When it’s time to fly again, make sure your aircraft doesn’t become part of this year’s aviation nightmare
By Robert Wilson
As if air transport didn’t have enough problems. With 70 per cent of the world’s airliner fleet mothballed for all or part of 2020, the latest syndrome to affect airline operations is aircraft waking up ‘grumpy’.
Distinctive failure modes are emerging in aircraft reanimated after being parked or stored for days, weeks or months.
Several examples are in the public domain:
- On 14 September a British Airways Boeing 777-200 flying from London Heathrow to New York JFK was climbing through flight level 250 out when the crew received indication of loss of oil pressure for the left engine and shut it down. The aircraft descended to flight level 100, dumped fuel over the sea and returned to Heathrow for a safe landing. The occurrence aircraft had been in long-term storage in Cardiff from June to 8 September 2020 and had been on its first revenue flight since coming out of storage.
- On 15 July an Alaska Airlines Boeing 737-800 was descending towards Austin, Texas after a flight from Seattle when the left engine failed. The crew took checklist action and landed safely. The aircraft had last flown on 9 July and had remained on the ground for six days before departing for the incident flight.
- On 16 June a Wizz Air UK Airbus A321-200 was accelerating at Doncaster Airport in northern England, when the crew rejected take-off at high speed due to unreliable airspeed indications. The aircraft slowed safely and returned to the apron. The UK Air Accidents Investigation Branch reported the flight was the first after long-term storage.
Other incidents are less mysterious, but still concerning:
- On July 29 silica gel desiccant packages and red tapes were found in the engine nacelles of an Aeroflot Superjet 100-95 after two weeks back in service. The aircraft had been laid up from early May until 15 July.
The US Federal Aviation Administration was sufficiently concerned by reports of single-engine shutdowns on classic and new generation Boeing 737s to issue Emergency Airworthiness Directive 2020-16-51 on 23 July. This directive required operators to check the engine bleed air fifth-stage check valve on each engine for proper and free operation.
‘This emergency AD was prompted by four recent reports of single-engine shutdowns due to engine bleed air fifth-stage check valves being stuck open,’ the directive says.
‘Corrosion of the engine bleed air fifth-stage check valve internal parts during airplane storage may cause the valve to stick in the open position. If this valve opens normally at take-off power, it may become stuck in the open position during flight and fail to close when power is reduced at top of descent, resulting in an unrecoverable compressor stall and the inability to restart the engine. Corrosion of these valves on both engines could result in a dual-engine power loss without the ability to restart. This condition, if not addressed, could result in compressor stalls and dual-engine power loss without the ability to restart, which could result in a forced off-airport landing.’
Airbus has recorded a spike in unreliable airspeed indications since the pandemic erupted in March. These included rejected take-offs and in-flight turnback incidents. Of the 15 occurrences of unreliable airspeed known to Airbus in this period, 11 were confirmed to have occurred during the first flight after a period where the aircraft had been parked or stored.
For the previous two years Airbus received an average of one reported event of probe obstruction on ground per month. However, all but one of the 15 reported events were caused by foreign objects obstructing the pitot air pressure line.
The Airbus publication Safety First says, ‘All of these events had positive outcomes thanks to the actions of the flight crews. It does, however, illustrate the challenges for returning aircraft to flying, after they are parked or stored and if the maintenance procedures are not followed in all cases.’
CASA’s Airworthiness Engineer Ben Cluff says modern transport aircraft have many components and systems that can be affected by insects, debris, or moisture. They include:
- the main pitot/static system used for determining airspeed and altitude
- secondary pitot probes used to control flight surfaces; these require airflow to operate;
blocked or damaged systems may not be detected until used
- engine external pressure-sensing probes that allow for engine operational calculations
- angle of attack sensors, which are required to be free moving within the airflow to allow aircraft systems to obtain precise angles of attack for safe flight computations
- total air temperature probes or outside air temperature probes require airflow through the sensor to derive an accurate temperature
- outflow valves and pressure relief valves used to control cabin pressurisation; these valves are predominately open on the ground
- avionics cooling systems have cooling duct or heat-exchanger outlets that may be blocked; without system cooling, heat build-up may lead to avionic failures
- electrically driven motors and actuators.
