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APU start technique question

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APU start technique question

Old 6th Jul 2019, 09:13
  #21 (permalink)  
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The book says START APU

Like with everything we do in life there are better and there are worse techniques.
We all know “that driver” that does everything we all do yet it’s unpleasant to drive with him/her.
Start, shift, accelerate, brake, corner, they do all that.
Yet their techniques are what makes it uncomfortable.

Flying single engine piston the POH says “advance throttle” it doesn’t state not to slam it open yet that’s a bad technique. A better technique is a 3 second count from idle to full power and the same back to idle.
Short crankshaft, swinging 60-70lbs weight aka prop and so on.

Your mom says to close the door behind you.
Implied is a non verbal warning not to slam it.

So the book says “START”.
Insight and technical understanding would warrant a particular technique....
So we just go braindead and turn the switch regardless of conditions or do we apply a light coating of mechanical empathy?

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Old 6th Jul 2019, 13:56
  #22 (permalink)  
 
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A cool down cycle is normally written into the APU ECU software. If the manufacturer thought that a warm up cycle was also required, it would be included.

Just hit the start button and let the APU take care of itself.
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Old 6th Jul 2019, 18:26
  #23 (permalink)  
 
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Originally Posted by B2N2 View Post
The book says START APU...
So we just go braindead and turn the switch regardless of conditions or do we apply a light coating of mechanical empathy?

B2N2, When you started this thread you asked for opinions. You got them.
Don’t now round on your contributors and call it brain dead to do the job as taught.
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Old 6th Jul 2019, 18:53
  #24 (permalink)  
 
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BN2, some advocate not lowering the gear whilst the flaps are still running.
Others won't switch on the bleeds immediately after the APU comes on line.
Some say don't cool the brakes with the fan, arguing they're more efficient when warm.
And other do the controls checks reaaaaaallllyyyyy slllllooooooooooowwwwwwww in order to prevent...I have no idea exactly what.

And now you tell us to have some mechanical empathy by warming up an APU that's designed to be started at -56 C.
Noted.
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Old 7th Jul 2019, 14:59
  #25 (permalink)  
 
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Originally Posted by B2N2 View Post
So the book says “START”.
Insight and technical understanding would warrant a particular technique....
So we just go braindead and turn the switch regardless of conditions or do we apply a light coating of mechanical empathy?
you assume the need for a technique without considering that perhaps there has been an analysis conducted by a team of professional engineers in an office working for the apu oem, the aircraft manufacturer and the airline whose sole job description might be to consider the effects of temperature on apu/jet engine performance, BEFORE apu certification and IN-SERVICE, on a worldwide fleet, with experience in a range of operating conditions.
An assumption is also made that somehow all pilots around the world have been applying the suggested ‘technique’, leading to the apu reliability rates that underpin etops certification.
No - I just don’t think so. I think pilots have been cranking straight away relying on inbuilt protections (as designed and certified) to auto-shutdown if the apu is unhappy.

There is nothing wrong with developing technique, but some measure of reason needs to be applied and tested, and I don’t think delaying an apu start for cold soaked apu because it is thought to be healthy for it, is reasonable. There is no evidence that supports the technique unlike the other examples you gave, which can be sensed immediately and reacted to.

all that said, I think the suggested technique is one of little risk with fod ingestion being the biggest issue but even then, apu’s generally have a screen that captures the crap.

Last edited by QuarterInchSocket; 7th Jul 2019 at 15:15.
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Old 7th Jul 2019, 15:14
  #26 (permalink)  
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Originally Posted by QuarterInchSocket View Post

you assume the need for a technique without considering that perhaps there has been an analysis conducted by a team of professional engineers in an office working for the apu oem, the aircraft manufacturer and the airline whose sole job description might be to consider the effects of temperature on apu/jet engine performance, BEFORE apu certification and IN-SERVICE, on a worldwide fleet, with experience in a range of operating conditions.
An assumption is also made that somehow all pilots around the world have been applying the suggested ‘technique’, leading to the apu reliability rates that underpin etops certification.
No - I just don’t think so. I think pilots have been cranking straight away relying on inbuilt protections (as designed and certified) to auto-shutdown if the apu is unhappy.

