Image: Les Galbraith via Jeff Chartier
History of this Aircraft:
Type: T2 Trainer
Delivered: 08 May 1956
Side Numbers: Reported as 878(M) and 848(M). Side No 971 was on the aircraft at the time of its crash.
History: T2 Trainer. Maintenance test flight 13 Nov 1956 after ECU change. Suspect Propeller Control Unit (PCU) failure. Port engine oversped, shut down but did not feather. Returned to Nowra, lowered U/C. Prop went into fine and “disked”. Lost speed and carried out emergency landing alongside 03 runway. U/C not down.
Disposal: Reduced to components and burned in the fire ground some time after the accident.
Comment and Analysis
In Patynowski’s book ‘Submarine Hunter’ page 79, Matt Jacobs talks about 724 Squadron and includes the following:
“Just prior to the Royal Tour of 1956, aircraft XA514/NW/971 of 724 Squadron was being flown by a senior pilot on exchange from the RN with pilot Lt x RAN, receiving endorsement training. While flying circuits they encountered problems with the rear propeller control unit. The PCU malfunctioned on the starboard engine causing the rear to go into a setting called ‘ground fine’ pitch, this in turn causing a disking effect and blanketing the front propeller slipstream (this could have been the other way around)…. The story continues to say the aircraft was seriously damaged saying ‘As I recall XA was reduced to spares and never flown again, probably going to the fire ground. Another Mk 1 was converted to a Mk 2 but I can’t remember which one.’”
This account is not factually correct.
The Board of Inquiry found that both piston seals in the port propeller pitch change piston had failed, with the result that the Propeller Control Unit (PCU) was only able to control the engine speed under conditions of light load (low airspeed and low shaft horsepower), but the engine tended to overspeed outside of those (light) conditions. This in itself was not a dangerous fault and was not the primary cause of the accident.
Commander Air’s summary of the Report was that the primary cause of the crash was the failure to engage the Fine Flight Pitch Stop, with the result that the pilots lost control of the aircraft when the undercarriage was lowered.
In order to understand this assertion, it is necessary to also understand the aircraft system.
The Gannet’s two propellers (one for each engine) were designed to turn at a constant speed in flight. To ensure this, the pitch of each prop was adjusted to absorb any power changes applied to it: a coarser pitch for higher power and finer pitch for lower. This was automatically done by each of two Pitch Control Units (PCUs) – one for each propeller.
A characteristic of the Gannet was that if a propeller’s pitch was allowed to go to the fully fine ‘ground’ position (about 6° of pitch) it effectively presented itself to the airflow as a solid disc, creating massive drag. This was demonstrated to pilots during training by lowering the undercarriage and shutting an engine down. The CPU would sense the very low power on the ‘failed’ propeller and would fine off its pitch to the full extent possible. The propeller would then ‘disc’, creating aerodynamic drag and a rate of descent of about 2,500 feet per minute, even with full power on the other engine. It could also result in control problems as the airflow over the tail empennage was disrupted.
So, the Gannet had a thing called the ‘Flight Fine Pitch Stop’ (FFPS). This was a pin which imposed a 21° minimum ‘Flight Pitch’ limit on its respective propeller. The pin was automatically inserted when the undercarriage was raised.
In normal operations, then, with the undercarriage up, the FFPS ensured the relevant propeller couldn’t ‘fine up’ to the point where discing occurred.
But if the gear was lowered, the FFPS pin was automatically withdrawn and the affected propeller could fine off, and ‘disc’. Of course this was of little concern if you were at altitude as there was time to relight the engine, or, indeed, if you were were within inches of the ground in the normal landing configuration But outside of these parameters (for example, in the circuit) this situation was not at all desirable. Indeed, we know of at least one such case that ended in disaster.
So, to safeguard against propellers ‘discing’ in critical parts of the flight envelope the pilot could override the normal system. This was done by means of switches in the cockpit (one for each prop), which prevented their respective FFPS pin from being withdrawn, even when the undercarriage was down. In this case the respective propeller could not reduce below 21° of pitch and therefore not ‘disc’.
It was vitally important that the pilot engage this safety mechanism if he suspected a faulty (and therefore unpredictable) Pitch Control Unit, or if he was contemplating single engine operations with the undercarriage down.
An example of how mis-management of the above system could cause an accident occurred to a different Gannet that was undertaking a display involving shutting down one engine and flying past the crowd with the bomb-bay open and radome lowered. The ‘shut down’ propeller was at Flight Fine pitch because the undercarriage was up. Unfortunately the pilot forgot to ENGAGE the FFPS override switch so that, when he lowered the undercarriage to land the FFPS pin was automatically withdrawn and the affected prop went to fine pitch and ‘disked.’ The resultant high rate of descent close to the ground caused the aircraft to crash and he was killed.
