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stilton 13th Aug 2010 03:32

Concorde question
Considering the era in which the Concorde was produced I am curious as to why there was no APU fitted. It would seem to have been quite an inconvenience at times.

Was there ever any consideration given to fitting one and was the decision against the installation solely a weight issue ?

M2dude 13th Aug 2010 08:45

Hi Stilton, that is a question that we all used to ask ourselves; not having an APU was a major pain in the butt for the fleet, particularly at charter destinations, where air start trucks, GPU's and air conditioning trucks would all have to be pre-arranged.
One problem with 'Conc' was always one of weight, (for every extra pound you carried, another pound of fuel was required) so any APU installation would have to have been light, and worth the extra weight. But the main problem was one of 'where to put the darned thing. The only suitable space available for an APU was in the tailcone, aft of the tail wheel. Now a ready supply of fuel would have been available either from the aft trim tank, #11, or from one of the two trim galleries. (For stability reasons, tank 11 was invariably left empty during ground transits). The real crunch however, was how to arrange pneumatic services from an APU: Tank 11 was directly forward of the tailcone, so this would have meant either ducting the pneumatics THROUGH the fuel tank (not a particularly good idea :)) or externally around the fuselage, which would have been 'draggy' to say the least.
You could still have had an APU powering hydraulics, and in essence electrics too (the emergency generator was powered from the Green System), but without pneumatics for engine starting and air conditioning, it would really have been a waste of weight. Still, it really is a shame that there was no APU.
Historically, there were 'sort of' aux power units fitted to development aircraft: The prototypes had two GTS's (Gas Turbine Starters), one in each nacelle pair, that could start the engines without an air start truck, but these never saw the light of day in later aircraft. The most unusual unit of all was the MEPU (Monogol Emergency Power Unit), located in the tail cone. This was manufactured by Sundstrand, and was fitted to all of the development aircraft. (A derivation of a unit fitted to the X-15!!). The idea was that if you had a four-engined flameout at Mach 2, this thing would fire up, power Green and Yellow hydraulics (plus the emergency generator, again from the Green system), and give you power and control down to a safe relight altitude. The MEPU was powered by Hydrazine rocket fuel (unbelievably unstable) and I seem to remember that the thing would run for about 8 minutes. There was no way that this monstrosity would ever be acceptable on a commercial aircraft, and so a conventional RAT was developed by Dowty for the production aircraft. (Also, the windmilling engines would give you full electrics down to Mach 1.1, and Hydraulics down to about Mach 0.7, so the thing had little practical use when supersonic anyway).
I hope this extended blurb helps answer your query Stilton. :)

galaxy flyer 13th Aug 2010 15:07

Yes, M2dude, but how long could you remain above M1.1 with a four-engine flameout while drifting down? I presume you would driftdown above M0.7. BTW, the RAT on the F-16 is hydrazine powered as was the ME 162 rocket interceptor.

M2dude 13th Aug 2010 17:53

Point taken GF, but it was discovered during development flying that that the Olympus 593 could be relit, given sufficient IAS, at almost any altitude within the normal flight envelope. The variable inlet would even be automatically scheduled, as a funcion of N1, in order to improve relight performance at lower Mach numbers. I certainly agree that you would decelerate and lose altitude fairly quickly under these conditions, however a multiple flame out was never experienced during the entire 34 years of Concorde flight testing and airline operation. There was, as a matter of interest an un-commanded deployment of a Concorde RAT AT MACH 2!! (The first indications of the event were when the cabin crew complained about 'a loud propeller sound under the rear cabin floor'. A quick scan of the F/E's panel revealed the truth of the matter). The aircraft landed at JFK without incident, and the RAT itself, apart from a very small leak on one of the hydraulic pumps, was more or less un-phased by the event. Although it sounds horrific, a prop rotating in a Mach 2 airstream, the IAS it 'felt' would be no more than 530 KTS at any time. The RAT was of course replaced before the aircraft flew back to LHR.
Not quite sure about your reference to the RAT on an F16 being Hydrazine powered; a Ram Air Turbine is just that, using the freely rotatting propellor to power hydraulics, electrics or both. Or do you mean the the F16 has an emergency power unit? Either way, it's fascinating stuff.
Yes, I do remember that the Germans used Hydrazine as a fuel during WW2: The father of one of our Concorde pilots was on an air raid to destroy one o the production plants there, this aviation business is such a small world.:)

galaxy flyer 13th Aug 2010 21:14


Thanks for the reply, Concorde expertise is always interesting. I should not have called the F-16 Emergency Power Unit a RAT, it is indeed not. The Concorde RAT was located aft between the engine pods, correct?

