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Helicopters and hot starts

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Helicopters and hot starts

Old 20th Nov 2019, 08:37
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Helicopters and hot starts


Fixed wing pilot only here so indulge me


Currently reading ‘ guns & gunships’ by Mark Garrison who flew the UH1 in Vietnam


He mentions the common practice of hot refueling between sorties then, it saved time and he emphasized that it reduced the possibility of a hot start with a brief shut down leaving you with high residual EGT prior to restarting


That all makes sense and of course it’s easier on the battery and fewer engine cycles


Just out of curiosity though, can you not motor a helicopter turbine with fuel off to lower EGT prior to fuel on ?
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Old 20th Nov 2019, 08:48
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You can motor it to lower the EGT, but a UH1 has a lot of weight up top that will turn and that canes the battery, in an area where the natives are a bit inhospitable the idea of an unnecessary shut down and the possibility of problems on start would leave me wanting rotors running every time.

I've only once flown a UH1, and never in a shooting gallery, but I do remember the EGT acceleration reallly got my attention. SASless will probably be along shortly to give an exact answer from experience.

SND
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Old 20th Nov 2019, 11:25
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To answer your curiosity, yes you can. The RFM usually specifies an ITT limit for starting prior to fuel ignition. If the engine is still hot after a recent shutdown, and the ambient is hot, a starter cycle limitation might not get you the pre-fuel ignition ITT temp drop you need. If there is an operational reason for hot refueling, such as being in hostile territory (whether in a military sense, or sitting on the top of a remote mountain where they can't send the cavalry to get you going if you break down, or whatever) then it's sensible to hot refuel.
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Old 20th Nov 2019, 12:34
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You certainly can motor a helicopter turbine to cool before fuel on. It's over 30 years since I last flew a UH1, but you could do so by running the starter motor with the twist-grip throttle closed.

In the Wessex, you could run the starter motor and chose when to "introduce fuel" by opening the HP Cock.

The Super Puma had a switch that basically allowed you to run the starter motor without fuel being introduced (memory fades even more here)

However, there is so much to be said for rotors-running refuels. Not least being that a "quick suck" of fuel will allow payload to be maximised, and need only take a few minutes.
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Old 20th Nov 2019, 12:48
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SAS’ CH-47 had an APU, so low battery leading to a hot start wasn’t a factor. In our UH-1 unit, the low battery situation sometimes did occur,and the operational solution was to get a battery from the nearest ship that had already cranked, get yourself started and get going. That was just one of the reasons that led the Army UTTAS requirement to include an APU.

Last edited by JohnDixson; 20th Nov 2019 at 12:51. Reason: Typo
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Old 20th Nov 2019, 13:59
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Originally Posted by stilton View Post

Fixed wing pilot only here so indulge me


Currently reading ‘ guns & gunships’ by Mark Garrison who flew the UH1 in Vietnam


He mentions the common practice of hot refueling between sorties then, it saved time and he emphasized that it reduced the possibility of a hot start with a brief shut down leaving you with high residual EGT prior to restarting


That all makes sense and of course it’s easier on the battery and fewer engine cycles


Just out of curiosity though, can you not motor a helicopter turbine with fuel off to lower EGT prior to fuel on ?
Throttle off and igniter breaker pulled for good measure. Doesn't take long to lower the temp enough for a start. The RR's want it below 150C before fuel. Most of the time it will come down enough on it's own during the start but if it has been less than a few minutes, better to cool it with a dry motor. FADECs now pretty much eliminate having to do this.
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Old 20th Nov 2019, 14:04
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On the Bell 206 Jetranger (and many other types) there is a limitation of max TOT for start. If your shutdown was short enough for this to come into effect the engine was motored until TOT fell below the limit, and then fuel introduced. It would light off with an enthusiastic, even abrupt woof and rapid rise of TOT which had to be watched carefully.
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Old 20th Nov 2019, 14:36
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Huh? that's an odd procedure, and not by the book unless I've miss understood your technique. If you're topping N1 to cool engine before open throttle then the mechanic will get upset, and so will the boss when the engine doesn't make TBO.
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Old 20th Nov 2019, 15:24
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Originally Posted by stilton View Post

Fixed wing pilot only here so indulge me


Currently reading ‘ guns & gunships’ by Mark Garrison who flew the UH1 in Vietnam


He mentions the common practice of hot refueling between sorties then, it saved time and he emphasized that it reduced the possibility of a hot start with a brief shut down leaving you with high residual EGT prior to restarting


That all makes sense and of course it’s easier on the battery and fewer engine cycles


