The Accident Chain
Sounds Like...? — The Final Step in Forging the Accident Chain
This is the tale of a PA31 gear-up landing. It starts with an open nose baggage door. But let's go back to the beginning to see how this crew came to do a belly flop and a 1000-foot slide with the sparks flying.
Step One — Design
The first Murphy in the aircraft design is the nose baggage door warning system. The door is equipped with a warning light that will also activate the Master Caution. The door light is activated whenever the nose door is unlatched and the Battery Master switch is ON. However, to activate the master caution light, the system must first be armed. To do that, the nose baggage door must first be latched. If it is not latched when the Battery Master is turned ON, the system is not armed.
Neither light activated during the accident sequence. Although post-slide checks showed both to be functioning normally.
Step Two — Design
The aircraft is equipped with both a landing-gear warning horn and a stall warning horn. Here comes design Murphy number two. Both are solid-tone warnings. The gear warning sounds at 510 +/- 25 hertz; the stall warning at 675 +/- 25 herz. The human ear does not easily discriminate between frequencies this close in range, making it difficult to determine quickly which warning is sounding the alarm — stall or gear? Perhaps one should be a steady tone while the other is an undulating one.
To lessen stress on the open door, the captain had flown the approach at a deliberately low airspeed. When the horn sounded during the landing flare, he assumed it to be the stall warning and he continued the landing.
Step Three — Maintenance
The baggage door closes using a key to turn the lock. Routine maintenance check might have detected the wear, leading to timely replacement.
Step Four — Living with It
On the day prior to the accident, the pilot did not have a key. He used his thumbnail to turn the lock. Tight fit?
Step Five — Writing It Up
There was no record in the aircraft logs that the lock had been reported as unserviceable.
Steps Four and Five go together. If you decide to live with it and don't write it up, it's not going to get fixed, and it's going to turn back and bite you.
Step Six — A Sense of Urgency
As the flaps and gear were retracted after takeoff, the captain noticed the nose baggage door ajar. He elected to make a tight teardrop turn back to land on the reciprocal runway (the winds were calm).
Noise from an open door can be distracting, but flying the aircraft and completing the checklist is vital. Over the past ten years, there have been 53 Canadian incidents involving open doors on all types of aircraft. The vast majority have concluded with uneventful landings. Some, when the pilots got distracted by the noise and sense of urgency, have resulted in gear-up landings. A few have had fatal results.
Step Seven — Cockpit Resource Management
The PA 31 captain decided to reduce approach speed by 10 knots to reduce aerodynamic loading on the door. He instructed the co-pilot to complete the landing checklist, but he concentrated on accurately flying the reduced approach speed. Therefore, he did not fully monitor the co-pilot's pre-landing checks. Evidently, the co-pilot was also distracted by the open door. He missed the Gear Down step in the checklist.
Step Eight — Confirming the Warning
Many checklists or Standard Operating Procedures require that the crew's first step in reacting to a warning be "Confirm," i.e. "Master Caution — Engine Oil Pressure." Check the pressure gauge, and then call for the checklist procedure to be completed.
Perhaps the PA31 pilot expected to hear a stall warning because of his low approach speed, but he did not confirm. He assumed and continued to a belly flop slide down the runway.
Originally Published: ASL 1/1997
Original Article: Sounds Like...? - The Final Step in Forging the Accident Chain