If a commercial airliner crashes you're very likely to hear that on the news,
but if a light aircraft of the type that's used for say general aviation, or those tiny
airlines that call themselves charter or air taxi services, if one of those crashes it's
much less likely to be widely reported, unless of course someone very famous was on that aircraft.
And there really is a different prospect climbing on a light aircraft as to a commercial airliner,
and one of those differences is in safety. It's almost axiomatic that flying is safer
than driving, you hear that all the time, but actually statistically, flying in light planes
is safer per mile traveled, but it's more deadly per hour traveled. And it's not like
mishaps with light aircraft can't be important, can't be something that, you know, is memorable,
it's just that because fewer people are involved they're more likely to be forgotten. But there are
some very interesting incidents that have occurred with light aircraft that are actually important,
and did impact the way that we do things, and safety, and are interesting stories. For example,
the incident that happened outside of King's Lynn in Norfolk, in the United Kingdom,
in June of 1993. A very interesting incident. History that deserves to be remembered.
At 6:31 p.m. on June 7th 1993, a Piper PA-31-325CR Navajo twin-engine light aircraft, took off from
Birmingham Airport in the United Kingdom, headed for Norwich. It was a regularly scheduled flight
operated by Prospero Air Charter Limited, which operated a twice daily round-trip scheduled
passenger service between the two cities. The pilot was 45 year old Edward Wyer, a former
RAF pilot who had flown the Panavia Tornado. Wyer was a veteran pilot with over 4,200 flight hours,
but had only 120 hours flying the Piper PA-31. Built by Piper Aircraft, the PA-31 had been
introduced in 1967 as a corporate and commuter transport aircraft. The design was popular in
the small-scale cargo and feeder line as well as corporate markets; nearly 4,000 were produced. The
PA-31 is powered by two six-cylinder air-cooled turbocharged Lycoming engines. The 325 CR version
was introduced in 1974, and so-called because it is powered by 325 horsepower counter-rotating
engines. That is the two propellers spinned different directions. While the plane had been
a good seller, there was a general drop-off in demand for general aviation craft in the 1980s,
and Piper ceased production of the PA-31 in 1984. The PA-31-325CR that Wyer was flying had been
manufactured in 1975. Prospero owned two Piper Navajos, but one had been engaged in survey work,
and the second was due for maintenance so the plane being flown that day was therefore
leased. That day was the first time that Wyer had flown that specific aircraft. He'd flown
the morning route, and then taken a short break, and then from the afternoon flight to Birmingham
all without incident. After a short turnaround he took off for the return trip to Norwich. The
PA-31 carries a maximum of eight people, and the flight that day was full with seven passengers
and the pilot. The plane was near its weight limit, carrying an estimated two thousand eight
hundred and fifty nine kilograms of the maximum two thousand nine hundred forty eight kilogram
load for the aircraft. Because the craft was small, no co-pilot or cabin crew were required,
and one of the passengers sat in the front next to Wyer.
He gave the passengers the normal pre-flight instructions, including the proper position to
brace in case of an emergency landing, and then took off of the short flight which was
just some 217 kilometers or about 135 miles. As the plane climbed after takeoff there was a
noticeable vibration that could be felt through the control column, and that was unusual,
Wyer said he had never felt such a vibration when flying similar aircraft, but the vibration
was slight and ceased about ten minutes into the flight, and Wyer was not concerned by it.
The weather was good, it was clear, what pilots in the United Kingdom call CAV Okay, meaning clouds
and visibility okay. At 6:57 Wyer contacted the Marham Military traffic zone radar at nearby RAF
Marham for a radar information check. He informed Mahram that he would be reducing altitude to 3000
feet as he began his descent into Norwich. He was near the port town of King's Lynn, 70 kilometers
from Norwich. As he reduced throttle to start the descent, suddenly there was a loud bang.
The plane immediately rolled uncontrollably to the right and went into a tight and steep
spiral dive to oblivion. After two spins, Wyer was able to get control of the spin by turning
the left full rudder, but the plane was still in a steep dive that he was trying to pull out of. At
7:01 he issued his first Mayday call. The Mahram zone radar controller responded with the heading
for the RAF Marham field six nautical miles to the southwest. Wyer made a second Mayday call,
he had lost power in both engines and was not going to be able to make it to Mahram. But the
noise and vibration of the plane was so loud that he couldn't hear the tower's response. He
was trying to reduce the rate of descent to get the plane to a safe glider speed,
but when the speed reduced to around 120 knots the plane again started rolling uncontrollably right.
He had to dive steeper then he wanted to keep the plane at a speed that allowed him to keep
control. Desperately he searched for a place to set down and started swinging left towards a green
field. There were power lines in the way, but he had no time, and there was not another choice.
The plane was now just gliding without power, it was shaking terribly. It was
taking all of his strength to try to control the plane! Something had wildly thrown off
the center of gravity of the aircraft, a situation for which he had never been
trained. He was flying by instinct! He looked out the window and was shocked to
see that something had somehow ripped the entire right-hand engine off of the plane!
And there was another problem, without power he had no hydraulics. He couldn't lower the flaps,
or extend the landing gear to land. There was a manual pump that he could use to produce enough
hydraulic pressure to lower the gear but there was no time, and he needed both hands-free to
control the plane. Trying to keep the nose up, and forced to dive faster than he preferred,
he was going to have to land the plane on its belly, in an open field. He narrowly avoided
the power lines and tried to level off over the field to reduce speed.
He yelled, “Brace! Brace!” and the passengers assumed the brace position, happy probably for
the first time in their lives, that they had taken time to get the pre-flight instructions.
