Concorde engine intake "Thrust"

[M2dude]

When Intakes Go Wrong Part 3

BA themselves had a serious incident, but unlike the OTHER airline this was not as a result of pathetically poor maintenance or airmanship, but of a strange defect coupled with a design anomaly. In the early 1990’s the ramp actuator brake assembly of #2 intake of G-BOAD detached in flight and travelled rearwards, seriously damaging the engine itself, which of course had to be shut down. This defect had never happened before and was attributed to a random failure, until three weeks later the same thing happened again. The aircraft was of course grounded until a fix was found (we could not even find out why this happened). It was known that a feature of the system called ‘HOLD’ would freeze the intake for a brief period (400 milliseconds) in the case of certain failures, inhibiting failure indications and control lane switching at the same time and it was postulated that maybe here maybe lay a clue. (just a theory at this point). Although no static failures could be detected (using specific electronic test equipment) we tried it during an engine run, and found that with a little vibration there was a high frequency repetitive failure of the spill door position resolver, which was replaced. The subsequent engine run proved good, with no further failure indications, and a test flight carried out with the same test equipment fitted, again with without any further failure indications. (It was a great flight too, just me, the three crew and my very large yellow test box at the rear of the aircraft). The aircraft was now allowed safely back into service, and when the offending spill door resolver pack was inspected in the overhaul workshop, a dry soldered joint was confirmed. Analysis showed that the failures were at just the right frequency to allow the ramp actuator brake to be applied, but after the HOLD time delay period expired the failure had cleared, and so the brake was released again, with no other indications, but a fraction of a second later the process would repeat itself. It was estimated that in the three week period between failures over a quarter of a million brake applications had been made, eventually the attachment bolts of the brake assembly failed as a result of metal fatigue. It was a horrible coincidence that this one little soldered joint could fail at just the right frequency to cause such mayhem; eventually stronger bolts were fitted to all aircraft and regular bolt replacements were routinely implemented.
During the first couple of years of Concorde airline operation there was a serious structural issue regarding the intake assemblies. Serious cracking was discovered at the ramp hinge area, and the cause was found to be an aerodynamic resonance issue in the Mach 1.4 to 1.8 flight regime. A short term fix was to strengthen the structure, coupled with a change to the control laws in this speed band. (The change to the intake software did have a performance penalty however). The long term fix was a profiled rear ramp leading edge, and more software changes coupled with a performance enhancing modification to fit a lower, thinned bottom lip to the intake.
The intake was nothing more than an aerodynamic balancing act, where you not only knew and controlled the position of the terminal shock system to within fractions of an inch you also hat to ensure that maintaining critical operation (the normal shock being at the narrowest part of the intake convergent/divergent duct) did not compromise other factors such as ramp angle and control schedule limits variable limits that could result in flow distortion and surge. The system always controlled everything to give you maximum performance, but would move slightly away from this point when necessary to preserve engine flow stability and safety; as was said before, a surge was always the result of something going slightly off 'tune' and was never just ‘one of those things’.
To find the root cause of a surge required a fair bit of forensic analysis; unlike AF, BA had a superb Plessey PVs1580 digital flight data acquisition and recorder (AIDS) system that monitored all the intake parameters; this enabled us to find the cause most of the time, by reading data from the Quick Access Recorder after the flight. (A fair bit of Midnight oil burned though; I still have bags under my eyes ). I remember that originally the singular most important parameter, inlet void pressure, was only sampled once every four seconds by the AIDS system, but once we showed that this was like an ETERNITY when you were trying to track down an engine 'hiccup', we got the sampling rate increased to once per second. Although even this was not ideal, it generally did prove to be sufficient for us.
One small drawback of the thinned and lowered bottom lip was a susceptibility to small pop surges at top of descent, when the throttles were retarded. You just had to be a little careful, and I seem to recall that the initial 'throttle back' minimum limit was 18 degrees throttle angle; zero being max throttle. These surges were very mild and did no damage, but as with all surges post flight engine and intake inspections were required, (no problems werer ever found) together with the usual forensic investigations, ‘just in case’.

Dude