Cluff says leaking antenna seals can be a difficult to detect source of moisture in communication and navigation systems and may create intermittent time-consuming defects.
The ‘waking up grumpy’ problem is a known issue in air transport that is being managed. But it is also a potential safety issue for sport and general aviation aircraft. Despite the vast difference in complexity between these and commercial aircraft, some of the same precautions need to be taken to prevent engines, airframes, and avionics from dangerous deterioration.
Getting a good rest
In aircraft as with people, there are two ways to avoid waking up with a headache. They are comfortable storage and considerate awakening.
Storage: a dry season
The real estate maxim of location, location, location, applies here. Unused aircraft fare better in dry climates than humid maritime climates because dry air provides less opportunity for corrosion, mould growth and fuel contamination. However, over the long term, dry air is more likely to dehydrate rubber and plastic seals.
Aircraft should ideally be parked or stored on a flat surface with the nose pointing in the direction of the prevailing winds to limit the effect of wind and gusts on the aircraft. Wheels should be chocked, and the pilot’s operating handbook or aircraft maintenance manual should be your guide on whether parking brakes should be engaged or left off.
Pitot heads must be covered with tightly sealing conspicuously tagged covers. Poorly fitting covers could allow moisture or insects into the pitot static system while giving the impression that it has been protected.
Aircraft should also be electrically grounded to protect their avionics from static electricity build-up or (the admittedly unlikely possibility of) lightning strike.
CASA Airworthiness Bulletin 85-021 Issue 1 – 29 March 2017 has useful information on preservation of unused piston engines, that you can use in conjunction with the manufacturer’s recommendations, which should be your first reference.
The bulletin says aircraft piston engines last longest with regular use and maintenance. Corrosion and contamination can begin within a few days for aircraft stored in humid coastal locations, but engines stored in more favourable conditions can go several weeks between flights without any ill effects. The bulletin concurs with the manufacturer’s recommendation that engines which won’t be flown for 30 days or more, should have a preservation regime.
However, the bulletin says some preservation measures do more harm than good to aircraft piston engines.
‘Engine ground running is not a substitute for regular flying, in fact, the practice of ground running will tend to aggravate rather than minimise the corrosion condition,’ it says, arguing that short, low-power engine ground runs do not heat the engine enough to vaporise water in the oil and therefore promote, rather than inhibit, condensation. The practice of pulling engines through by hand merely wipes the protective film of oil from an engine’s cylinders and cams and followers, increasing the potential for corrosion and causing increased wear when the engine is started again.
The bulletin also goes into detail about moisture formation in unused engines and discusses the correct type and freshness of engine oil for preserving unused engines.
Awakening: rise and shine
The consequences of several months hibernation on the ground may include:
- low tyre pressures
- low-battery voltage
- fluid leaks and/or low levels
- insects/nests, especially in pitot/static ports
- rodent damage to wiring
- bird nests
- dust on airframe and windows
- expired emergency and first aid kit items.
Each of these should be checked and remedied, if necessary, before you fly the aircraft. It means returning to the sky must be more than a Sunday afternoon spur-of-the-moment decision.
Qantas Engineering’s head of line maintenance, John Walker, told the Australian Financial Review it took about 200 hours of labour to recommission a Boeing 737 after extended storage. The effort involved disinfecting the cabin, flushing the potable water system, checking and lubricating the landing gear, running up the engines and testing cabin environment and flight controls.
Even then results could be variable. Walker told 9News, ‘Sometimes they wake up cranky (with system fault codes needing attention) and sometimes they wake up in a good mood. It just depends on the aeroplane.’