DANCING A SHIMMY

By Brian SpurwayAirframes


A shimmy is a dance move in which the body is held still, except for the shoulders, which are quickly alternated back and forth. When the right shoulder goes back, the left one comes forward –but we riggers have another version, as we were taught at Halton:-

“Shimmy is an oscillation in aircraft landing gear that can occur both on landing and take-off, typically in a band of velocities. It causes excessive wear on components and can cause accidents.”

At Halton we were taught all about “aircraft wheel shimmy” and the various design features used to prevent, or at least minimize, its occurrence. Little did I know then that very much later in my career I was to encounter this shimmy effect quite seriously and, along the way, experience a couple of less serious incidents.

229 OCU, RAF Chivenor, March 1959: I was working 2nd line, mainly involved in rectification rather than the Minor servicing also carried out in our hangar. An aircraft would come into our domain from 1st line with a defect that could be quickly resolved, so enabling it to get back out and into the hands of an eagerly awaiting student pilot, or an instructor, without utilising the line fitters themselves whose priority was to keep the flying programme going – it was said that, between the two Sqns that made up the OCU, up t0 ninety sorties a day often took place. I can vouch for that as I spent some time on the line with the live firing section.

Hunter Mk4, WV410, was rolled in after its pilot had reported excessive vibration, most likely due to main wheel shimmy, during his landing run. Finding no significant reason for the shimmying other than worn tyres my JT mate Dave and I simply replaced both main wheels, brake discs and pads, checked out that nose wheel self-cantering was adequate and added a new nose wheel; just for luck –no expense spared in those days.

WV410 was returned to the line and we went off to another job. Maybe we’d get some feedback after its next flight, maybe not. It can’t have been more than an hour later that the “Crash Alarm” sounded, and yes, it was our WV410! It had been allocated immediately to an Iraqi student pilot who, just a matter of a week earlier, had landed another Mk4, WD400, without bothering to lower the undercarriage. He had once again experienced difficulty in landing; this time, having at least remembered to lower the undercarriage, he had touched down short of the runway – in fact on a sandbank in the middle of the R. Taw - bounced up a beach and scraped across the airfield’s boundary embankment depositing all our newly fitted bits on the way! What was left of the aircraft then ended up nicely placed bang in the centre of the runway threshold, whereupon it burst into flames. Probably communicating with Allah - whilst the aircraft blazed around him – the young Iraqi just sat there in his cosy live M&B seat, not even bothering to open his canopy. The fire crews arrived, quickly doused the fire and extricated the completely uninjured student pilot; gossip later spread around that if he had opened his canopy the flames would have engulfed him. Sadly several of those young Iraqi students never got to leave Chivenor alive, but I have oft wondered if any of those who did get through their UK training survived long enough to fly during the first Gulf war, albeit by then in their fifties.

24 Sqn RAF Colerne, sometime in 1966: We did our best on 24 to operate what was known as a “Crewed-up” system; my crew was led by a lovely chap called Pete Bates – nicknamed “Master”, but I never understood why - who was a training captain responsible for the continuation training of the many Sqn co-pilots. This was good for my amassing flying hours but didn’t get me my share of “Duty Free” booze, did it? On this occasion the young co-pilot flying the Hastings had just adequately demonstrated his three-engine landing ability to Pete; we taxied off the runway and stopped ready to restart the shut-down engine. To my total embarrassment, and Pete’s chagrin, rather than opening the cowl-gills on the three hot engines, as part of my after-landing checks, I switched off all three ignition switches. There was a deafening silence – initially – as the three Hercules 216s shut down; then Pete really let rip at me - I had no excuse to offer him.