There is nothing wrong with developing technique, but some measure of reason needs to be applied and tested, and I don’t think delaying an apu start for cold soaked apu because it is thought to be healthy for it, is reasonable. There is no evidence that supports the ‘technique’ unlike the other examples you gave, which can be sensed immediately.
Thank you, enlightened response.
Im a very simple carbon based life form and mostly water at that. Easily confused.
So getting told off 180 degrees apart confuses me lol.
I think I can put this to bed now.



As far as the brain dead comment, we have all flown with brain dead people.
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Old 7th Jul 2019, 15:19
  #27 (permalink)  
 
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I don't have a dog in this fight, but interesting discussion anyway. Let's say you land and go to the ramp within 5 minutes. How much warm air actually gets to the APU? Wouldn't you have to be moving pretty fast to get enough ram air to actually warm up the APU?

On the 320, some of our airplanes display the APU temp even when it's off in flight. Opening the APU flap at cruise in -50 degree air doesn't change the indicated temperature (was just around freezing if I recall), but that could simply be the lower limit of the sensor.
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Old 7th Jul 2019, 17:20
  #28 (permalink)  
 
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B2N2 is clearly referring to 747 operations. Opening the 747 Classic APU inlet door immediately after landing and in some cases on approach was a common practice especially after long haul flights to improve APU starting. The 747 Classic APU is not used for any emergency or back up functions so much of the chatter above is irrelevant. In fact the 747 Classic APU is not certified for in flight starting if the ram scoop on the inlet door is deleted. Most operators deleted the ram scoop per Boeing SB to eliminate a small drag penalty. There is a placard on aircraft with the scoop deleted that reads "APU IN FLIGHT STARTING PROHIBITED. In Flight Operation After Ground Start Allowed".
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Old 7th Jul 2019, 17:30
  #29 (permalink)  

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No issue with warming up the lubricant and various movable parts to pamper the thing. Point being made multiple times already is that opening the flap AFTER landing and delaying the start by 4-12 minutes for such un-soak achieves precisely nothing towards that direction.

Mileage, timezone and jetlag may vary.
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Old 7th Jul 2019, 18:15
  #30 (permalink)  
 
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Originally Posted by CV880 View Post
B2N2 is clearly referring to 747 operations.
I did consider the 747CL along with the 737CL both of which share similar characteristics with respect to issues relating to starting reliability. My rubbish talk is geared toward the modern machines and despite etops apu controls not applying to the heavies as is the case with some of the lighter twins, the learnings relating to apu performance from the more stringent controls of an etops program I would reasonably assume to apply to apu’s more broadly. Airlines, apu manufacturers and airplane manufacturers do monitor many metrics including MTBF of components on an apu both on-wing and as a result of overhaul etc. They’re watching! If there was a trend, they’d react... or at least, that is the expectation from historical occurrences and experience.

to op - apologies if my prev post has caused offence. I see many people try to reinvent the wheel, some of the suggested shapes that get churned out aren’t very well thought out. The latest rediculous one came from a chap who saw nothing wrong with leaving the apu bleed on with packs switched off on the ground (737NG). I can’t recall if there is mechanical risk to apu health in this config, but I do know that it results in approx 40% loss of efficiency, half that (20%) if only one pack is used for the 131-9b on 737ng, (off the top of my head); I can’t remember where I read it so I wouldn’t be able to produce evidence even if I tried! Hah.
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Old 7th Jul 2019, 22:56
  #31 (permalink)  
 
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Originally Posted by CV880 View Post
B2N2 is clearly referring to 747 operations. Opening the 747 Classic APU inlet door immediately after landing and in some cases on approach was a common practice especially after long haul flights to improve APU starting. The 747 Classic APU is not used for any emergency or back up functions so much of the chatter above is irrelevant. In fact the 747 Classic APU is not certified for in flight starting if the ram scoop on the inlet door is deleted. Most operators deleted the ram scoop per Boeing SB to eliminate a small drag penalty. There is a placard on aircraft with the scoop deleted that reads "APU IN FLIGHT STARTING PROHIBITED. In Flight Operation After Ground Start Allowed".
But the issue is the same on all aircraft, from 747 to Cessnas: the urge of some pilots to invent procedures based on unverified, unofficial conjecture, sometimes called mechanical sympathy our even airmanship...


