When Intakes Go Wrong Part 2
There were many concerns regarding the design integrity of the system, where performance and stability had to be both predictable and safe. The designers at British Aerospace (and to an extent Rolls Royce also) had hundreds and HUNDREDS of flight test hours ahead of them; remembering also that a digital control system had to be 'invented' and flight testing begun in less than two and a half years; the original analog system being totally discarded as far as development went.
The biggest worry for the intake design team was however the STRUCURAL integrity of the intake itself, in the event of engine surging, and this was a worry that was well justified by events. In 1971, the French prototype 001 was undergoing tests when on cancelling reheat (At Mach 1.9, not 1.7 as was the eventual 'norm') the associated engine suffered an N1 overspeed and surged, along with it's neighbour. (001 like it's British sister 002 used the original Ultra Electronics constructed analog intake control system; the digital system being still two years away from taking to the air). Unfortunately in the case of this surge, the intake design was found to be seriously under engineered and the surge over-pressure spike caused the failure of the entire forward ramp attachment assembly. A large section of the forward ramp was blown forward and over the top of the wing (we are doing Mach 1.9 remember) and pieces of the rear ramp were swallowed by the engine, which was already in deep surge. The engine was seriously damaged and flamed out, but miraculously was re-lit at Mach 1.5, of course only to surge again, and was then shut down manually.
After the above event the structure was substantially improved mechanically, in order to prevent any such surges causing failures within the linkages and hinges. The biggest surge fear was always the over-fuelling surge which produced the highest overpressures of all, and it was always a concern that the structure could suffer terribly from such an event. Although the electronic engine control system was carefully designed to avoid such a thing, in normal airline operation it was hoped that this would never happen; IT DID!!:
In early 1977 a Concorde (NOT British Airways) was flying at Mach 2 when the crew noticed a mismatch on engine parameters; the fuel flow of one of the engines was depressed compared to the other three. Unfortunately it was decided instead of following normal procedures that a little 'experimentation' would be tried, and the F/E pulled the N1 overspeed protection system circuit breaker. There was a loud bang, and the aircraft was buffeted by some quite severe yawing forces, the autostabilisation system working flat out to prevent violent yawing of the aircraft. The engine was subsequently shut down as a precautionary measure and luckily the aircraft landed safely. Although there was a little damage to the frame of the intake, generally the structure had stood up well to a very serious event brought about quite honestly by abject stupidity. (There was a malfunction of the overspeed system, causing the system's own 'butterfly’ valve' to partially close off from it's normal wide open position in order to limit the fuel flow. In response the 'normal' engine control system's butterfly progressively opened to attempt to keep N2 on schedule. The overall effect was a wide open normal fuel valve and the overspeed one, in 'series' with the other guy partially closed off. When the F/E tripped the C/B the overspeed valve, that for obvious reasons was not rate limited in any way, to fly back to it's normal wide open position in about 150 milliseconds. So we have two valves, and both of them are wide open, and before the 'normal' valve has a chance to react, MAXIMUM fuel flow is fed to the engine, hence the violent surge.
There was another violent event that was in many ways far more serious: In late 1977 another NOT BA aircraft was undergoing maintenance at CDG, and part of this maintenance input was to replace an intake hydraulic actuator. Now this rather substantial unit lived high in the roof of the intake, just above the forward ramp and drove two screw jacks, winding in and out at ether side. The two screw jacks were coupled to the twin torque tubes (that rotated and subsequently moved the ramps up and down) with a pair of trunnion blocks, these being secured to the screw jacks with a large nut and bolt. The trunnion blocks were very substantial, being designed to absorb any shock loads that an engine surge could provide. Unfortunately the engineers in Paris forgot to fit, yes THEY LEFT OUT the two trunnion blocks, and the only thing attaching the two torque tubed to the actuator were the nut and bolt. (In this condition the thing would have rattled around like crazy, it's a 'surprise' that no one spotted it. (Or even not to mention followed the maintenance manual). The aircraft departed from CDG for JFK, and miraculously attained Mach 2 cruise without incident, that is until there was a system defect on the adjacent intake; this defect was a failure of the servo valve that operated the spill door that was located in the floor of the intake. (There were several servo valve failures in the early years of operation until modifications remedied the problem). The spill door would normally only ever open at top of descent when the throttles were reduced for decelleration, or in the event of an engine shutting down. Now the control system had quite adequate and sophisticated protection for such an falure event, and, due to the same spill door servo being mechanically common to both control channels, (but with independent control windings) froze the intake as both channels failed, preventing any further movement of the surfaces. I seem to remember that the crew drills just allowed a single reset attempt; every time you did a reset, all failure were over-ridden for just over 100 milliseconds, to give the system a chance to get online. While this time delay is in effect, the door was allowed to drive open, before the protection system freezing things up again. Unfortunately, the F/E performed a completely unauthorised rapid series of multiple resets, ech time the door travelling downwards, each time pushing the terminal shock system further aft in the intake, until the inevitable happened and the engine surged. Quite naturally this induced a 'sympathy' surge into it's neighbour intake, the one with the trunnion blocks missing. The surge loads hit the ramps, and without the two vital trunnion blocks in place, the two attachment bolts (which were never designed to absorb such punishment) sheared. The two ramps then freely fell downwards, the front ramp hitting the floor of the intake violently, shattering the honeycomb structure as well as seriously damaging the intake lower lip. Pieces of intake went straight into the compressor, taking out the engine and it's associated temperature probe. (Which unfortunately at the time was being used by the adjacent engine). Fortunately the aircraft, although with serious intake damage, was close enough to JFK to make an emergency descent, and land safely. The intake itself was so badly damaged that is had to be returned to Filton for repair. (Where all Concorde intakes for both countries were built on a separate production line, and fortunately in this case that line was still making intakes). It was highly fortunate that the intakes were still being constructed in late 1977; the wrecked assembly spent several weeks back in the assembly jigs. But things could have been far worse; it was lucky in this event that this pattern of poor maintenance and airmanship did not cause more serious damage.
The intake things that went bang in the night story IS TO BE CONTINUED
Dude
Last edited by M2dude; 24th Sep 2010 at 14:55.