Sorvad..no probs I left to fly the VC10..a gentleman’s aircraft..no one twigged.

What Central wrote. Most "low bypass" turbofans had multi-stage fans prior to the bypass duct (and fan inlet guide vanes), and routing to a single 'mixed flow' exhaust nozzle is a design choice for the nacelle and has little to do with the engine configuration (every JT8D installation I'm familiar with used a single mixed flow exhaust). This is different from a 'pure jet' where the entire engine flow goes through the burner stage (if not always through the burner itself).
"High bypass" turbofans were the next evolution - the first widely produced example being the JT9D - single stage fan, no inlet guide vanes. Many high bypass turbofans used a mixed flow single exhaust (common on the RB211 engine). In theory, there is a performance advantage to a single mixed flow nozzle, although this tends to be negated by pressure loses in the mixer and the extra weight of the longer exhaust.

I flew the T3 for a year in the late 1970s. We used the boost maybe 1 in 10 take-offs. From memory:

The engine had a three-position control switch, spring loaded to a central off position. The active modes were FLIGHT IDLE and CLIMB, There was a 5-position thrust gauge: OFF, GROUND IDLE, FLIGHT IDLE, CLIMB, TAKE-OFF. During taxi out the engine was lit by the press of a button and auto-accelerated to GROUND IDLE. Entering the runway for take-off the switch was blipped to FLIGHT IDLE. On opening up the Speys the boost auto-accelerated to TAKE-OFF thrust.

At noise abatement the Speys were retarded to climb thrust and the boost control blipped to CLIMB. At 6000 ft it was shut down by the press of a button.

Once shut down in flight the boost could not be relit (although I believe this shortcoming was rectified in later years) . . . which led indirectly to the infamous Malaga incident - the guys shut down the boost at 6k on climbout after which one of the Speys gave up the ghost, so they diverted to MAD. Instructed by ATC to go around on short final the T-bird wouldn't climb with 1½ of its 3½ donks dead & flew a very low level circuit before successfully landing off the next approach. It transpired that the crew were operating outside the WAT limit, although there was no way they could have known that at the time.

The boost engine's reliability was poor. I recall doing ATH to LHR with a u/s boost on a warm day. Boostless, we couldn't lift the required fuel so had to tech stop GVA - highly embarrassing. And a winter OSL to LHR off a slushy runway. Full load of pax but no prob using the boost & contaminated runway perf. Light boost just before entering runway. It works! Blip switch from ground idle to flight idle - still working! Capt opens up Speys - and boost promptly dies. Back to the terminal. Eng sucks his teeth. 'Nah, can't do anything here, boys - you'll have to take it home boostless'. Rework contaminated rwy perf - we can take 40(?) pax. Luckily the cull was done by the ground staff rather than us. So we guiltily left behind a load of disgruntled pax.

Still an engine, just not used for propulsion !

That's not too bad, in fact, A340s have five APUs :}

The BAe 146 has 4 APUs and a Hair Dryer.:)

This thread has been a delight, thank you to all who contributed!

Like Discorde , I went ''Tridexterous'' [ All 3 varieties of T-bird ] for the last 3 of my 12 Trid yrs . By then we were taught about the WAT limit for Hot and High airfields .
GVA ... embarrassing ? Surely not with the best lunch allowance in the network .
Memory is not as good as his , regarding operation , but sounds about right .
Winter ops out of GLA , snow , fog 'darn 'Sarf , so Prestwick div. + 1 hrs holding fuel thought wise ..
Holding point 05 , boost fails to go ! ,,,,, Next 1 hr+ sat there 11000+ HP RPM , burning off excess fuel down to Boostless TO wt. Sorry Paisley for the noise .
Because of the boost position , rudder ht is reduced . Thus does memory say X-wind limit reduced ? Or even less allowed from one side than the other ? [ or was that ATP ?]
CS , sorry original ground school on Trids. was in 1973 , so no KNs . .
Seem to remember ,
T1Cs Spey 511s 11000lb thrust
T2Es and T3Bs Spey 512s [W?] 12000lb thrust . Was [W] water/meth inj.?
Rb162 boost engine , 5000lbs thrust , with plastic fan blades .
Helped deliver T3 G-AWZZ on her final flt to BHX for the fire dept to practice on . Min fuel , full chat And the boost , gave us a sprightly t/o !

PB , all civil transport a/c have to be able to lose one engine on t/o , and make 30' ht.[ 15' on wet rwy ] by the end of the rwy .
DR , memory gone but is a B744 equipped with a PWC double PT-6 APU ? Making her a 6 holer .
Thus T3s were 5 holers !

rgds condor

PS. Wing Cracks and solutions is another story ..
Plates bolted on under the wings , increasing wt. reducing payload , increased fuel burn .
The very sophisticated flight recorder , was brilliant in persuading the germans that the Frankfurt rwys. were the roughest in Europe .
Several 'G' spikes on t/o and ldg . Thus helping to crack the Trid. wings .

rgds condor .

Condor …yes you were correct wrt different crosswind limits but don’t understand the relevance of performance A remark re screen height.
Water..iirc someone put meth in a 1-11 which didn’t end well (it burns).
The WAT limits were always available in the takeoff performance calculation manuals (vol3?) but you had to use a bit of creative thinking to apply them.
Twas only one of my many chats with the ex head of the fuel conservation group (known for his descent wizz wheel) that I realised that contingency fuel was already included in our trip fuel and that, along with alternate fuel, was calculated at max landing weight rather than planned weight. I had always thought how economical our operation was as the fuel score was often positive.
He of BOAC fame also mentioned that they had invested in computers for flight planning whilst our flight planning used letraset, printed a load off, which we had to amend by hand until the batch ran out.
Mind you BOAC did go to a lot of weird and wonderful places further afield than Sauchiehall Street.
rgds ace