The flight crew reported for duty at Bournemouth Airport at 3am on Friday the 7th of May 2004. They were flying a thirty-three-year-old Fokker F27 Mark 500, registered in the UK as G-CEXG, to the island of Jersey and back.
The Fokker F27 Friendship was designed as a successor to the DC-3, which had been popular with KLM and other commercial operators after the war. Although a total of 787 Fokker F27 Friendships were produced between 1955 and 1987, by 2004, they were becoming a rare sight and by 2009, only 82 F27s were still in service. These days, you are more likely to see the aircraft on display at a museum rather than flying.
As the flight crew walked up to the F27, they saw engineers charging the aircraft’s pneumatic system. Surprised, the captain asked what they were doing, as this was not normal procedure and they did not know of any issues with the aircraft.
At the same time, the first officer entered the cockpit and spoke to the engineer there. He asked if there was a pneumatic leak. The engineer said no, there was no leak.
The engineers outside explained to the captain that there had been pneumatic problems on another aircraft, and so the operator had decided to charge the pneumatic systems on the entire fleet.
Typically, aircraft use hydraulics, using pressurised liquids, but the F27 uses pneumatics as more economical to maintain. At the heart of a pneumatic system is a pressure or vacuum-creating engine-driven air pump.
The F27 has two pneumatic systems: the main system and the emergency system. Pneumatic systems provide fluid power using pressurised air or gases. These are charged using compressors driven by the engines.
The main pneumatic system supplies the pressure that operates the landing gear, the nose-wheel steering and the wheel brakes. The emergency system supplies pressure to the alternate landing gear extension and the emergency wheel brakes, ensuring that if the main system fails, it is still possible to drop the landing gear and stop the aircraft.
The aircraft also carries compressed air in storage bottles with check valves. If there is an interruption in the supply from the engine compressors, the system charges from the compressed air supply.
Back in the cockpit, the first officer noticed that the brake bottle pressure was slightly low and remained concerned that there was an issue with the pneumatic system. He pulled out the pneumatic isolating valve pin.
The pneumatic isolating valve is used in the case of a pneumatic leak. The valve (actually, two valves operated by a single control rod) isolates the main system and the main system storage bottle. The idea is that you don’t want to let the pressure bleed out, so by isolating the main system, you preserve the remaining pressure until you need it.
Once the main system is isolated through pulling the pin, the other parts of the pneumatic system are cut off. Specifically, the landing gear, the nose-wheel steering and the wheel brakes cannot be operated from the main system supply once the main system is isolated.
If there was a leak, it was a good idea to isolate the system while doing the checklists and start-up, saving the pressure for when they needed it. The first officer was stopping the bleeding so that they would have enough pressure left when they needed it. If there wasn’t a leak, then theoretically, it would do no harm to isolate the system until they were ready to move.
The captain entered the cockpit. The first officer showed him that the brake bottle pressure was slightly low.
Here’s where it gets weird: He didn’t mention that he’d pulled out the pneumatic isolating valve pin.
The captain started the engines and the two pilots began the AFTER START checklist. Checklists like this are high-level documents. There is no room and no need to explain every detail, instead the checklist is a summary of the actual check required, where the Operations Manual gives the full checks needed. So for example, in the F27 AFTER-START checklist, it says
Pneumatics….Checked.
The Operation Manual expands this:
- Check all 3 pneumatic gauges
Minimum pressures for take-off: Brake: 1500, Main: 1500, Emergency: 2500
Confirm the pneumatic isolating valve is IN. Both pneumatic low-pressure warning lights should be out.
It is the captain’s responsibility to start the engines. Then the first officer reads out the AFTER START checklist and the captain responds to each item.
The captain agreed that pneumatics had been checked, without noticing that the pneumatic isolating value was pulled out. The first officer knew it was pulled out, but did not push it back in or mention that he’d pulled it out.
As they began to taxi to the runway, both the captain and the first officer noted that the pneumatic pressures were abnormally low but they agreed that they were still within limits, as in every case, the pressure showed as still in the green “acceptable” band.
It’s hard to imagine what the first officer was thinking, other than that he completely forgot what he had done, despite having a conversation with the captain about the low pressure as they moved from parking to the taxiway. This is utterly perplexing to me, having taken the action directly as the brake bottle pressure was slightly low. Pulling the pin likely contributed to the continued low pressures they saw, but still he did not consider his action or think to mention it.
Suddenly, the F27 veered hard to the left. The captain steered full right and applied right brake. It didn’t help. The F27 was running straight off the taxiway.
With his left hand on the steering, trying desperately to straighten the aircraft, the captain didn’t dare let go to reach for the emergency brake. This was an understandable split-second decision but ultimately the wrong one, as the emergency brake was fed by the emergency pneumatic system, which was still working. He had no way to know it, but that was the only braking he still had.
The wheel brakes did nothing. Steering did nothing.
No doubt completely bewildered by completely losing control of his aircraft which was now careening off the taxiway, the captain suddenly spotted the pulled out pin. With his right hand, he quickly pushed it back in. It was too late.
They drove off of the runway and bounced over the grass. The F27 bumped along for another 50 metres (165 feet) and finally came to a halt.
The captain shut down the engines and then called for engineering assistance. At the same time, air traffic control called emergency services to report an aircraft stopped on the grass.
The nose gear of the F27 was so damaged that even when they restarted the engines, it was not possible to taxi forward or even to tow it by the nose back onto the runway. Instead, they had to drag the aircraft tail first off of the grass and back to the hangers.
How can we explain such an elementary mistake?
The flight crew had not expected any servicing of the pneumatic systems. The investigation confirmed that they were right to question what the engineers were doing and to be concerned about the integrity of the aircraft’s pneumatic system. The entire issue might have been avoided if the flight crew had been given advance notice that the entire fleet was having their pneumatic systems charged by the engineers overnight.
The engineers outside of the aircraft had explained to the captain why they were charging the system; however, the first officer was in the cockpit and likely did not hear why the system was being charged. When he saw the low charge for the brake bottle pressure, he became concerned. He decided to isolate the main pneumatic supply and storage bottle in case his suspicions were right and there was a leak in the system.
This decision should have been communicated. To my mind, the biggest failure here is that the first officer said nothing of his action and never thought to push the pin back in when they taxied from parking.
However, the final cause of the accident must rest with the captain. After the captain started the engines, he was responsible for confirming each item on the AFTER START checklist. When it came to checking the pneumatics, he clearly expected the pneumatic isolating valve would be IN and he did not check that this was actually the case. Not until they were driving straight off of the taxiway did the captain realise that the pin was OUT. As a result, he lost pneumatics leading to the failure of the nose-wheel steering and wheel brakes. He couldn’t keep the aircraft on the taxiway without them.
The incident details were published as a part of AAIB Bulletin No: 10/2004.
It would have been nice to hear from the first officer whether he had forgotten that he’d pulled out the pin or simply didn’t understand the effects that isolating the main system would have.
The operator changed the checklist as a response to this accident, expanding the initial item to check pneumatics into two: check the pneumatics AND confirm that the isolating valve pin is pushed in. The AAIB recommended to the Civil Aviation Authority and to the manufacturer of the F27 that other operators of the F27 be alerted to this checklist change.
