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Old 22nd Aug 2010, 06:32
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Biggles78.


No apology needed, I can't think of anything I have read on this forum that can compare to the delight of this thread.
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Old 22nd Aug 2010, 09:09
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I feel like the fog is begining to clear and I am getting a slight understanding of how she flew. I was hung up with her flying at Mach speeds where as she was flown at an IAS (specific the the profile she was in). The Mach speed, especially when high, was a result of the temperature and not because she was f a s t ! The altitude flown was due to temperature and weight of the areoplane. This is true of all aeroplanes but due to the extreme environment this was more true of Concorde?

The subsonics have issues with Coffin Corner (I think I read that one Airbus model had somehting like 7kts between the high and low end of the envelope when up high); did Concorde have this "problem"?

I remember reading the BA Concorde flew with 2 Captain Pilots (and of course the most important Flight Engineer) and when I was watching The Rise and Fall of the Concorde, I was looking for the 4 bars in the RHS. Didn't see one but on the Air France Concorde the RHS pilot had 3 stripes. Was this correct or are my "little grey cells" confused?(sorry can't type a Belgium accent )

I don't know why this popped into my head but what was her glide ratio if all the engines stopped? Maybe because I remember from my early training being told the a B707 had a better glide ratio than the PA28-140 I was learning in. Now that was an eye opener at the time.
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Old 22nd Aug 2010, 10:32
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Keep the stories coming!

Just wanted to add my voice to those encouraging you to continue... this thread is great stuff! What a fascinating ship; makes my day job on the B744 look plain in comparison

I too would like to ask what her idle thrust glide ratio was? From Bellerophon's post:
For a straight in approach, in zero wind, on a standard day, from FL600 to touchdown, typical figures would be something like a track distance of around 200nm, flying time of 22 minutes and 3,500kg of fuel.
Sounds like a typical airliner 15:1 glide ratio? (but down from FL600 in 22 minutes ?)
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Old 22nd Aug 2010, 12:16
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From Chariots to the Concorde, how far we have come. Words won't describe the feeling the pilots felt while flying her. This has to be in the top three threads on PPRuNe. Thank you so much for making my day.
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Old 22nd Aug 2010, 12:29
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One more question, could the Concorde lose pressurization, descend to some low level (FL180 or below, perhaps FL100) and make it to scheduled destination or would a divert to Shannon or Gander be required? What was a low level cruise speed?
We never had a case of lost pressurisation, ever. The cabin windows had dual laminated panes; an inner pressure pane and an outer thermal pane. We had dual systems that kept the cabin at a max diff' of 10.7 PSI, the engine bleeds pushing about 200lb of air per minute into the cabin. This allowed you to fly the cabin at an altitude of around a 6000' maximum only, right up to TOD. If you HAD to fly subsonically, the ideal was Mach 0.95 at FL290. (Subsonically the aircraft had similar range to normal, but took well over twice as long of course). If however you had to shut down an engine, your range deteriorated quite dramatically, and a diversion was usually sought.

