Aircraft News Weblog

Tag: icing

  • Continental Airlines Dash 8 Aircraft

    dash-8The tragic accident of the Continental Airlines Dash 8 has generated a lot of speculation about the role of ice in the loss of control and subsequent crash of the aircraft. I am not going to suggest any cause of the accident as there are many knowledgeable people involved in the investigation that will come up with the answers.

    I would like to explain to those who are interested, why ice makes such a difference in the handling of the aircraft.  To understand the problem you have to have an idea of the theory of aircraft flight.

     The wings have a shape which produces lift as they travel through the air. However, they also produce drag and they have a weight that is a gravitational force.  The weight of the wing acts through the centre of gravity; this is the point that the wing would balance on if it were removed from the plane. The centre of lift is where all the lift forces could be measured and  opposes and is normally behind the center of gravity. Because it is behind the centre of gravity there is a twisting force to lower the front of the wing. If this were not opposed then the aircraft would not be able to fly.

    The tail is the place where the opposition is generated. The tail surfaces are designed to generate a downward force to oppose the twisting force of the wing and keep the aircraft level.  So, everything is in balance.  There are times when more lift is required, for the aircraft to take off or fly slower so it can land.  To achieve this flaps on the back of the wing can be lowered to increase the size of the wing and produce more lift.

    When the flaps are lowered the centre of lift moves rearwards and the twisting force of the wing increases, this is normally opposed by moving the trim on the tail to increase the opposition force and raise the nose. There is a range of trim available which is sufficient for normal operation.

    This is the way the aircraft is designed to operate. When ice collects on the wings it changes the shape of the wing so that some of the lift is lost, it also increases the weight of the wing which can move the center of gravity forward.  The build up of ice on the tail will add even more weight to the aircraft and reduce its ability to oppose the twisting force on the wing. The result is the same as lowering the flaps without the added lift that would be generated. The nose would be lowered and  the way to deal with this would be to trim the tail to increase the downward force.

    This is exactly what the autopilot does. As the ice builds up on the wings and tail the autopilot trims the tail to increase the downward force. This can be a very slow and insidious process as the autopilot will use more trim to keep the aircraft level. If the crew are not aware of the situation they may find that the trim limit has been reached before they  know they have a problem.  If they then change the status quo by lowering the landing gear or the flaps the auto pilot will be unable to trim the aircraft and will disengage leaving the crew with a very dangerous nose down attitude for which instant recognition of the problem and the correct response will be needed to avoid a loss of control.

    This situation is normally also at low altitude and airspeed and is very hazardous. Most operators regard icing very seriously and when in icing normally instruct the crew to fly the plane manually to make them aware at an early stage of the amount of trim they are using to control the attitude. They can then decide if they need to climb out of the icing conditions or take other action to avoid increasing ice build up. There are different systems on different aircraft which either prevent ice forming (anti-icing) or remove it as it forms (de-icing). De-icing that uses air to inflate boots to distort the leading edge of the wing and break up the ice which has formed was the system on the Dash 8 in Buffalo. This system requires the ice be allowed to build up to a certain thickness before the boots are inflated as thin ice will not break off. An automatic timer will inflate the boots periodically and there is a manual system if the automatic periods are too long.

    The tragic loss of the aircraft in Buffalo has brought the problem of icing into the spotlight but aircrews and airplanes have been dealing with it ever since flying began and there are many safeguards built into the system. I hope this has shed some insight on the hazards of ice on aircraft and the means to combat it.

  • Boeing 777 Trent engine problems with ice.

    The US Federal Aviation Authority have now issued an Airworthiness Directive (AD) concerning the problems with ice in the fuel system of Boeing 777-200 and 300 airplanes fitted with Rolls Royce Trent 800 engines. The AD requires the crew to monitor the fuel temperature within 3 hours of top of descent and if it is below a certain temperature they must increase the engine power to prevent the formation of ice in the main tank fuel supply system. They can do this by stepping up to a higher altitude or increase the airspeed. There is no requirement to carry out this procedure if the aircraft is not fitted with Trent 800 engines.

    There is also a procedure for the circulation of the fuel on the ground by a certified mechanic to prevent the formation of ice.

    This AD is a result of the problems encountered in January 2008 by a British Airways Boeing 777 which failed to respond to a command for increased thrust while the aircraft was on approach to London Heathrow airport and landed short of the runway. The interim report suggested ice formed in the main fuel tank supply system during low fuel flow conditions in cruise flight. The ice then dislodged and caused a restriction in the fuel supply because of high flow conditions due to the lower altitude during approach.

    No doubt there will be more measures to prevent the formation of ice from the engine and aircraft manufacturers but in the mean time the FAA are counting on this procedure to prevent the recurrence of the fuel restriction and loss of engine power.

  • British Airways Boeing 777 Crash at Heathrow – Report

    Ice has always been the enemy of aircraft, it builds up on the wings and reduces lift and it lies on the runway and reduces braking and directional control when landing, but now there is a new hazard. After nine months of investigation the Air Accidents Investigation Branch (AAIB) have decided the cause of the British Airways Boeing 777 landing short at Heathrow and crashing was due to ice in the fuel. All the passengers and crew escaped the aircraft with only minor injuries apart from one man who broke his leg.

    It has been known for years that there is water in aviation fuel and when the temperature drops, due to the aircraft operating at altitude, ice crystals form in the fuel. There have been chemical and mechanical solutions to this problem and it is a largely unknown problem in large commercial aircraft.

    The fuel passing through a Trent 800 engine like the one that was powering the BA aircraft goes to an oil/fuel heater in the engine fuel system. This takes hot oil from the engine to a matrix of small pipes and passes fuel around the pipes on its way to the fuel control unit and the burners. This is normally sufficient to melt any ice that may be in the fuel so it will not interfere with the operation of the engine. The theory of the investigators is that ice formed before the engine oil/fuel heater and reduced the amount of fuel available for the engines to use.

    The High Pressure fuel pumps that are downstream of the oil/fuel heater showed evidence of cavitation. This is a condition that happens when the impellor of the pump has an insufficient fuel supply to boost to a higher pressure and mechanical damage is caused to the pumps. Clearly there was low fuel supply to the engines but where was the restriction? Because of filters, low-pressure pumps and valves that are all in the fuel supply system it has not yet been determined as to where the problem lies.

    Why it occurred is also a mystery as there has never been a case of fuel starvation due to icing in the fuel system of a commercial airliner. The Flight Data Recorder has shown that within 6 seconds both engines reduced power to a value below that needed to maintain flight and the automatic systems and the pilots took the required actions to supply more fuel but to no avail as it could not get to the High Pressure Pump.

    The low fuel flow at cruise power settings on an especially cold, long flight may have caused a build up of ice in an area in the fuel system which, with the high fuel demand in the landing phase of flight, detached from its location and caused the restriction. This theory has not yet been proven despite extensive and imaginative testing. The reason it is being put forward is every other scenario has been eliminated. As this is a preliminary report we will have to wait some time, maybe years, before the final report is issued and the cause is known. In the mean time the FAA and the manufacturers of the engine and aircraft are going to look at ways to mitigate the effects of long, cold flights on the fuel system.