What I found interesting is that the AC generators would remain on-line at all; they drop instantaneously at subsonic speeds and the associated N2 rpm. I believe the hydraulics on the 747 will power flight controls down to a pretty low IAS.

Four engine flameout is a very unlikely event, unless one runs into a volcanic cloud.

stilton 13th Aug 2010 22:36

Thanks M2Dude for your interesting and informative reply.

TURIN 14th Aug 2010 20:17

For what it's worth, I read a book years ago. "By the Rivers of Babylon" I think it was.
It involved a couple of El-Al (Yes I know hard to believe) Concordes that had APUs fitted in the area normally used as the forward baggage hold. Small it would have to be to fit methinks.

No idea if the author had researched it or just made it up to fit the story.

M2dude 14th Aug 2010 22:15

Galaxy Flyer
Thanks very much for your comments. It's true, that while supersonic, a windmilling Olympus engine would have sufficient N2 to keep all servics on line. (The hydraulic systems on Concorde also operated at 4000 PSI). The RAT itself was 'said' to be good down to approach speeds, fortunately we never had to find out if that was true. (Although the thing was tested routinely using a hydraulic rig to drive it and check the variable pitch speed control). Thr RAT was in fact located and stowed in the fwd part of the R/H inboard elevon Powered Flying Control Unit Fairing. It was an absolute work of art by Dowty, to make the device fit into such a small space.
Yep, an ash cloud would be particularly bad news, particularly at FL600 :ok:
You are most welcome, thank you for posting this topic also. These forums are a wonderful way for all of us out there in the aviation world to share and learn interesting information from each other.:)
I remember reading By the Rivers of Babylon many MANY years ago. The terrorists, I seem to remember, had a bomb fitted inside Tank 11 (the rear trim tank) during construction 'before it was welded shut'. Not sure if the author had researched how aircraft were built, but still I guess it sold a copy or two. (Well at least you and I read it).:)

Christo 15th Aug 2010 14:22

Not quite sure about your reference to the RAT on an F16 being Hydrazine powered; a Ram Air Turbine is just that, using the freely rotatting propellor to power hydraulics, electrics or both.
I think he got confused with the F16 JFS which does use hydrazine but like you mentioned, is most certainly not a RAT :)

ChristiaanJ 15th Aug 2010 14:54

I've read "By the Rivers of Babylon" too..... and there is some fact behind the fiction.

History does not relate if and where the El Al Concordes would have had APUs.

However, history DOES relate, that the two Concordes ordered by Iran Air WOULD have had APUs (which would have made sense in the Middle East).
Now, Iran Air was the very last company to cancel its orders, and by the time they did, Concorde 214 (now G-BOAG) and 216 (now G-BOAF) were already well underway (they ended up flying initially as white tails).

As a result, both 214 and 216 still have the mounting holes and fittings for an APU. Don't ask me where, but experts know where to find them to this day....

M2dude 16th Aug 2010 11:09

Both A/C 214 (OAG) and 216 (OAF) were Variant 192 A/C (British Unsold A/C). 216 was later converted to a 102 (British Airways) Variant, where 214 more or less stayed as a Variant 192. I'm not disputing what you say about possible APU mountings (I guess it would HAVE to be at the front section of the lower cargo hold somewhere) but I for one have certainly never seen any evidence of them. I'm still trying to imagine where the air inlet and exhausts would have to be arranged, not to mention pneumatic services ducting/hydraulics. Wouldn't it be interesting to find out? :)

Dan Winterland 16th Aug 2010 14:17

The original BOAC/BA VC10s didn't have APUs either. The ones converted to K4s for the RAF had to have them fitted.

ChristiaanJ 16th Aug 2010 14:18

Originally Posted by M2dude (Post 5872914)
Wouldn't it be interesting to find out? :)

It would be, yes!
I can try asking around, but can't remember my original source - it probably came up during a discussion of the MEPU on Delta Golf or on 01 several years ago. I very much doubt it would be mentioned explicitly in the SRM (Structural Repair Manual).
IIRC the MEPU lived in the tail (I'll look it up), so it's not impossible the APU was planned in the same location.

ChristiaanJ 16th Aug 2010 15:37

Just looked up the MEPU in the 01 AFM, and indeed it's located in the tail cone. I'll scan the page this evening.
In the meantime, here's the MEPU exhaust in the tailcone of Delta Golf, courtesy of Neil Walker.



ChristiaanJ 16th Aug 2010 18:53

Just scanned this off the Concorde 01 (G-AXDN) Flight Manual, and cropped as much as possible.