It's great that Bellerophon is posting here again; we need a steely eyed Concorde pilot's input here (not just the boffins/nutters and nerds [that's me ]. To touch more on a couple of his valid points;
Fuel burn: The aircraft would naturally require less fuel as she became lighter and as a consequence gently climbed to maintain cruise Mach number, this is what the engine control system was doing all the time, even though the throttles were wide open it was 'tweaking'.. BUT, the decreasing IAS as you climbed, due of course to the reducing density, just like any other aircraft meant that drag was reducing too, so it was a combination of both of these factors, reducing weight and reducing drag.
Flying controls: It was a slightly weird but wonderful arrangement; pilots inputs would move a servo valve in the hydraulic relay jack, the jack would move in response and drive both a resolver AND mechanical linkages. The resolver ourput was sumed with the flying control position resolvers, and the error signal was fed into an autostab' computer, where it was summed with stabilisation demands (primarily axis rate and acceleration). The autostab computer would the directly drive the surface, and the reducing error signal would reduce the demand etc. While all this was going on, the mechanical linkages would slavishly follow, but as long as you were in FBW (what we used to call 'signalling') mode, these mechanical inputs were de-clutched at the PFCU, so did nothing at all. Only if there was an EXTREMELY unlikely failure of BOTH FBW channels would these inputs be clutched in and the flying control group (rudders, inner elevons or outer and mid' elevons) would then be in Mechanical signalling. The system redundancy was checked after engine start on every flight. But to reinforce what Bellerophon stated, there was no mechanical reversion here; without hydraulics you had nothing. Another aside here; the designers, being paranoid like all good designers (no offence Christiaan ) were worried what would happen if the controls would somehow jam up. A jammed mechanical flying control input run itself would have no effect on FBW operation whatsoever, due to spring boxes being fitted to the runs. A 'Mech Jam' light would be set, together with a separate red light and audio warning, but this was all. But to completely protect against the aircraft was fitted with a Safety Flight Computer (SFC) system. The idea was, if a control axis (pitch or roll only) jammed up, the captain could press down on a switch light set between the two halves of his control wheel, (at the centre of the 'W') and the Emergency Flight Controls would activate. Strain gauges at the front of the control wheel, two sets on each control column for pitch and roll axis, would input into an SFC that would covert the control force into an elevon demand. These commands were then fed into the autostab' computers, and hence directly into the controls. (A little like L-1011 CWS in a way). There was a little test button that was used to test this system, again after engine start. So although the controls were jammed, the aircraft could still be flown. (Never used in anger I'm pleased to report).
But there was a problem; if this system was inadvertantly used, the results could have been catastrophic, as the system was extremely sensitive indeed, and full elevon movement could be enabled with only moderate effort. Because of this hairy prospect some safeguards were obviously put in place. The first safeguard was an interlock in the autostab' engage logic; If the switchlight had been inadvertently selected beforehand (the light was green by the way) you would not be able to engage pitch or roll autostab's (both channels too) so you would not be going flying until that was fixed. The second safeguard was a little more subtle; A plastic, frangible cover was fitted over the switchlight, unless the captain pressed reasonably hard the cover would prevent the switchlight from being pressed. At least that was the theory, in practice this little bit of plastic could be a pain in the ass . It was carefully fashioned, and I seem to remember BAe charging the airlines a few hundred pounds each for these things. If some wally fitted the cover upside down (and unless you were careful it was easy to do) THE THING WOULD NOT BREAK!! I remember at Fairford in 1976, G-BOAD was on pre-delivery flight testing, and the late great test pilot John Cochrane was doing a test of the system. The cover on this occasion HAD been fitted upside down, and of course he could not plunge his thumb through it and engage the EFC button. After trying everything, in the end he removed a shoe, took out his pen, and smashed the plastic cover until it broke. (It's OK, the autopilot was engaged at the time). Unfortunately, his combined shoe/pen emergency device also wrecked the switchlight as well, so the system still could not engage. (There was only a switchlight on the captain's side). After he landed and he confronted us all with his dilemma, he was shaking; not with rage but with laughter. (This was the great John Cochrane, sometimes the dour Scotsman but he was always able to see the lighter side). After that event, careful instructions were issued regarding the fit of the cover, and it was modified and made a little more frangible.

Last edited by M2dude; 23rd Aug 2010 at 00:02. Reason: will engineers ever learn to spell?
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Old 22nd Aug 2010, 13:18
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...The altitude flown was due to temperature and weight of the areoplane. This is true of all aeroplanes...

Sadly, it isn’t, as subsonic aircraft are allocated a specific cruising flight level and often - for example on the North Atlantic Track system - a specific cruising Mach number as well, and no deviation from that clearance is permitted without specific permission from ATC. Obviously everyone flight plans at the most economic heights and speeds for their aircraft type, but in busy airspace not everyone gets what they want!

