Plane Truths and Knee-jerk Reporting

Now that the wreckage of the Beaver floatplane has been fished out of the waters of Jerusalem Bay, in Sydney’s Hawkesbury River system, it will be only a short time before the Australian Transport Safety Board investigators determine that the plane stalled at a low level and went into an incipient spin. The crushed noses of the two floats indicate that the Beaver dived almost vertically to the bottom of the bay. While the front of the plane including the cabin has been crushed, the rear fuselage and tailplane are almost intact.

There’s nothing like an air accident to reveal the ignorance and lack of experience in what passes for daily journalism today.

A clutch of ABC tyros repeatedly referred to the crash as a ‘ditching’ – the technical term for a pilot setting an aircraft down on water in an emergency as carefully as possible.  But that was eclipsed by a Channel 9 report that had police divers hoping to recover the ‘black box’ – a device not carried, or required to be carried, in such a light aircraft. The Guardian reported that Sydney Seaplanes had been operating joy flights for eighty years.

When the Sydney Morning Herald turned up the fact that the floatplane, registered VH-NOO, had been written off as ‘destroyed’ 21 years ago when in its previous incarnation as a crop-duster landplane, VH-IDI, it was the signal for the ignorant to pontificate about the dangers of ‘old’ aeroplanes. ‘Destroyed’ in the ATSB summary merely meant that the plane had suffered substantial damage when it flipped onto its back and was no longer airworthy. It was certified recoverable by an aeronautical engineer, and was neither too difficult nor expensive to re-construct to certifiable condition.

What is more than likely is that aircraft in the Sydney Seaplanes fleet of Beavers and Cessna Caravans were maintained to standard and as good as new for the light duty sight-seeing tasks they are called upon to perform. From witness descriptions, engine failure and aircraft break-up in mid-aid could immediately be ruled out as causes of the accident, which claimed six lives.

The dive to the water immediately after making a banking turn to the right strongly suggested a stall common to the Beaver type.

Like the venerable DC-3 Dakota of even greater vintage, the Beaver has had an enduring place in civil and military aviation. Designed and built by De Havilland Canada as a rugged short-take off and landing (STOL) aircraft for the Canadian outback, the Beaver quickly earned its place worldwide. A total of 1,657 planes were built, and 980 of them were supplied to the U.S. Forces. Some went also to the British Army before production ended in the mid-1960s. VH-NOO was among the last off the line, first taking to the air in 1964.

In any air accident, the place to start is with the flight manual, often referred to as pilot’s notes. The manual sets out all the specifications, dimensions, fittings and instruments of the aircraft — and, importantly, advice on its flying characteristics. For the Beaver, the relevant numbers specify normal operating speed limit is given as 243kph, landing speed of 72kph and stalling speed with flaps up (as in normal flight) of 96kph.

Paragraph 4.11 helps to explain the Beaver’s popularity with pilots and why hundreds remain in use:

“Stability is good about all axes. The aircraft is easy to fly and is docile down to the stall. Controls are normally effective throughout the airspeed range. The aircraft can be trimmed to fly hands-off from climb to maximum speeds.” 

But there’s a warning in Paragraph 4.11.5:

“The stall is gentle at all normal conditions of load and flap and may be anticipated by a slight vibration which increases as flap is lowered. The aircraft will pitch if no yaw (sideways swing) is present. If yaw is permitted, the aircraft has a tendency to roll. Prompt corrective action must be initiated to prevent the roll from developing.”

As every pilot knows, there is an inherent relationship between speed, angle of bank and the load factor the airframe can withstand. The tighter the turn, the greater the angle of bank has to be, the more stress on the structure, particularly the wings. But because the wings are no longer horizontal, the lift generated by the inner (lower) wing is reduced, and that wing may stall. The Beaver Manual gives pilots a neat table which warns to beware of steep turns.

What it all means is the steeper the turn, the higher the speed has to be to avoid a stall. And as both increase, so does the load factor, which may reach the design limit for the plane.

When Nat Nagy, director of the Australian Transport Safety Bureau, was interviewed on television, he was at pains not to draw comparisons with other Beaver accidents. But there is no doubt this is one of the first things his investigators will look at. They will know, as I was soon able to learn, that in Canada there were twelve similar accidents in which Beaver floatplanes stalled, spun and crashed. A total of 31 people lost their lives and another 19 were injured.

The most recent accident, in August 2015, was analysed in great detail by the Transportation Safety Board of Canada in Report A15Q0120.  This summary quote seems apposite to the Sydney Seaplanes crash:

“In the controlled conditions of certification, the stalling of the DHC-2 was described as gentle. However, as is the case of many other aircraft, a stall in a steep turn under power triggers an incipient spin with few or no signs of an impending stall, and the flight path changes from horizontal to vertical. In low-altitude flight, stalling followed by incipient spin, no matter how brief, prevents the pilot from regaining control of the aircraft before impact with the ground.”

It’s only fair to point out that in the Canadian accident cited above, the pilot lost control while circling at a dangerous and illegally low level at a spot where a family of bears had been seen  All indications are the Beaver that crashed into Jerusalem Bay was only banking, and not necessarily steeply. The questions to be answered relate to the speed and power setting for the angle of bank.

In their investigation, the Canadians had the benefit of the pilot’s GPS device which had recorded the position of the aircraft every five seconds on the last twenty flights he had made. From it, they were able to construct a three-dimensional graph of the flight, and calculate his likely angle of bank. They were also able to compare his fatal flight with previous trips.  They found he had flown almost exactly the same route many times before; only the last time he banked a little too steeply, and stalled.

So one of the critical things for a pilot to watch in his turns is his air speed. The airspeed indicator in a Beaver is a dial in which the needle moves clockwise from zero mph at the 12 o’clock position to 250 mph at 11.30. But the gradations are not evenly spaced. For most of the instrument, there are marks every 25 mph, but between 0 and 75 mph the gradations are compressed. This makes it difficult to easily read speeds in the critical stalling zone. Several observers have commented on this and asked if it contributes to a pilot’s lack of awareness of his speed.

Several public statements have appeared to the effect that the investigators hope to recover video or photography from the passengers’ phones, but this is unlikely to help. What they will be looking for include: the position of the flaps (flaps are lowered for takeoff; if still down, they reduce the risk of stalling in a turn); the needle of the turn and bank indicator; the reading on the airspeed indicator; the throttle setting and any GPS recording. They may also try to find where the passengers were seated; three of them were heavy men who could have affected the centre of gravity if sitting in the rear seats.

In 2009, the U.S. Federal Aviation Administration (FAA) suggested that the Beaver’s stall characteristics in an “aft CG “(centre of gravity) condition are unstable and unpredictable, and that what it called ‘flight excursions’ with an aft CG are often unrecoverable at low altitude. These, and many other clues will give the ATSB much to work on to determine the cause of the crash.

Geoffrey Luck, a pilot and former ABC journalist, wrote memorably in 2014 for Quadrant Online of his own brush with death as a novice aviator. Follow this link to his memoir, Down to the Wire