M2dude 17th Aug 2010 09:50

Hi Christiaan, yes THAT was the MEPU (Good photo of G-BBDG by the way). As far as fitting an APU in the tail cone, I still personally think that UNLESS you are prepared to pass a sizable pneumatic duct through a fuel tank, (Remember that tank 11 occupied the entire rear fuselage between the rear cargo aft wall and the front of the tail cone). then I don't think that this was really on. (It's quite possible of course that I'm missing something here, it comes with age :}).
As far as the MEPU goes, all it really did was drive 2 hydraulic pumps; the Green System then powering the 40 KVA emergency generator; unless you are going to use the APU for engine starting and ground air conditioning, then I honestly don't think that there would be much point. It's interesting also to note that the MPU, being a rocket motor, needed no air intake, and as it was not driving any huge loads, the exhaust duct dould be quite narrow. :)

ChristiaanJ 17th Aug 2010 13:33


Nice set of photos of "The Thing" here :
MEPU at MAE at le Bourget.
This one is at the Air and Space Museum at Le Bourget, near Paris. My guess is that is was a spare, since the manufacturing date is 1973. 'SA flew in January '73 and 'SB in December '73.
IIRC, Delta Golf arrived at Brooklands with the MEPU still in place; I might have a photo.

As to the installation, we're obviously thinking along the same lines....

However, there were already several conduits through tank 11, such as hydraulics for the tail wheel, various electrics, and the 'backbone' fuel manifolds, that ended at the fuel jettison port in the tailcone.
A couple of fairly substantial air ducts would only have displaced a few hundred kgs of fuel at the most, out of the more than 10,000 kgs in tank 11.

And yes, of course, the whole point of the APU would be to have independent ground start and ground airco available, so clearly an APU would have been bigger and heavier than the MEPU (which was only just over 80 lbs), plus the problem of the air intake and bigger exhaust.
I'd have to get the drawings out to see how easy or difficult it would have been to fit one in the available space.

Since the tailcone was BAC, and both 214 and 216 were built at Filton, I wonder if anybody there still remembers?

M2dude 18th Aug 2010 11:27

Thanks for the MEPU link, that really brings back memories (or was that nightmares :rolleyes:). I remember at Fairford, a small drop of Hydrazine leaked onto the hangar floor; the next thing you heard was a really loud crack, and a after the smoke cleared, there was a sizable hole in the floor.
I'd still really like to know what the 'thoughts' on this APU issue actually were. Although as you rightly point out tank 11 already had a fair amount of 'plumbing' running through it, we are talking here about a duct with sufficient size that can provide enough mass flow to turn over an Olympus engine to at least between 10 and 20% N2. You are looking at an least 10" diameter duct, not including the copious amounts of thermal insulation surrounding it, as well as an extremely sensitive overtemperature protection system. (This tank is going to be near empty, filled with fuel vapour). I'm not really convinced that this idea would even be considered by the CAA/DGAC/FAA etc. for safety reasons alone.
Still, it's food for thought though :)

Biggles78 18th Aug 2010 15:19

10,000kg in a trim tank? :eek: No, I am really not that stupid to think it was all used for trim but I am beginning to realise just how little I knew about this technological wonder of the skies. Also wish someone had recorded her being rolled (like the B707 when being displayed). Now that would be something that would stand along side the noise abatement takeoff or maybe not. The T/O is impressive!!

M2dude and ChristiaanJ, please keep posting any anecdotes that you remember about this incredible aeroplane. It really is fascinating learning about the technical side from those who actually knew her.

ChristiaanJ 18th Aug 2010 20:24


Re the MEPU at the Le Bourget museum...
The story I just got was that it was taken off F-WTSA or F-WTSB at Roissy for a fault and replaced (both 'SA and 'SB operated out of Roissy around '74 / '75 for things like route proving, etc.).
It got left on a shelf in a store, and was only discovered again in 2003 during the "big clean-out" and was saved 'in extremis' by somebody who recognised it for what it was, stopped it from being 'binned' and took it over to the museum.

Originally Posted by Biggles78
10,000kg in a trim tank? No, I am really not that stupid to think it was all used for trim

Initially of couse it was. It was not until the return to subsonic, towards the end of the flight, that the contents of the n 11 trim tank were moved forward again to the other tanks.
So yes, you're right, essentially all of it was "useable" fuel, it did not serve only for the trim.

Also wish someone had recorded her being rolled (like the B707 when being displayed).
Don't we all....
Jock Lowe seems to have stated there is a photo.... and we all still wonder if there is some footage taken from the Lear Jet during the filming of "Airport 79". But none is publicly known to exist ... we just know it's been done!

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