Think of your flight plan as being Angelina Jolie, and your ATC clearance as being your wife. Your flight plan is what you’d really like to have, but your ATC clearance is what you’re going to have to live with!


... altitude flown was due to temperature and weight of the areoplane...this was more true of Concorde?...

Subsonic aircraft could equally benefit from using cruise-climb techniques (early long range aircraft crews knew all about cruise-climb techniques and used them when able) but with the large number of subsonic aircraft now using the world’s airways it is impractical for ATC to allow them to drift up and down at will, and so they are assigned specific cruising altitudes.

Few other aircraft got up to Concorde’s cruising levels, and so ATC were able to issue much more flexible clearances to her.

A typical Concorde ATC clearance would have allowed her to accelerate to M2.00 whilst operating within a "block" of altitude, rather than at a specific flight level. Typically this block clearance would have been to operate anywhere between FL450 up to FL600 without restriction.

So, unlike subsonic aircraft assigned a fixed cruising altitude such as FL350, Concorde could, and did, drift up or down, and was thus able to remain at the optimum altitude for the prevailing conditions throughout most of the flight.


... I remember reading the BA Concorde flew with 2 Captain Pilots (and of course the most important Flight Engineer)...

Concorde operated, as did all 3 crew aircraft in BA, with a standard crew of a Captain, F/O and F/E.

A small number of trips had two Captains on board (or two F/Es for that matter) when training or checking was going on, or an extra crew member was carried for PR purposes, but otherwise, the vast majority of occasions, just the standard crew was on board. Everyone preferred it that way, especially the F/O and F/E!


... The subsonics have issues with Coffin Corner (I think I read that one Airbus model had somehting like 7kts between the high and low end of the envelope when up high); did Concorde have this "problem"?...

Have a look at this picture of G-BOAE, cruising at her maximum certificated altitude of FL600, en-route to Barbados on 16 August 2003:





The available IAS speed range is shown on the ASI, and lies between the yellow and black Barbers Pole, currently indicating 440kts, and the white bug set to 300kts, the VLA (Lowest Authorised speed) at this altitude.

The available Mach speed range is shown on the Mach meter, and lies between the yellow and black Barbers Pole, currently indicating M2.05, and the yellow bug which indicates the lowest Mach number allowed for the current aircraft CG position (the AFT limit) currently showing M1.35.

So, given that at her maximum altitude she had a speed range of 140kts IAS and a Mach range of M0.7, we can see that coffin corner was not a problem!


main_dog


...I too would like to ask what her idle thrust glide ratio was...

By my calculations, the figures quoted for a straight in approach, give an average glide ratio of around 20:1, however these were for a standard decel/descent, and on Concorde the early part of the decel/descent was not flown at idle power.

A considerable amount of power was left on initially, around 94% N2, for various reasons, and only below M1.0 were the throttles usually selected to idle.

I hadn’t noticed it until now but there does not appear to have been a chart giving glide distance at idle thrust!

However, since the speeds to be flown during the “4 Eng Flame Out” procedure were not too far from the normal decel/descent speeds, I’ll hazard a guess (and that is all it is) that the glide distance from FL600, with no thrust, would have been about 150nm, giving a glide ratio of around 15:1.
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Old 22nd Aug 2010, 14:03
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Originally Posted by M2dude View Post
[Re microprocessors on Concorde]
ChristiaanJ, for once my friend you're not quite correct. The Plessey PVS1580 Aircraft Integrated Data System, fitted to all BA aircraft from mid' 1977 used a microprocessor in the data entry panel. In the mid-80's, a fault interrrogation module was fitted to the Engine Control Units; this used a 4 bit Intel 4004.
You're right, M2dude, I should really have written that there were no µPs on board when she first went into service (1976), and that they only slowly filtered in afterwards.

