When coming to our stand, we usually cut off the engines immediately according the checklist. In the flight safety checklist, it says, however, at "shutdown" that throttles should be cut off after two minutes (original text:"Throttles...off(after allowing ITT to stabilize at minimum value for two minutes)). Now, I've never seen any jet letting the engines run for an additional two minutes when coming to a stop. I was thinking that from touching down until reaching the parking stand, two or more minutes usually elapse with throttles barely above idle so that enough cooling time should be provided for the engines. How do you handle that? And: does it make a differenc if you cut off the engines one by one or if you cut off both at the same time?
Depends on the engine model and the manual recommendations.
Most of these kinds of recommendations are related to nice-to-do to prolong engine life. I could second guess the actual reasons depending on the engine but then I could be wrong.
From the distant past...
Something to do with burning off carbon deposits from some thingymajig (pickup brushes) in the engines? May be a turboprop thing though.
Allowing the engines to cool and stabilize at these temperatures is normal. barit1 is correct. Check with the engine manufacturer, along with your POH, to get the correct procedure.
It is important to let temps stabilize, both after landing, but also before takeoff. You'll find the EGT margins will increase with proper engine warm-up before setting takeoff thrust.
In terms of numbers....let me be clear, it's important to follow your SOP....not mine....we flew the CFM56 and the IAE2500. We found the engineering people to be right. (Little surprise.) Allowing the engines to warm up at idle or next-to-idle for five minutes (preferable 10 minutes in cold weather) gave much better EGT margins on takeoff. If the engines had been previously run within the past two hours, the engines were considered to be already 'warmed up'.
The takeoff warm-up really, normally, applies to single-engine taxi. But, in some circumstances in General Aviation, you have short taxi times with immediate takeoff clearances. In these cases, the times apply to both (all) engines.
After landing, the timing starts when the engines are pulled to idle. So, on approach, at whatever point the engines go to idle, that's when the timing starts. (If you use reverse thrust, the timing starts after engines come to idle after reverse. If you use idle reverse for landing, again, timing starts when you are at idle....whether in forward thrust, or idle reverse.)
For our company's SOP, the required cool-down prior to engine shutdown was three minutes. (I'm now retired, so I have no idea what they're doing these days.) For the airlines, this is an issue for single-engine taxi (usually). For General Aviation, again, depending on where you are, you may have short taxi times....the boss is in a hurry, shut down the left engine ASAP, let the boss run to his ride to the meeting. Well, that may be hard on the engine(s)......again, follow your company's SOP. If your company has no SOP that addresses these issues, well, then, that's not your problem, is it?
Fly safe,
PantLoad
Oh, your second question....on the Bus, we shut down one engine at a time.
Let's assume your engine idles at 65% N2. If you ever get the chance to experiment a wee bit, note the ITT at idle. Then nudge it up to 70% for a minute or two, let it stabilize, and note ITT again. Nudge it up to 75% and repeat.
Compare the ITT's at these points; and don't be surprised if the (stabilized) ITT is actually cooler when slightly above idle. This is because the engine is pretty inefficient at idle, and efficiency improves pretty quickly as you advance out of idle. This gives you a hint why SE taxi saves fuel!
But I wouldn't use this as an excuse to deviate from published limits.
I'm a rated cj series engineer and having done the williams course I'd say the following:
The 2 min cool down is only a recommendation to relieve stresses in the engine prior to shutdown. I strictly observe this if we've been doing high power checks where we may have the requirement to go from full power, thus high ITT, to shutdown asap. If you've had a gentle taxi into the stand with little use of power, I'd be happy for an immediate shutdown once. As you are most likely aware the use off manual attenuator stow on the non FADEC cj models will facilitate this.
And to answer the second part of your question, you should always shut them down "staggered" - not both simultaneously. If you kill them both at once the generator control units will have a brief fight with each other, whereas if one generator has dropped off line cleanly before the second runs down they both give up gracefully and don't stress the components.
The Emergency Checklist has a procedure for 'High or Sustained ITT on shutdown' (or words to that effect), which involves pressing the start button and letting the engine motor for 15 secs before disengaging. So another good reason to stagger the shutdown, as CJ Driver suggests. Having said this, I have never experienced a high ITT on shutdown in over 30 years of Citation flying!
Completely agree about taxying in after landing with minimal power. As previously suggested, use of the thrust attenuator switch to stow them will normally give sufficient thrust at idle to keep the bird moving, except in strong headwinds and/or an uphill slope. Remember to put the switch back to AUTO on shutdown.
Thanks for your inputs. We recently were forced to have some parts exchanged at our right engine and after that I pay more attention to the little things concerning the engines.
On our little Saab, we have a cooling down period of 2 minutes just as described above. According to Saab or GE who makes the engines, this is to 1) cool down the engine and 2) distribute the heat evenly in the nacelle.
After we shut the engine down, it is suggested that we either start the engines within 2 minutes or after 10-12 minutes. After the engine has stopped rotating, the heat transfers to the upper part of the nacelle, expanding the upper part of the rotor/spool. This causes the whole rotor with turbine and compressor assemblies to bow ever so slightly wich in turn causes them to rub against the casing. With time, this wear will degrade the engine performance.
low n' slow's items apply in principle to all engines, great or small. The numbers may vary model to model, but a gas turbine's a gas turbine.
The "rotor bow" phenomenon (convection within the cases causing static spools to be temporarily unbalanced) was first described to me about 1970. Seems to me that if one engaged the starter for a few seconds to stir things up, you could overcome this issue. But I've never heard of this as an approved procedure.
Linear Mode