Just out of curiosity though, can you not motor a helicopter turbine with fuel off to lower EGT prior to fuel on ?
Yes, you could engage the starter motor, pull air through the engine to cool the residual heat, but why would you want to go to the bother of a shutdown and restart just for fueling? "Hot" refueling, that is with the engine running, were routine.
Battery, weak starter motor, aggressive start settings and/or a mistake in the procedure- the pre-start required one to close the the throttle against the idle stop, click the stop off, roll through it and then guess where the throttle was just below the stop. My memory is that a lot of hot starts involved discovering that the throttle was above the idle detent, the start was going hot, the pilot would 'close' the throttle only to hit the detent and fuel continued into the engine as it exceeded limits; or the battery would be depleted by the time the pilot realized the mistake and tried to click through the idle detent to shut the fuel off.
I hated flying other aircraft besides my assigned one. I knew my crew chief well, I knew the pride he and the gunner took in THEIR aircraft, I knew the aircraft idiosyncrasies 'Spares' were often not flown with a regular crew who had a lot invested in keeping it top notch.
There could well be a lot of human fatigue in the equation as well, we worked day/night rotations- I was grounded for exceeding the maximum 30 day running flight time total for about half my tour. The running sum varied between 110 and 140 hours, and you have to remember that my crew chief and gunner were always along on the flights, they had pre- and post-flight duties. There were occasions when crew were running 24, 36 hour continuous flight assignments, snatching the occasional nap. I've had pilots fall asleep at the controls...
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Old 20th Nov 2019, 16:31
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Although you are cooling the hot rotating parts in a heat soaked engine that is being motored, you are cooling the tgt/mgt/tot probes more than anything else because they have little mass. If you've never done it, motor the engine and then just let it sit and watch the temp climb back up. My point is that if you have the time, if you have the battery juice and if you are not wortied about having some other start issue at a remote or dangerous site then by all means shut down, blow down your temp when you restart. But hot refuel accidents with jet fuel, although they do happen and can be very ugly especially with closed circuit systems, are very rare if done with proper precautions. Keep the fuel truck clear of rotors, have a big extinguisher handy and ground/ bond /earth the piss out of everything and hot fuel away. Gasoline is another story. I'd never hot fuel with it unless it was life or death.
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Old 20th Nov 2019, 17:45
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Originally Posted by Sir Niall Dementia View Post
You can motor it to lower the EGT, but a UH1 has a lot of weight up top that will turn and that canes the battery, in an area where the natives are a bit inhospitable the idea of an unnecessary shut down and the possibility of problems on start would leave me wanting rotors running every time.

I've only once flown a UH1, and never in a shooting gallery, but I do remember the EGT acceleration reallly got my attention. SASless will probably be along shortly to give an exact answer from experience.

SND
All you're turning is the Compressor and accessories with the Starter Motor. All the rest of the stuff that turns is due to Scientific Magic!
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Old 21st Nov 2019, 08:39
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Thanks for the informative and interesting replies, another question


Is it possible to start and / or run a helicopters turbine engine(s) without having the rotors engaged, can they be ‘clutched in or out’ ?
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Old 21st Nov 2019, 08:41
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Yes, on certain helicopters (if they are specifically designed for it). The S76 is one.
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Old 21st Nov 2019, 08:41
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Originally Posted by Cyclic Hotline View Post
All you're turning is the Compressor and accessories with the Starter Motor. All the rest of the stuff that turns is due to Scientific Magic!
Brilliant! After 30 years and more hours than I'd care to remember I ve still never understood so many things from the exams

SND
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Old 21st Nov 2019, 09:33
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Originally Posted by Bravo73 View Post
Yes, on certain helicopters (if they are specifically designed for it). The S76 is one.
The S-76 has a very powerful rotor brake; although everything remains "engaged", it can prevent the rotors from turning at engine idle. It's very useful when embarking/disembarking passengers. In strong & gusty winds, it's reassuring to be able to start the engine/s to idle without the rotors turning. Once they're stable, releasing the rotor brake and advancing the ECLs gets the blades quickly through that uncertain phase where you're not really in control of their flight path.

The original RAF Puma HC1 used to have the facility to disengage one of the engines and run it as an APU. Unfortunately the de-clutch mechanism proved to be unreliable in flight (!) and so the facility was removed.
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Old 21st Nov 2019, 09:46
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Gazelle ( Sa431/342) has a clutch so always start the engine then engage rotor system
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Old 21st Nov 2019, 10:30
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Originally Posted by ShyTorque View Post
The original RAF Puma HC1 used to have the facility to disengage one of the engines and run it as an APU. Unfortunately the de-clutch mechanism proved to be unreliable in flight (!) and so the facility was removed.
The famous Seaking is also equipped with the so called Accessory Drive System, but still in use...

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Old 21st Nov 2019, 10:52
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Unfortunately the de-clutch mechanism proved to be unreliable in flight (!) and so the facility was removed.
The Puma de-clutch was a lever by the No.1 input drive on the gearbox. It had a locking pin that kept it closed and also to keep it open (drive disengaged). Two worthies, JA & MC, IIRC, failed to do this when they closed up and started the engine leading to some very expensive grinding noises. At that the facility was removed to avoid temptation.

Around the same time the RAF locked their cockpit doors so that the pilots had to crawl through the back to get at their seats; still a unique arrangement against the thousands of Pumas/Super Pumas around this world where they get through an convenient door.

The Super Pumas has an option controllable from the cockpit. The controls are similar to the Rotor Brake and Safety Lever mounted on the other side of the throttle box. A Safety Lever unlocks the disengagement lever which then slides the sleeve in the gearbox and disengages the rotor drive. The Safety Lever is then reapplied to lock it. The power turbine must be stationary during the engage/disengage process.

For those not fitted and on the Puma HC2 the outline** of the lever travels can be seen on the throttle panel.

** Always a good question to a trapper giving you a hard time.
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Old 21st Nov 2019, 12:08
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Originally Posted by Bravo73 View Post
Yes, on certain helicopters (if they are specifically designed for it). The S76 is one.
S76 has a clutched tranmsission?! Lynx, gazelle yes, but S76?
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Old 21st Nov 2019, 12:22
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Originally Posted by Phoinix View Post
S76 has a clutched tranmsission?!
No, just a powerful rotorbrake to stop the powerturbine out of idle...

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