Another example on the BA aircraft, of course, in full view of the pax, were the "Marilake" cabin displays that showed Mach, altitude, speed, etc. that replaced the earlier Mach-only displays, and where everybody just HAD to have their picture taken once at Mach 2. Each of the four displays (two up front, two at the back) had a micro-processor.

Not sure when those were first fitted.... it was during one of the cabin re-do's and livery changes.
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Old 22nd Aug 2010, 15:35
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Re the questions about depressurisation, this may be useful.



It shows the emergency descent profile (solid line, 'Avion'), and the resulting effect on the cabin altitude (dotted lines) in the cases of one window ('hublot') blowing out with either three or four air conditioning packs ('groupes') operating.

As the graph shows, in the worst case the cabin altitude rises to about 40,000ft for about two minutes before starting to drop again, which is survivable when breathing oxygen.

It was studies like this, that lead to the small windows on Concorde. Keen spotters may actually notice that the windows on the prototypes are bigger than on all the other aircraft

The diagram is taken from "The Concorde Story" by Chris Orlebar, but the original was so pale that it was uncopyable, so I did redraw it, in answer to a question by a French friend (hence the legends in French).
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Old 22nd Aug 2010, 15:47
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Originally Posted by Biggles78
...The altitude flown was due to temperature and weight of the areoplane. This is true of all aeroplanes...
Sorry Bellerophon, a badly worded question from me but you gave a really good analogy. Gonna remember that even though I don't like my Flight Plans have collagen filled lips. I will see if I can reword it to make it comprehensible.

Thank you for the Instrument Panel image that I have now added to my collection. What is the Yellow Arc on the Mach metre that starts at about M1.12?
Do you remember if you had a signifigant headwind at that stage? I notice that the G/S is 1,139kts was this fairly standard for an East-West flight? (DUH me. Just read the fastest crossing was an east-west direction. Winds must have been quite favorable) I am now guessing the displayed G/S would be fairly typical, plus or minus a bit.
The Glide Ratio, even if it is a highly educated guess, is impressive. I would not have expected it to have been about the same as a B747. How many more times is this Lady going to surprise me with her performance.

Also notice the ball is slightly off to the left even though it is still inside the lines. Was this normal or does it need a tad more rudder trim? Can't imagine it is really out of balance.

Originally Posted by Bellerophon
Everyone preferred it that way, especially the F/O and F/E!
Was that because of the tight fit on the flight deck or because we really don't like others in our workspace?

ChristiaanJ thanks for the CoG diagram. That I am still getting my head around. There is a large range at the bottom and top of the speed range but fairly narrow in the mid speed range. Seems like 165T was a less complex balancing act than it was at 105T.

The center rear fuselage gear unit, what was that for? I have seen it deployed on many occasions but I can't for the life of me remember if it was during T/O or LDG however it didn't seem to be extended every time the aeroplane flew. Was this used during loading so she didn't accidently "rotate" at the ramp or to avoid a tailstrike during LDG? I can't imagine an over rotate during T/O.

And a big Thank You to Bellerophon for sharing his knowledge with this thread.
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Old 22nd Aug 2010, 16:30
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Re Mach 2 ....

In the earliest days of the project, Concord(e) was described as a Mach 2.2 airliner.

Once the RR58 alloy arrived, and the first thermal fatigue tests were underway, Mach 2.2 appeared as somewhat optimistic, and to assure an acceptable airframe life, the Mmo (maximum operating Mach number) to be certified was brought down to Mach 2.04.

Interesting question just asked by somebody on another forum....
Why Mach 2.04 ? Why not Mach 2.10, or Mach 1.96 ?
With thermal fatigue still being a field that was only starting to be explored, was that a fully technical choice.... or was there a commercial aspect ?

Mach 1.96 would again have meant a few more hours life for the airframes, and would not really have made a significant difference in the flight duration.
But think of the huge difference between "more than twice the speed of sound" and "not quite as fast as twice the speed of sound".....
Mach 1.96 would simply not have "sold"......

I have no answer to the question who finally decided on '2.04', and I don't think many of the people that wrote the "TSS spec" are still with us, so we'll probably never know.


