Unreliable Airspeed and the Hidden Risks of Aircraft Storage

The dramatic effect that Covid-19 has had on the aviation industry has grounded an unprecedented number of aircraft. They have been placed into storage whilst the world waits to recover. The pandemic emerged without warning, and some operators were likely not prepared for what was coming.

Now travel bans are lifting, airports are reopening, and airlines are scrambling to return aircraft to the skies.

EASA recently released a disturbing Safety Information Bulletin. There has been an alarming trend in the number of aircraft experiencing unreliable speed and altitude indications during first flights after storage, caused by contaminated air data systems.

The result has been multiple rejected take offs and airborne returns. Most of the events have been caused by nesting insects in the pitot static system – even after covers were installed.

Modern flight instruments provide large amounts of information to crew with great precision, while automation makes flying transport category aircraft almost routine. Flight envelope protections and aural/tactile warnings keep us safe even in most abnormal scenarios.

At the heart of all of this is the air data computer (ADC) – a small piece of hardware that needs accurate information from outside of the aircraft to work correctly. They are the “Achille’s heel” of modern electronic flight information systems. In a nutshell, these small computers obtain and process information from the aircraft’s pitot static system, and supply critical systems with information such as airspeed, altitude and temperature.

Like all computers, they don’t think for themselves. They are only as accurate as the information they receive. So, when the pitot static system is contaminated, they can only respond to what they sense. They can’t look out the window.

History has shown that unreliable airspeed events are dangerous:

February 6, 1996. Birgenair Flight 301, a Boeing 757, departed Puerto Plata in the Dominican Republic, on a routine flight. During the climb out, the Captain’s airspeed indicator began to increase dramatically. The autopilot reacted as designed, and increased pitch to reduce airspeed, while the auto-throttles reduced power.

In the meantime, the co-pilot’s ASI indicated a dangerously slow airspeed which was decreasing. Almost simultaneously, an overspeed warning was generated. The autopilot reached the limits of its programming and disengaged. The stick-shaker activated, warning the confused crew that the aircraft was flying critically close to a stall.

The Captain responded by applying full thrust. The excessively high angle of attack resulted in insufficient airflow to match demand and the left-hand engine flamed out. The right-hand engine developed full power and the aircraft entered a spin. Moments later the aircraft became inverted, before impacting the Atlantic Ocean. The three pilots had 43,000 hours of experience between them.

A Mud Dauber Wasp entering the pitot tube of a 737.

The cause of the accident was a blockage of a single pitot tube. The likely culprit was the black and yellow mud dauber – a small wasp known to nest in artificial cylindrical structures. The aircraft hadn’t flown in 20 days.

The threat of similar events is greatly increased by improper storage techniques and rushing to return to service.

Getting aircraft flying again is a complex process and presents major risks. It is up to operators to ensure adequate procedures are in place to accomplish it safely. They must anticipate the difficulties and rapid adaptation to internal procedures that this entails.

Don’t know where to start? We don’t blame you. Thankfully, EASA has published guidance which can help mitigate some of these risks. Here is a brief rundown of their recommendations:

Assemble your A-team. Everyone needs to be onboard. Flight operations, CAMOs, maintenance organisations, type certificate holders and aviation authorities are your first port of call. Find out what needs to be done for each individual tail number and communicate with human resources for manpower, supply chain for the tools, and flight ops for hangar spacing and crewing. Think about who you need to talk too and get started early.
Similar aircraft stored in similar conditions will invariably behave in the same way. Safe return to service begins with good data. It is vital that defects are reported and linked. If a nest is found in an aircraft’s pitot tube, the odds are there will be many more. The data needs to be analysed, and operating procedures (such as additional checks) need to be changed to reflect it.
Storage Procedures. It is possible that aircraft were not fully stored in accordance with manufacturer procedures. Implement a rock-solid audit programme to make sure things are being done properly. EASA recommend extra inspections, ground runs and flight testing of at least ten percent of aircraft before release to service.
Storage Environment The storage environment presents significant hazards to airworthiness. Insects, sand, salt, dust and humidity can all damage aircraft. There may not have been enough protective covers to go around. Was there biocide in the fuel? Is it even useable? It is advised that extra checks be carried out on aircraft parts that are susceptible to contamination, particularly pitot/static systems. Get additional support to add those inspections.

Jet Fuel contaminated with bacterial growth.

Remote Storage This presents unique challenges. Engineering services may be limited, and staff may become overwhelmed with the large number of aircraft waiting to become airworthy. You may need to send additional manpower or require ferry permits to move aircraft around. Is enough equipment on hand to complete extra checks?

A Boeing 777 in deep storage in the Australian outback.

Time. Nothing happens in a day. Commercial time pressure is a major risk factor. Getting an airplane airworthy can cause delays and rushing has a profound effect on safety. Plan ahead and make sure your deadlines are realistic. Communicate them with your staff to ensure confidence.
Inappropriate decision making. This is hazardous, particularly with unfamiliar procedures. Storage on this scale has never happened before and answers may not be in existing manuals. Key personal may not be immediately available to help. Remind staff not to act alone and create a team responsible for making decisions in this challenging scenario
Limited staff experience. Remember that this has never happened before and you may need the help of staff who are new to your organisation. Make sure they are aware of internal procedures that they need to know beforehand. It is a good idea to properly supervise them and assess their work.

Big teams: It takes a lot of people to get aircraft back in the air.

The elephant in the room. Covid-19. The virus has changed the way we can work. Staff can’t move around as freely and there may be restrictions on how many people can work together. You may need to plan ahead and establish isolated teams who work remotely if practical.
Overdue maintenance. Airworthiness directives, MELs, routine maintenance, inspections, ground runs, test flights. There is a lot to do. Start with comprehensive airworthiness reviews of each individual tail number.
They will be under the same pressure that you are. Communicate with them ahead of time and check their availability.
Pilot training. It is likely they are uncurrent, and operating aircraft which have just come out of long-term storage. Simulator training should be relevant to the challenges they will face in the current operating environment. Consider critical systems vulnerable to damage in storage and the affect that these might have on the first flight. In other words, expect the unexpected and provide them with the ability to react quickly and with confidence.

Covid-19 has created a lot of unknowns in our industry. Amongst the noise of statistics and global media, it is important to remain vigilant to the risks specific to aviation that the virus has created. Most of us will have heard by now that aviation itself is not inherently dangerous, but terribly unforgiving of complacency. Never has this been more important than when returning 75% of the world’s fleet from storage to the skies.

Unreliable Airspeed and the Hidden Risks of Aircraft Storage