What’s that got to do with shimmy? Well, I restarted all four engines, probably unnoticed as we were at the far end of the airfield, and we taxied back for another take-off; just one more circuit and landing, and then I would have to face up to an “interview” with my boss – at least – before risking a coffee, and some fitting comments, in the crew room. After the circuit the young co-pilot performed a masterly three-pointer which was completely destroyed by the intense shimmying from the tailwheel as we ran down the track with our teeth juddering in our skulls – very, very unpleasant indeed! Why? Well Pete, himself, had failed to select the tailwheel lock as part of his pre-landing checks – the Hastings trailing-link tail wheel was self-centring but was also fitted with a hydraulic lock, and we now knew why!. We agreed, there and then, that it was now “Even Steven” and neither event would ever be mentioned again – there would be no chat with my boss and my coffee break would be comment- free! I didn’t even snag the aircraft for the excessive vibration caused by the shimmying – I should have really, but at least both Pete and I saved our embarrassment; we continued to fly together on the Hastings, and later the Hercules.

Years later, and much water under the bridge, I was very close to my 55th and about to retire from BA/Caledonian; my last rostered flight in a Tristar was from Gatwick to Toronto then, two days later, back to Gatwick via Birmingham.

Birmingham Airport, very early on 31 Oct 1993, Tristar 100, G BBAE: In gathering dusk we had been pushed back off the stand at Toronto Airport and had flown through the night – proper “Red-Eye” stuff – to land at Birmingham before daylight. The co-pilot had been in control and, with the Tristar having “tick-tocked” across the pond for seven hours, he had carried out an immaculate ILS and a smooth touch down. It all became a whole lot less smooth from there on in, though! As soon as the nose-wheel contacted the runway all hell broke loose, intense vibration, such as paled my previously mentioned Hastings shimmying experience into nothing, caused everything on the flight deck not tied down to fly all over the place. Sitting right above the nose wheel we felt that vibration somewhat, believe you me! It was obvious we had a serious problem on a runway of limited length for a Tristar. My immediate thought was that one of the nose-wheel tyres had burst. The Captain, aware that we were in a Tristar 100, rather than a 200, with only he having a steering wheel, took control and braked heavily; his attempt to control things using the steering wheel failed and the severe vibration continued until we came to a stop, not a comfortable distance from the end of the runway! ATC started to get excited and made us aware that our main wheels were smoking – in their estimation much more than usual - whilst insisting we vacate the runway ASAP! It became obvious straight away that the nose-wheel steering had failed and we were unable to taxy anywhere; we were stuck there on the active runway! There were a few warning lights showing where we didn’t want them to show but, hey, they could wait until after we’d shut down the engines, assessed the possible brake fire, and considered deploying the passenger escape slides. I had the APU running so essential services were still running. Fair do, the fire tender arrived quickly and powdered the main wheels and a BA gang arrived in front of us with a towing arm –the only trouble was they hadn’t brought a head-set with them so communication was difficult. Eventually we were towed off to the stand and were able to offload our full complement of passengers, half of whom were delighted that they had reached their destination with the other half not so happy as they were, like us, faced with coach rides down to Gatwick. So my last rostered flight never took place and my professional flying career had come to an ignominious, and booze-free, termination.

During our push back at Toronto the steering disconnect pin had been removed – whether it had been necessary to do so is a moot point – and obviously had not been refitted securely. During the seven hour flight the pin had inched its way out until finally, just over the runway threshold at Birmingham, it had fallen out allowing the torque-links to separate. Looking at the first image it can be seen that the nose wheel axle has no trailing element at all; therefore the hydraulic steering seems to be the Tristar’s only prevention against shimmy. Steering is accomplished by the linear action of the hydraulic jacks transmitting through the torque links to the lower rotating part of the assembly, so our nose wheels were free to do their thing; and they certainly did just that!

In the first image you can see the steering mechanism has ended up fully to one side and something - no idea now what it was – is hanging loose; the little red arrow on the second image shows a crack in part of the assembly. Other than those two I could see no further damage at the time and, having left the company, I never did hear of anymore - other than a couple of nice letters from Management, one of which I’ve included.

In retrospect we were extremely lucky that the pin remained in place during take-off as that sort of severe vibration would have been been considered serious enough to abort; we were at maximun take-off weight, so aborting at high speed, without nose-wheel steering, would have been very interesting, to say the least! Continuing the take-off, without knowing exactly what the problem was, could have been even more interesting; I didn't even want to consider that possibility, thankfully we were where we were, safely on the ground and I could now go home - job done!