And along the very same lines, another snippet.....

In 1985, during a major cabin upgrade, BA installed the "Marilake" displays, that showed Mach, altitude, groundspeed, etc. in place of the simple Mach-only displays that Air France kept until the end.
Nice display, complete with microprocessors.... you must have seen photos.

Of course everybody wanted their photo taken next to the display saying "Mach 2".
So these display were subtly programmed to read "Mach 2.00" as soon as the Mach number was above 1.98, and they stayed there....even if the aircraft went to Mach 2.03 or beyond.
A tiny bit of cheating... but commercially it made a lot of sense, of course.

Like the earlier BA cabin displays, the Air France displays only showed the Mach number, and they were little more than "rescaled" digital voltmeters that directly displayed the 0-12V Mach signal from the Air Data Computer. They tended to flicker a bit from 2.00 to 2.01 to 2.02 and back, but at least they didn't "cheat". And I still proudly have a photo of myself with a "Concorde grin", at Mach 2.03 !
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Old 22nd Aug 2010, 19:35
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I have yet another couple of questions and I hope all you Concorde experts don't mind me taking up your valuable time.
As regards fuel burn: was there any difference between each indvidual airframe and if so was it significant enough to be considered when calculating the trip fuel? Also did different engines also have slightly different fuel consumption?
Whilst on the subject of engines, I just wondered how many were required to keep the BA Concorde fleet flying? What sort of useful life could be expected from the engines?
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Old 22nd Aug 2010, 19:56
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Fascinating thread!

I think (along with the other PAX that day) that I can claim to have been faster on Concorde than anyone else.

Despite many trips, I only flew on BOAB once (sad I know) and there was obviously a malfunction of some sort as the speed (on the Marilake display), instead of stopping in the normal range of 1260-1320 mph continued steadily climbing to 1990 mph.

This was the second "fault" as we had previously begun the flight from JFK with a low speed RTO due to one of the computers disagreeing with the other 2 on takeoff. Despite the slow speed we still had to wait 10 mins for the brakes to cool.

I have it all on Video! (The RTO and the Speed anomaly)
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Old 22nd Aug 2010, 21:04
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Originally Posted by Nick Thomas View Post
I have yet another couple of questions and I hope all you Concorde experts don't mind me taking up your valuable time.
In my case, my time is no longer THAT valuable, being retired for several years !
And I enjoy answering those questions, if and whenever I can!
As regards fuel burn: was there any difference between each indvidual airframe and if so was it significant enough to be considered when calculating the trip fuel?
There WERE differences... after its misadventure at Dakar, F-BVFD definitely consumed more fuel, although IIRC he was already reputed as a gas guzzler even before that incident.
It was one reason why, when Air France withdrew an aircraft from service, Fox Delta was the first one to go.
Also, due to the gradual improvements in production methods, and minor redesign, the last British production Concorde, G-BOAF, was about a ton lighter than the first one (G-BOAC). While the differences weren't huge, they were noticeable.
Also did different engines also have slightly different fuel consumption?
Of course... but there I have no figures at all, and I doubt the differences, evened out over four engines, were really significant.
Whilst on the subject of engines, I just wondered how many were required to keep the BA Concorde fleet flying? What sort of useful life could be expected from the engines?
Interesting question, and I hope somebody will come in and answer it.
According to 'Wikipedia', 67 engines were built, which would mean, in theory, 64 engines for 16 aircraft and 3 spares....
In practice, of course, fewer aircraft flew at any one time, so the statistics are different, but even so, a lot of engine swapping went on over the years.
As to the MTBO, I don't know... it's not my field at all....
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Old 22nd Aug 2010, 21:12
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Originally Posted by norodnik View Post
....there was obviously a malfunction of some sort as the speed (on the Marilake display), instead of stopping in the normal range of 1260-1320 mph continued steadily climbing to 1990 mph.
Nice one....
That would have been about Mach 3 !!!!!!
(Without taking head or tail wind into account.)
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Old 22nd Aug 2010, 22:15
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Christiaan J


What happened to the Concorde at Dakar ?

