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Tailwinds And Engine Temps

Thu May 03, 2007 9:38 am

Is there a noticible increase in engine temps on jets when taxiing or facing into a significant tailwind? Obviously in flight it's not an issue but I was thinking that the air being pushed into the rear of the engines may have an affect on the internal temps.
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RE: Tailwinds And Engine Temps

Thu May 03, 2007 9:57 am

Thats an interesting question. I have no experience operating engines, so I cannot fully answer, but, I do know that the speed of the exhaust gas even at idle is considerable. I don't have it in front of me, but I believe its well over 100 mph. I doubt that even a very strong wing would have any appreciable effect.

Curious to hear from operators.
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RE: Tailwinds And Engine Temps

Thu May 03, 2007 10:04 am

Before FADEC, turbine engine starts with a tailwind had to be monitored had to arrest a runaway hot start in progress before it did any damage.
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RE: Tailwinds And Engine Temps

Thu May 03, 2007 12:43 pm

I sitll have some pilots that ask for their jets to be turned into the wind before starting if the winds shift. In most cases they're flying something older. However there are some old salts out there that insist on it in the newest aircraft because it's just the way they've always done it.

I have yet to see an aircraft have a hot start becuase of a light breeze up it's tailpipe.
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RE: Tailwinds And Engine Temps

Thu May 03, 2007 12:56 pm

Quoting Pilotpip (Reply 3):
I have yet to see an aircraft have a hot start becuase of a light breeze up it's tailpipe.

Not a light breeze more like 10-15kts but I have seen it happen on a TPE331.

Quoting Pilotpip (Reply 3):
. However there are some old salts out there that insist on it in the newest aircraft because it's just the way they've always done it.

I've seen Challenger 300's with a stiff enough breeze that had the FADEC not allow a light off. On days when it gets pretty windy I have seen many times aircraft while not have a hot start come very close. A white smoke starts to come out through the fan.
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RE: Tailwinds And Engine Temps

Thu May 03, 2007 9:34 pm

Interesting stuff everyone, thanks for the replies.
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RE: Tailwinds And Engine Temps

Thu May 03, 2007 9:49 pm

This reminds me when was in the military I visited Florennes AB for a week or so and noticed they put some kind of blanket behind the exhaust of their MirageV fighters when starting up its engine, maybe for some similar reason?

Edit for spelling

[Edited 2007-05-03 14:50:35]
[edit post]
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RE: Tailwinds And Engine Temps

Fri May 04, 2007 9:25 am

On the Lockheed JetStar’s there is a 10 knot tailwind restriction for starting the engines.
There is a trick that some pilots used by starting the engines with the thrust reversers deployed to prevent the wind from blowing up the tailpipe.

Normally you could not start the engines this way because to deploy the thrust reversers the power levers must be back in the idle position and the thrust reverse levers pulled up into the deploy position, but to start the engines the power levers must be firmly back into the off position and the thrust levers have to be down in the stowed position in order to place the power levers in the off position, which is done by pulling up and back past idle on the power levers. The trick was to first place the power levers in the idle position, then extend the reversers using the electric hydraulic utility pump, the pull the circuit breakers for the thrust reversers and then pull the power levers back into the off position, the thrust reversers would deploy because they are independent of the engines. By starting the number 2 engine, which is normally the first engine to be started because it has the main hydraulic pump on it, hydraulic pressure will come up as soon as there is engine rotation. Once the engine reaches about 30 or so percent rpm after ignition, you can then push in the circuit breaker to stow the reverser as the engine come up to its normal idle. This was not an official Lockheed procedure, I never did it but I have known pilots who have done it. With just the one engine running, as long as it was clear behind the airplane and the ramp is level, you can move the airplane and turn it so the wind is not blowing up the tailpipe and then start the other engines normally. If not then the second engine, number 3 would be started the same way.

The straight older turbo jet JetStar’s had no FEDEC system or any engine over temp warning so engine starts had to be monitored vary carefully. For starting once 10 percent RPM was achieved, the power lever was moved from the off to the idle position, this then allowed fuel to be pumped into the start fuel manifold and activated the ignition system. Both the EGT and RPM gauges had to be watched and both had to accelerate up evenly. During a hot or hung start the RPM would stop accelerating but the EGT would show an increase in temperatures and unless the engine start was aborted, the engine would over temp.

Any over temp was cause for engine removal, disassembly and inspection so it could be a very costly mistake. One bonehead chief pilot who I worked for, before I started working there tried to start the engines with a 25 mph wind blowing up the tailpipe. They had just got the JetStar and this was his first jet airplane he flew. He did not watch the gauges closely and allowed the first engine to over temp before he aborted the start because it hung up on starting and the engine had to be removed and sent out to repair. The company should have fired this idiot then and there.

I have had a few hot starts happen to me, all but one were caused by low batteries usually because someone left the battery switch on to long and drained the batteries, the other was a bad main fuel control unit. The JetStar is equipped with dual 24 volt, 34 amp hour nicad batteries. Nicads maintain an even power discharge until almost depleted so if the batteries give out during engine start there is not enough power to continue powering the starter, yet all the fuel needed for starting is being pumped into the engine. Normally at this time a ground power card is needed to start the engines. Once 2 engines were started, the airplane could be taxied and the generators would charge up the batteries and along with the 2 generators on line there was enough power to start the remaining 2 engines. Not all Jetstar’s had APU’s, if they did the APU generator, which was the same one that was on the engines would usually top off the batteries within a few minutes of starting the APU. Starting the APU needed a lot less battery power than starting an engine so even if the batteries were low, usually the APU would start ok. The APU had an over temp system and would automatically abort the start before any damage was done. If the airplane did not have an APU and there was no ground start cart available then basically your were $hit out of luck until you got the batteries out and recharged. One Jetstar I worked on had an APU and the other did not and the APU made life much easier.

Starting an engine after aborting because of a hung start requires that the engine to be motored for about 30 seconds to blow out all the fuel that accumulated in the engine and then to initiate the start. Once the engine started it would ignite the remaining fuel in the turbine section and it would shoot flame out about 10 or more feet behind the engine. It did no damage but was very impressive especially at night and for the uninformed they thought the engine was on fire.

JetStar Trivia

The JetStar has a unique battery start system, both batteries are wired in parallel but during the first 2 engine starts using battery power, the batteries are automatically wired up to series for increased battery power. This is done through the series parallel start relay. When an engine is started as soon as the voltage drops to 14 volts, the batteries are switched to parallel and as the engine spools up and the battery load decreases when the voltage reaches 32 volts the batteries switch back to series. This can be seen especially at night as the cockpit lights first go dim, then get brighter and then go dim again as the relay switches from series to parallel to series.

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