He kept the nose up so the tail hit the ground first. He managed to level the plane pretty well,
but as the speed slowed, it started to roll again. The left wing hit the ground, cut a
deep gouge in the field that caused the plane to spin 90 degrees left, but it kept sliding,
now sideways to the right, for another hundred yards, 300 feet! When it stopped everybody
exited the plane quickly for fear of a fire! But there was no fire. And Wyre looked around
to see that astoundingly, all seven passengers were safe and uninjured. He was able to crawl
back into the plane in order to radio RAF Mahram that he had crashed landed in a field.
The plane was a mess. The right engine was completely missing, torn off at the
firewall. There was a gaping gash more than two feet long to the nose luggage compartment,
and the spinner on the left propeller was missing and the left propeller blades were mangled.
Somehow both engines had come apart at the same time. What had happened to them in flight?
The responsibility for figuring that out belonged to the Air Accidents Investigation Branch of the
Department of Transport. The first step was to recover the missing parts of the plane.
The right engine, including his propeller missing one blade, was found about six kilometers from the
crash site. Items from the nose baggage bay were found nearby. The missing blade of the propeller
from the right hand engine, as well as the pitch change piston from the left engine, were found
later by a farmer. The left-hand propeller blades were largely intact except for one that had broken
off about a third of the way up, and was part of the wreckage trail. You’d think the blades were
mangled during the crash, but they showed evidence that they had encountered some hard object while
they were rotating under power in a pitch set for flight. That means that the propeller blades
on the left engine were not merely twisted by the crash, but had apparently been deformed by hitting
something while the aircraft was in flight. This allowed the Air Accidents Investigation Branch
to piece together the dynamics of the accident. In flight, the propeller hub of the right engine
had fractured, causing one of its three blades to detach. That caused what was described as a
massive out of balance force that literally caused the right engine to be torn from the
wing. Meanwhile, the detached blade had flown through the nose of the plane and struck the
left propeller blade, tearing off its spinner and pitch change piston, which then flew through
the left propeller blades, twisting them, and causing the loss of power in the left engine.
That all happened in an instant, and resulted in what the Air Accidents
Investigation Branch described as, “An immediate and critical loss of control.” And of course,
this all speaks very highly of the pilot, Mr. Wyer, who was able to regain control
of the aircraft and successfully crash-land it, despite the fact that it had largely come
apart in midair. But it raised an important question. Why had the propeller hub failed,
and what could be done to prevent that in the future?
Examination of the damaged engine and failed hubs showed that the failure had started with
the deformation of the threads in the grease nipple hole. A hole in the hub that allows
grease for the hub's bearings. The propeller manufactured by Hartzell Propeller Incorporated,
had been hardened and formed using a process called Shot Peening, a process somewhat similar
to sandblasting where cold metal is bombarded with small pellets which compresses the metal,
essentially making it harder. But the grease nipple holes were not covered during the process,
and the Shot Peening had not only deformed the threads on the nipple holes, but also made it
harder to detect cracks emanating from those holes since they were obscured by the peened metal. The
metal fatigue therefore had not been identified in the maintenance and inspection process,
even though prescribed procedures had been followed. The vibrations that were felt as
the plane was climbing were the result of the metal fatigue and crack spreading and a rapid
rate. As they had not been felt by previous pilots of the craft, that indicated that the
crack spread quickly, and occurred in between the regular maintenance schedule, making it
essentially undetectable. The plane maintenance was normal and within specifications. Only the
extraordinary skill of the pilot prevented the failure of the propeller hub from having a fatal
result. It has to be described as one of the most impressive crash landings in aviation history.
The most disturbing part of the incident is that the flaw in the propeller hub was
a problem that had already been identified. In fact other such hubs had already failed,
and the Hartzell Propeller company had redesigned the hub to move the nipple hole to a place where
the metal was thicker, and thus less subject to stress. All such propellers built by Hartzell
after 1983 used the new design. Hartzell had provided guidance both for inspection procedures,
and a warning that unusual vibrations could be a sign of impending failure.
Hartzell had strongly recommended that hubs manufactured before 1983 be replaced.
But the only FAA directive regarding the service bulletin had stressed the increased
inspection schedule, it hadn't stressed that the manufacturer had recommended replacing
the entire propeller hub. And moreover, while the service bulletin did mention the importance
of those strange vibrations, that was sent to mechanics but not to pilots. Operators like Wyer,
who were flying the aircraft, had not been told of the meaning of those strange vibrations. The
Air Accidents Investigation branch came up with four major recommendations based on the
experience from this particular incident. Number one, if a service bulletin was relevant to pilots,
as well as mechanics, extra effort should be put into making sure that the pilots
get the information. Number two, if a manufacturer updates a service bulletin,
then the FAA should also update the FAA directive that was involved. Number three,
that the FAA should consider making mandatory the replacement of parts where that is strongly
recommended by a manufacturer, and it might affect the safety of the aircraft. And number four,
all operators of the PA-31 should do a detailed stress analysis of any pre 1983 propeller hubs.
Despite not being manufactured since 1984, Piper PA-31s are still popular
among collectors and private pilot's. Although, if you are purchasing one,
I would certainly check to make sure the propeller hubs were the post 1983 design.
And in any case, that crash landing outside of King's Lynn in Norfolk in June of 1993,
represents an exceptional bit of piloting, one of the more impressive crash landings in
aviation history. And for that reason alone, it deserves to be remembered.
I hope you enjoyed this episode of the History Guy, short snippets of forgotten history between
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