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Old 22nd Aug 2010, 22:54
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Originally Posted by stilton View Post
What happened to the Concorde at Dakar ?
The story has never been fully elucidated....
But in brief, F-BVFD made an extremely hard landing at Dakar in November 1977, with a vertical speed in the order of 14 ft/sec (with 10 ft/sec being the formal limit).
The result was a major tailstrike, ruining the tail wheel and some of the tail structure.
The aircraft was repaired, repatriated, and put back into service, but it was the first one to be withdrawn from service when the Paris-Dakar-Rio route was closed.

In the end it was scrapped in 1994... only a section of the forward fuselage still remains in the stores of the Air and Space Museum at Le Bourget (Paris).
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Old 23rd Aug 2010, 08:28
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What is the Yellow Arc on the Mach metre that starts at about M1.12?
This is the minimum Mach number that can be flown with the existing CG. (which would be around 59%). Just as the CG indicator (not shown in this photo) gave minimum and maximum CG for a given Mach number, the Machmeter gave a reciprical indication also). You can also see that as the aircraft is not flying at Vmo any more, being at Mach 2 cruise, that the VSI pointer is now away from the orange and black Vmo bug. At our 'not so coffin corner', now that the aircraft is at maximun alllowable altitude, Vmo would naturaly coincide with Mmo; the orange and black Mmo bug being shown at Mach 2.04. This really superb photo taken by Bellerophon gives a graphic illustration of what the panels looked like at Mach 2. Note that the with the TCAS VSI Concorde retained it's original linear VSI also. (Miust have beeen the only aircraft flying with FOUR VSIs. (The originals had to be retained due to the fact that the autopilot Vert' Speed Mode error was derived from the indicator itself. As far as TCAS goes, R/As werer inhibited above FL300 (on acceleration this would coincide with the aircraft becoming supersonic, and the mfrs would not countenance the aircraft doing extreme manoeuvrs as a result of TCAS RAs at supersonic speeds).
The center rear fuselage gear unit, what was that for? I have seen it deployed on many occasions but I can't for the life of me remember if it was during T/O or LDG however it didn't seem to be extended every time the aeroplane flew. Was this used during loading so she didn't accidently "rotate" at the ramp or to avoid a tailstrike during LDG? I can't imagine an over rotate during T/O.
The tail wheel was lowered for all 'normal' gear cycles (not stby lowering of free-fall). It was designed to protect the bottom the nacelles in the case of over-rotation, but in practical terms the thing was a waste of space (and weight) and a simple tail skid (used on the prototypes) would have sufficed. Any time that the tail wheel contacted the ground, it would ALWAYS collapse, damage the tailcone structure and in fact aforded no protection whatsoever. Fortunately these events were EXTREMELY few and far between. The biggest problem with the tail wheel was a major design flaw: On gear retraction the assembly would retract in sequence with the nose and main gear, and as it entered the opening in the tailcone, it would release over-centre locks that were holding the spring-loaded doors open. The doors would then firmly spring shut behind the gear assembly and finish the job. UNFORTUNATELY this was a very poor design; if for any reason one of the two doors had not gone over-centre on the previous gear lowering, it would be struck by the retracting tail wheel gear and cause structural damage to the local skin area, that would have to have a repair done. Unfortunately these events were not quite so rare, and several measures were tried to reduce the chance of this happening. Although not a safety issue, it was an issue that was a total pain. (As a matter of interest, G-BOAC had this happen on one of it's first test flights out of Fairford in 1975).
Nick Thomas
As regards fuel burn: was there any difference between each indvidual airframe and if so was it significant enough to be considered when calculating the trip fuel? Also did different engines also have slightly different fuel consumption?
As ChristiaanJ said, the last two BA aircraft WERE lighter than the others, and would be preferred aircraft for certain charters. But that is not to say that any aircraft could not happily do ANY sector. We fortunately had no distorted airframes in the British fleet, so this was never an issue. There was very little spread, regarding fuel consumption between different engines; one of the best parts about the Olympus 593 was that it hade very little performance deterioration with time, it was an amazing piece of kit.
Whilst on the subject of engines, I just wondered how many were required to keep the BA Concorde fleet flying? What sort of useful life could be expected from the engines?
Time on wing for the engines was a real variable. Each engine was built up of modules, each one of these had a seperate life. In the early days of operation, time on wing was quite poor, and MANY engines would be removed on an attrition basis. One of the early failure problem was the fuel vapourisers inside the combustion chamber were failing, taking bits of turbine with it!! A Rolls Royce modification that completely changed the design of the vapouriser not only solved the problem completely, but also increased the performance of the engine. As the engine matured in service time on wing greatly improved, and in service failures became a thing of the past. A 'trend analysis' was done after each protracted supersonic flight, where engine parameters were input into a propiatry RR computer program, that was able to detect step changes in the figures, and if this were the case, more boroscope inspections were carried out. The OLY time on wing was nothing compared to the big fan engines, but the conditions that it operated under bore no comparison. Not really sure about absolute figures on this one Nick, I'll ask one of my Rolls Royce friends and see if I can find a figure.