Maybe he should ask the captains exactly which parts of the APU need warming up. The core of a cold soaked APU will never reach anywhere near ambient temperature in the ten minutes it takes to taxi to the gate IMHO.
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Old 7th Jul 2019, 23:33
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The latest rediculous one came from a chap who saw nothing wrong with leaving the apu bleed on with packs switched off on the ground (737NG). I can’t recall if there is mechanical risk to apu health in this config, but I do know that it results in approx 40% loss of efficiency, half that (20%) if only one pack is used for the 131-9b on 737ng, (off the top of my head); I can’t remember where I read it so I wouldn’t be able to produce evidence even if I tried! Hah.
Sorry for hijacking...

What about having packs off and APU bleed on? With or without APU running? 8 years now on the 737NG and I thought I had pinned down the essentials.
​​​​​​​
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Old 8th Jul 2019, 01:13
  #33 (permalink)  
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Originally Posted by PENKO View Post
But the issue is the same on all aircraft, from 747 to Cessnas: the urge of some pilots to invent procedures based on unverified, unofficial conjecture, sometimes called mechanical sympathy our even airmanship...


Maybe he should ask the captains exactly which parts of the APU need warming up. The core of a cold soaked APU will never reach anywhere near ambient temperature in the ten minutes it takes to taxi to the gate IMHO.

Amen.......
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Old 8th Jul 2019, 03:04
  #34 (permalink)  
 
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The technique of turning the APU to ON for a bit of time before turning it to START comes from decades ago and has nothing to do with ‘heating’ the APU.

It’s to let the APU start sequencing happen. Mx decades ago said quick application to START could generate the FAULT light we’d often get after the first attempt. “Second start normal.”

The FAULT rate decreased significantly with a pause at the ON position. Over time guys started putting it to ON but not starting it until later, sometimes as long as the entire taxi-in. I’ve done that for decades and haven’t had an APU FAULT during start for years. I’ve seen new FO’s get the FAULT light. I tell them to cycle OFF then ON and leave it for awhile. The next attempt is normal. “Want me to write that up?” “No. It works with patience.”
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Old 8th Jul 2019, 08:03
  #35 (permalink)  
 
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Never done this or seen a reason to


This is the same sort of reasoning as delaying APU start after landing until the last possible moment to save a few pounds of fuel


Had a few FO’s do this, most of them forgot to crank it altogether, I’d be busy parking then we’d realize we had to keep an engine running until it was up and running


Didn’t save much fuel with that technique..
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Old 8th Jul 2019, 08:20
  #36 (permalink)  
 
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Maybe you should write it up, force Boeing to come out with a new SOP.

Or maybe things have improved over 'decades'.
Or maybe Boeing decided in their wisdom that an occasional fault does not warrant the added complexity and forgotten APU starts.
Or...

I appreciate what you're saying, but the byproduct, as you clearly see in this thread, is that you leave your FO's confused. And when they become captains, they will pass that confusion on to the next batch of FO's, citing warming up times and mechanical sympathy.

I occasionally see a clear example of this when young FO's on my Airbus refuse to select flaps directly after lowering the gear, parroting the 'mechanical sympathy' line, based on an SOP from an earlier version of the Airbus they never flew. The SOP changed before they were born, but some captains decided to stick with it, and voila......
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Old 8th Jul 2019, 08:49
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Originally Posted by Romasik View Post
Well, I’ve heard from maintenance, that their procedure was to start the APU 3 min after Master Switch pressed ON (Airbus). I’m not sure what is the technicality behind that. So I started delaying APU start after landing and since then never had an APU FAULT and second attempt.

Airbus (320) FCOM states “Wait at least 3 s before selecting APU START pb-sw.” and as stated by someone else here that’s probably just for the ECB to perform it’s power-up test before crew initiates the start sequence. Never heard or read about 3 min.

CP
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Old 8th Jul 2019, 10:05
  #38 (permalink)  
 
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Only pilots could complicate the simple act of using a switch that has 3 positions - on, off and start.
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Old 8th Jul 2019, 10:11
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Originally Posted by Occy View Post
Only pilots could complicate the simple act of using a switch that has 3 positions - on, off and start.


Some people think too hard. I wait a few seconds before starting the APU, as recommended (A320). I've seen more than one person push the start switch immediately after the master though.
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Old 8th Jul 2019, 11:28
  #40 (permalink)  
 
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Only pilots could complicate the simple act of using a switch that has 3 positions - on, off and start.
Crying with laughter

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