Keeping aircraft stored in the desert in full airworthy condition requires a time-consuming routine checks and maintenance

Another beautiful day in the valley expected today, with highs in the 60s, and lows tonight into the 40s. No rain forecast for the rest of the week, but relative humidity this evening could reach as high as 30%."

A typical January-day weather broadcast for the high-desert communities of the USA's south west. While most desert residents welcome some rain in the outlook, another arid day is good news for one section of the business community - the booming airliner storage industry.

Although temperatures in California's Mojave desert and neighbouring regions of Arizona and New Mexico vary greatly from night to day, and season to season, the one constant is low rainfall and relative humidity. Moisture levels are low throughout the region, with relative humidity usually staying below 40% most of the year. It rarely creeps above 50% during winter nights - or even rainstorms. On typical summer afternoons, the relative humidity drops to around 10%. This intense dryness makes it the best location in the world for short-, medium- and long-term jet storage.

Strict programme

A dry environment, however, is only the starting point for successful storage, as BAE Systems Victorville Flight Systems vice president and general manager Joe Vreeman points out: "People think they park there and that's it, but it isn't. A lot of damage can be done to an asset if it is not stored properly, and we follow a strict maintenance programme for those aircraft expected to be sold or put back into service."

Maintenance programmes are based on Air Transport Association-standard specified guidelines, and are outlined for in Chapter 10 of each stored type's maintenance manual. "Based on these same basic guidelines, each operator also develops their own maintenance and storage programmes which we follow," says Vreeman. After an aircraft lands for storage, an engine run is first performed to verify performance parameters at the end of the current phase of service. "Once it arrives we have seven days to de-activate it. That gives us time to seal inlets, pitot static ports and cover windows to prevent sun damage. We also have to carefully and clearly identify things we have put onto the aircraft with streamers to make sure it doesn't return to service with things added on," he adds.

"Of course not all are projected to return to service, and some will be parted out or disassembled," says Vreeman. If the aircraft enters storage with the eventual aim of either being sold or returned to service, it is kept in full airworthy condition by being subjected to a set of routine tasks at intervals of seven, 14, 21 and 28 days. The seven-day check includes rotating tyres to prevent flat spots and the hardening of bearings. Tyres are marked with chalk to indicate position since last turn, and are moved by either towing the aircraft or raising it on jacks. The exposed parts of the struts are also cleaned to remove dust and corrosion, and some systems are powered up either using a ground power unit or the aircraft's own auxiliary power unit.

Controls are exercised to lubricate seals at the ends of the hydraulic actuators to prevent them from drying up and shrinking. "They tend to wither away," says Vreeman. The 14- and 21- day events are essentially a repeat of the seven-day cycle. The 28-day interval involves turning on the aircraft systems, lights and air conditioning system. "The idea is to ventilate the aircraft to prevent the build-up of moulds and other contaminants," he adds.

Engines are run more periodically, usually at three-month intervals for aircraft in mid- to long-term storage. The skin undergoes corrosion inspections at 28-day intervals. "Although the low humidity is favourable for that, there could be leakage from galley or lavatory areas and corrosion would continue, so we have to keep a look out for that," says Vreeman. Hydraulic reservoirs and fuel tanks are not drained, and a bio-bore additive is added to the fuel to prevent the growth of microbes in the fuel system. "It's really an 'anti-bug' measure, particularly for those aircraft that have operated in very humid conditions."

Engines on aircraft which are stored for an extended time, typically one year or more, are drained of oil and other fluids as part of a "pickling" process. The liquids are replaced with oil and fuel containing chemicals which prevent freezing and corrosion, and protect seals and systems. Inlets and exhausts are also tightly covered. For airframes in longer term storage, the struts are covered with special aluminium foil to reduce exposure. The tyres on the longer term stored aircraft are still rotated, and the control surfaces exercised. "If it is in a mothballed state we will put external power on it," Vreeman says.

Interior attention

Cabin interiors also demand attention, and temperatures are closely watched to prevent heat damage to internal fittings and upholstery. If the outside air temperature exceeds 37ºC (98ºF), two or three cabin doors must be cracked opened by around 90mm (3.5in) for up to 2h per day to prevent heat build-up inside the aircraft.

To re-activate an aircraft, the hydraulic and pitot static pressures are checked, all moving surfaces are lubricated and exercised, and other pre-flight checks performed. "It's basically the same as a 'B' check, but if the aircraft has been on the ground longer than six months we'll typically put it on jacks and do a gear swing." When an aircraft with a less certain future has been stored and then reprieved, more extensive upgrade and maintenance work, such as replacement of the gear, can be performed.

Costs for storage are based on man hour labour rates and rent rates for the land. "It varies greatly depending on the maintenance programme, and how much of a state of readiness an owner wants it kept in," says Vreeman. Mid-term storage costs for a Boeing 747 would, for example, be "not quite double" that of a much smaller McDonnell Douglas DC-9. Short-term storage costs range from a "few thousand per month" to half that for longer periods.

Source: Flight International