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Old 23rd Aug 2010, 10:51
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One thing I have noticed on take-off is the way the throttles are handled. Application of take-off thrust is done by slamming the throttles forward rather than the traditional ease them up method used on most other aircraft.

Why.
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Old 23rd Aug 2010, 11:59
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nomorecatering

...I have noticed...Application of take-off thrust is done by slamming the throttles forward rather than the traditional ease them up method used on most other aircraft...Why...

It does seem rather brutal at first glance doesn't it, especially if you are used to handling piston engines with care!

Firstly however, unlike most jet aircraft, you never set reduced thrust for take-off but always set full power, which on Concorde meant that the throttles had to be fully forward, as far as they would go.

Secondly, you were not actually controlling the engines as directly as you might think. Various control units between the throttles and the engines would electronically regulate everything for you, especially spool-up rates, temperatures and thrust levels, even keeping #4 eng throttled back initially to 88% until reaching 60kts.

In effect, you were really just operating a switch telling the computers to set full take-off power.

And how slowly do you turn on a switch?
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Old 23rd Aug 2010, 13:20
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The reason that #4 engine was limited to 88% N1 on take-off was an interesting one, down to something known as 'foldover effect'. This was discovered during pre-entry into service trials in 1975, when quite moderate levels of first stage LP compressor vibrations were experienced at take-off, but on #4 engine only. Investigations revealed that the vibrations were as the result of vorticies swirling into #4 intake, in an anti-clockwise direction, coming off the R/H wing leading edge. As the engine rotated clockwise (viewed from the front) these vorticies struck the blades edgewise, in the opposite DOR, thus setting up these vibrations. The vorticies were as a result of this 'foldover effect', where the drooping leading edge of the wing slightly shielded the streamtube flowing into the engine intake. #1 engine experienced identical vorticies, but this time, due to coming off of the L/H wing were in a clockwise direction, the same as the engine, so were of little consequence. It was found that by about 60 KTS the vorticies had diminished to the extent that the N1 limit could be automatically removed. Just reducing N1 on it's own was not really enough however; some of this distorted airflow also entered the air intake through the aux' inlet door (A free floating inward opening door that was set into the spill door at the floor of the intake. It was only aerodynamically operated). The only way of reducing this part of the problem was to mechanically limit the opening angle of the aux' inlet door, which left the intake slightly choked at take off power. (The aux' inlet door was purely aerodynamically operated, and diff' pressure completely it by Mach 0.93).

Last edited by M2dude; 24th Aug 2010 at 08:31. Reason: A few corrections
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