When intakes go wrong
We've talked quite a bit here about surges and such, so I thought it might be useful to look at what was involved with Concorde 'when things went wrong', in that direction as well as a few other intake horror stories.
The variable air intake control system on any supersonic aircraft is fraught with pitfalls, and downright dangers, the main areas of concern are those of surge and unstart.
A surge, resulting from the breakdown of stable compressor flow, can occur in just about any type of gas turbine engine, however with a supersonic aircraft the effect are really dramatic; The typical characteristics can be really quite disturbing. We get a loss of thrust, an increase in turbine entry temperature and most alarmingly of all a short sharp inlet overpressure, followed by a fairly small longer duration under-pressure. The problem is with any such surge is that unless the cause of that surge is corrected, then the whole process can repeat again and again.
Unstart is far more serious than a simple surge; the inlet shock system is expelled forward of the inlet, with severe 'surge like' effects. Unless changes are made to the intake geometry itself AND ALSO engine demand is reduced, the inlet and engine will not recover and very severe cyclic yawing forces will continue to act on the airframe.
In the case of a Mach 2 surge on Concorde, the most commonly occurring event would be due to a control system malfunction. (The malfunction itself could be an intake ramp going slightly off schedule, a mis-matched engine N1 and N2 at a given temperature or even wing-induced flow distortions. Above Mach 1.6, a surging engine would always cause it's neighbour to surge also. The dynamics of all this, 'a typical surge' are as follows (again quoting from 'The Concorde AICS':
An engine surge is the result of a breakdown of stable airflow in the L/P or H/P compressor sections of an engine. Intake induced surges are exclusively L/P in nature and are brought about by excessive airflow distortions/induced turbulence that can be caused by a number of reasons. A surge will usually result in a sudden sharp rise in EGT, a brief dip in N1 and an almost total collapse of P7 (LP turbine exit total pressure). More dramatically some quite severe over-pressure pulses are generated, that travel forward, into and out of the front of the intake. (During Mach 2+ intake development flying, if an engine surged when the aircraft was operating in darkened conditions, balls of flame could be seen from the flight-deck that were actually OVERTAKING the aircraft at a colossal speed!!). A single surge is relatively short in duration, only about 200 m/sec' or so but can place both the engine and the intake structure under severe trauma. During a typical surge, there is an initial over-pressure of around 50 m/sec' duration that peaks at approx' 10 PSIG, being followed by an under-pressure of about 150 m/sec' duration, peaking at about -2 PSIG. After this point the engine will either recover or the surging will repeat, surges often do tend to be oscillatory in nature and typically repeat at about 8-10 Hz. When you consider the relatively high cross-sectional area of the ramp surfaces, it can be appreciated that very severe peak loads are induced into the whole ramp mechanism from these over-pressures. In the case of oscillatory surging, the effects are even more serious. (The ramps in this case being 'hammered' with severe oscillatory air loads that result in 'organ reed' vibrations of the ramps, at very high peak structural loads).
The surge drill used by Concorde crews involved reducing power on the affected engines, until the surging stopped, and when the power was re-applied this was done quite gingerly. (I will leave it to the guys to tell you all how not easy this wa, but the thing was that this well rehearsed drill did in fact work well, and normal service was normally resumed afterwards.
As far as unstart went on Concorde, well I#m pleased to say that there was no such thing 
. The intake was designed from the outset to completely self-starting, and so unstarts were fortunately completely unheard of.
I'll continue the surging saga and things that went bang in the night in my next post.
Oh, and ps:
EXWOK
Wasn't 202's divider robbed to go on AB after the latter had a fairly big failure of an engine? Again - no question of airfram damage outside the cowlings. In this respect I reckon the Conc was more robust than many conventional designs.
It was OAF, and you are quite right. (OAF's damaged titanium centre wall was removed, 202's was first modified to airline standard and fitted to OAF, the centre wall from the retired French pre-production aircraft 102 was fitted to 202 (to enable the aircraft to remain airworthy) and finally the damaged OAF centre wall fas fitted to 102 in the museum SO THAT THE ENGINE DOORS COULD AGAIN BE CLOSED. (A neat game of musical centre walls). I may have repeated in another thread that when this event happened (as the result of a first stage LP compressor blade loss in 1982 that an FAA inspector was quoted as saying the no other commercial aircraft could have survived the same titanium fire that OAF did.
EXWOK, as far as your statements regarding the safety of our aircraft go, I'm with you 1000%.
Dude 