A full copy of the BOI can be found in the National Archives in Melbourne but is not available on line. Excerpts of key pages can be found here. BOI Summary
The pilot in command of the aircraft lost his flying category as a result of this accident. He subsequently co-wrote a book entitled “Breaking Ranks” (Random House Australia 2005, ISBN 1 74051 315 0), in which he stated:
“That summer was very hot and decisions concerning flying depended on the engine requirements together with weather conditions. In the case of a Gannet trainer, which I was to test following the fitting of a new propeller assembly, the engine power was marginally above the minimum requirement. To test the controls and instruments in the instructor’s cockpit I asked the senior instructor to accompany me. As we climbed after take off, the port propeller became uncontrollable, and at 5,000 feet I closed both throttles and reduced speed. As I opened them again, the port revolutions rose further, and I immediately attempted to shut the engine down and feather the propeller. The propeller continued to rotate in superfine pitch, and I tried to relight the engine to provide control. At 6,000 feet I levelled the Gannet and was able to operate the port engine and control it partially through throttle application. I advised the control tower of the situation, and asked for a clear circuit in case of further cause for concern in landing.
I found that I needed to leave flaps and undercarriage lowered in order to induce drag while maintaining engine control for descent. I entered the landing circuit downwind at about 130 to 140 knots, and reached to engage the emergency flight pitch stops to prevent reverse tongue in case of engine failure. As I did, there was a sudden change of engine noise and an increase in revs on the port engine. ‘We’ll never make it,’ my passenger shouted, referring to the distance to the duty runway and, aware that there was no time to hesitate, I turned to the non-duty runway. Then he shouted ‘Lower the undercarriage!’ to which I replied, ‘Jesus, no!’ as we were still descending and unsure of aligning with the runway in a safe position for landing. Our speed was 105 knots when the aircraft became uncontrollably wing heavy as the torque took control. It rolled beyond 90 degrees and appeared to be rolling on its back. My passenger was screaming ‘Pick it up! Pick it up!’, which was heard over the radio by the tower and other aircraft.
At fifty feet, now believing that I was about to die, I applied full rudder and full aileron, and ran the electrical aileron trim through full travel. The aircraft began to roll level and, as it slowly responded to the controls and the wings were level, I decided that the only chance for my passenger to survive was to avoid hitting the ground with one wing first. As the wings levelled, cutting both throttles, I pushed the control column fully forward, and the Gannet hit the ground alongside the runway. I was thrown into the instrument panel as my seat fragmented under the 17G impact force recorded on the accelerometer as it jammed. Instantly discharging the fire extinguishers into the engines, I switched off the fuel and electrics, jettisoned my hood and helped my passenger, who was unscathed, out of his cockpit. As I led him away, I said, ‘Thank God I did not lower the undercarriage!’ He did not reply. The medical officer examining me later said that I did not suffer shock because I had accepted death.
I was grounded, pending an enquiry. When I gave a full account of the incident to Lieutenant Commander Gill Campbell, acting Commander (Air), I told him that we would have been killed if I had lowered the undercarriage. He said, ‘The undercarriage was down, Pedro!’ Gill later assured me of his belief that the undercarriage had not been lowered by me, and offered to testify on my behalf to that effect.
My family’s support kept me going while I waited for the inquiry and watched as Gill Campbell, my staunchest supporter, was appointed elsewhere. Then the composition of the board of inquiry was published. It was not good news. My judges would comprise Acting Commander R.T. ‘Potter’ Power, from Admiral Buchanan’s staff, Lieutenant Commander Peter Goldrick and Lieutenant Bill Caws, an air engineer whom I trusted and respected. Of the three, only Goldrick was a pilot, and Power was a passed-over lieutenant commander: that is, he had not been promoted to commander within the period of eligibility. He, too, was largely ignorant of flying. Yet I still naively expected that truth and justice would result from the inquiry.
The engineers from de Havilland acknowledged that the propeller had been assembled incorrectly and the control fluid drained during take-off, causing the entire episode to occur. I requested that one who offered to give evidence on my behalf be called, but it was denied. Gill Campbell was not questioned on the position of the undercarriage. Junior pilots who were airborne and heard the hysterical voice of my passenger, and the control tower staff who recorded him, were not called. I made a detailed statement about the problems I had encountered in my relationship with the commanding officer of 724 Squadron, in response to vitriolic evidence he had given about my flying ability and attitude.
Left to ponder the outcome, I had occasion to visit Admiral Buchanan’s headquarters in Sydney. Lieutenant Commander Jimmy Bowles, an experienced pilot, ex-squadron commanding officer and the aviation adviser to the admiral, showed me, in complete confidence, a copy of the transcript of evidence. He said, ‘You’d better read this and see what you’re up against.’ He took a risk and I respected his confidence. The transcript, typed by a WRANS writer, had been altered in ink, so that my responses were changed from positive to negative and vice-versa in critical parts. I was appalled, but could do nothing without disclosing the source of my information. Before leaving, Gill Campbell had endorsed my log book with a ‘six’ assessment.
The Navy eventually reached its decision. I was stripped of my flying qualification and appointed to HMAS Warramunga, a Tribal Class destroyer, commanded by Commander A.M. Synnot RAN. Assuming that I had been deemed responsible for the accident, I then protested officially to the new captain of HMAS Albatross, Captain V.A.T. Smith, and expected his support. I asked that I be charged and tried by court martial if I were adjudged guilty of causing the accident. Smith just assured me that he thought it was a misunderstanding and I should raise the matter though my new captain in Warramunga. But I could not let this slur on my conduct go unanswered. I gained an interview with Rear Admiral Gatacre, deputy Chief of Naval Staff, at Navy Office, who said that although sympathetic to my problem, he was powerless to help. Who then did have the power, and from where did it derive?”
Readers can reach their own conclusions in the light of the BOI report and the pilot’s own observations.