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Heat and Dust

Posted 25 November 2009 · Add Comment

The Middle Eastern nations have spent billions on building some the most modern and well-equipped airport terminals in the world. But the operators face some uniquely challenging conditions out on the airfield, writes BRENDAN GALLAGHER


During the summer the ground temperature can exceed 50 degrees C and there’s dust, sand and stones to contend with.

Those were the words of George Cooper, chief executive of Kuwait-based premium carrier Wataniya Airways, as he explained some of the major hurdles facing the region’s operators.

“These conditions are not normal elsewhere. Europe has pristine grassy runway verges – we have desert just about everywhere we fly,” added Cooper. “The dust and sand affect engines and other systems, and stones blown on the taxiways and runways are bad for aircraft tyres and wheels.”
Wataniya keeps its small but hard-working fleet of Airbus A320s rolling by the simple expedient of stocking more wheels and tyres than would be needed in a less demanding environment. The effects of fine wind-blown sand and dust are mitigated by more frequent changes of the filters for the engine oil and the air intakes of the air-conditioning system, just as they are in cars. But the region’s blistering summer heat, never encountered in more temperate climates, calls for more drastic measures.
“We had to specify brake cooling fans, which the standard A320 doesn’t have,” said Cooper. “Our turn-rounds average an hour in Kuwait and can be even less elsewhere on our route network. In very high ambient temperatures that’s not long enough for the brakes to naturally cool down below the 300 degrees C limit for taxiing and take-off.”
Pre-departure brake temperature is limited for at least two reasons. Braking performance falls off with brake temperature, with the result that beyond a certain figure it would be unequal to the most extreme test, a maximum-weight rejected take-off. Then there is the risk of fire. The Skydrol hydraulic fluid in the pipes inside the landing-gear bays auto-ignites at around 400 degrees C – the airworthiness authorities were keen not to have hot brakes anywhere near this potential fire hazard.
The fans have another, non-safety, function. The wear rate of the carbon brakes now common on airliners tends to peak at temperatures between 70 and 200 degrees C, depending on the manufacturer. Judicious use of the fans can help to maximise brake life by keeping brake temperatures in the desired range.
They can also contribute to operational efficiency and reduced fuel burn. Relatively cool brakes can begin to approach the limiting temperature as a result of a long taxi-out, particularly when there is a slower aircraft in front and the brakes have to be applied repeatedly. Airbus pilots can solve the problem with the fans, retract the gear and high-lift devices promptly and carry out an efficient climb. In other types, the gear must be left down for longer than optimum so that the brakes cool in the airflow.

Airbus and rival Boeing differ on the need to offer brake cooling fans as an option. The European manufacturer believes that an active onboard cooling capability makes it possible to optimise the size of the brakes and achieve an overall weight saving. But the US company says its thermal testing has proved there’s no need for built-in fan cooling of the carbon brakes on the Boeing 737NGs, and declares that it saves weight as a result.

The Boeing philosophy is evidently to leave the fans on the ground, as emerged earlier this year when it admitted that a modification of the 787’s brake temperature sensors would be required. The company expects the twinjet’s quick turn-round performance to be constrained until the change is made, and says that in the meantime ground-based fans could be used at the gate to help with brake cooling.  
Suppliers of this kind of equipment include France ’s Samifi Babcock. Specialising in mobile air-conditioning for air transport, the company exhibited a new mobile cooling unit at the Airport Build and Supply Exhibition in Dubai earlier this year.
Though Boeing has set its face against built-in brake fans, it has acknowledged that another extreme condition – blowing sand – can call for extra measures. “We are always ready to respond to customer suggestions,” said Seattle-based spokeswoman Beverly Holland. “In 2006 we sent a service bulletin to BBJ operators noting that environmental conditions such as blowing sand could sometimes cause air-conditioning systems to become less efficient. Our suggested remedy is a centrifugal cleaner to filter the air going to the air-conditioning packs from the auxiliary power unit or engine bleed systems.”
If brake fans and air-conditioning modifications can be treated as optional extras, that’s far from true of the upgrades offered by CFM International, supplier of engines to both Airbus and Boeing. “The fan and compressor sections are affected by ground debris,” said strategic communications director Jamie Jewell. “Sand can erode the compressor aerofoils. And airborne sand, dust and other small particles can affect the hot section.”
The Snecma/General Electric joint venture designs its engines to tolerate these conditions, and also offers modifications to operators who are particularly affected. “The fan inlet area and fan blades can be designed to reduce the intake of foreign objects into the core of the engine,” Jewell explained. “Coatings on the compressor blades can be effective against erosion. And in the hot section, cooling passages and circuits can be sized to reduce the potential for blockage.”
Some of these features are standard on all CFM powerplants, others are offered as spares: “We don’t produce engines with a different bill of materials for each customer. If the operator wants certain modifications, they are installed after delivery or during a scheduled shop visit. There is strong demand from affected airlines for these product improvements because they extend on-wing time and reduce operational costs.”
The package of modifications, and its cost, varies from region to region, depending on specific conditions. “Factors such as the composition and average size of sand grains and other particulates vary from region to region, producing different effects on the engine,” Jewell explained. “The primary regions of concern are the Middle East and North Africa , India and China . Other areas exhibit problems that are unique to themselves – extreme cold is a challenge, for instance. But in general, operators spending a lot of time in these three regions have the most to cope with.”


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