entrained and, as it has a higher density than the fuel, it takes the form of droplets, or puddles of water lying on the bottom of the fuel tanks. Free water can also be found in the fuel filters and stagnation points within the fuel delivery system.
1.18.1.3 Estimated water content of fuel on G‑YMMM
Based on the temperature of the fuel, it was estimated that the fuel loaded at Beijing would have contained up to 3 litre (40 ppm) of dissolved water and a maximum of 2 litre (30 ppm) of undissolved water (entrained or free). In addition, it was estimated that a maximum of 0.14 litre of water could have been drawn in through the fuel tank vent system during the flight to Heathrow. This water would have been evenly spread throughout the fuel and would have been in addition to any water remaining in the fuel system from previous flights. These quantities of water are considered normal for aviation turbine fuel.
1.18.1.4 Formation of ice in fuel
As water cools it freezes and forms ice as follows:
Dissolved water: Any water that is still dissolved in the fuel at low temperatures will not form ice because the water molecules are still chemically bonded to the fuel. Dust particles in the fuel could provide a nucleation point for the formation of water droplets that could then form ice. However, at low fuel temperatures the concentration of dissolved water is very low and therefore the amount of ice formed by this mechanism would be small.
Free water: Free water forms ice as it is cooled below its freezing point and within the aircraft fuel tanks the cooling mechanism is the effect of the TAT on the lower wing skin; it is the water closest to the wing skin which freezes first. From the examination of two other B777 aircraft, by the AAIB, it appeared that, in the main fuel tanks, ice forms around the rivets, access panels and structure adjacent to Rib 8 and it was very difficult to release some of this ice from the bottom of the tank. For the ice to release it is necessary to increasethe temperature of either the fuel or the lower wing skin above the melting point of the ice.
At the point, in the accident flight, when the engines did not respond to the demand for an increase in power, the fuel temperature was -22°C and the TAT was 12°C. Photographs of G‑YMMM taken as it crossed the airfield perimeter show the inboard sections of the lower wings skins, which form the main fuel tanks, covered in frost which indicates that the wing skin was very cold; therefore, there was no release mechanism for any ice that may have formed on the bottom of the fuel tank.
Entrained (suspended) water: Entrained water in fuel will freeze and form ice crystals, which turn the fuel cloudy. Because the density of the ice crystals is approximately the same as the fuel, the crystals will generally stay in suspension and drift within the fuel until they make contact with a cold surface. Due to impurities in the water the ice crystals will not start to form in the fuel until the temperature has reduced to around -1°C to -3°C. As the temperature is further reduced it reaches the ‘Critical Icing Temperature’ which is considered to occur between -9°C and -11°C. The ‘Critical Icing Temperature’, is the temperature at which the ice crystals start to stick to their surroundings. As the temperature is further reduced to -18°C, the ice crystals start to adhere to each other so that they become larger, with the risk of blocking small orifices.
The temperature range over which ice crystals in fuel adhere to surfaces, and each other, is sometimes called the ‘sticky range’. From observations made during the sub-scale testing, the investigation defined the ‘sticky range’ as being between -5°C and ‑20°C. …….”
(TO BE CONTINUED >>>>>>)