Jet engine run-up

This forum has been developed to discuss aviation related topics.

Moderators: lilfssister, North Shore, sky's the limit, sepia, Sulako, I WAS Birddog

Post Reply
User avatar
FenderManDan
Rank 6
Rank 6
Posts: 490
Joined: Wed Aug 03, 2011 10:40 am
Location: Toilet, Onterible

Jet engine run-up

Post by FenderManDan »

I haven noticed this in a while. Except the b757 doing the run-up in St. Marteen. Just courious why not anymore. As a kid flying often the dc9 and b727 to vacations, I always noticed the crew did the full static run-up after the push back. Is this something to do with the particular airline SOP or something that needed to be done on the older jets with the tail mounted engines?

Cheers
---------- ADS -----------
 
rubberboot
Rank 2
Rank 2
Posts: 65
Joined: Mon Jul 26, 2010 6:42 pm

Re: Jet engine run-up

Post by rubberboot »

Some of the JT8D model engines had Automatic Reserve (ie. JT8D-17AR) Not sure if this is what you are refering to. As I recall, they go through a self test, and the power would momentarily advance to 1.??EPR, then spool down. I don't think it came close though to T/O power. I have never worked on this system, but have heard of it.

r/b
---------- ADS -----------
 
jet a1
Rank 6
Rank 6
Posts: 402
Joined: Sun May 09, 2004 12:35 pm

Re: Jet engine run-up

Post by jet a1 »

the crj's at jazz would always do a little run up of the engines after de-icing...
---------- ADS -----------
 
Gannet167
Rank 7
Rank 7
Posts: 589
Joined: Sat Jan 19, 2008 12:23 pm

Re: Jet engine run-up

Post by Gannet167 »

It really depends on performance and conditions. APR tests are done below max t/o power, but at a fairly high power setting. If the take off is predicated on requiring APR power, on the older planes you generally have to get a valid test before brake release. If you're at the max weight it's very likely that you need the engines at t/o power before brake release to get the required performance - either due to rwy length or climb gradient. On the newer aircraft, I'm guessing they're over powered enough (or lightly loaded enough) that they don't need to run the engines up. Newer planes also have multiple APR computers or FADEC stuff that compensates for the requirement to do an APR dynamic test on the runway before rolling. Having big fans sitting at max t/o setting and a ground speed of 0 really eats into profit margins.
---------- ADS -----------
 
User avatar
Nark
Rank 10
Rank 10
Posts: 2967
Joined: Thu Feb 19, 2004 6:59 pm
Location: LA

Re: Jet engine run-up

Post by Nark »

Yep newer hi-bypass engines are extremely 'electronically' monitored that run ups aren't needed.
FADEC controls everything on the engine, a runup isn't needed.

We have it in our OpSpecs that only one full power takeoff is required per month. All others are generally done with a reduced power settings.
---------- ADS -----------
 
1000islander
Rank 1
Rank 1
Posts: 37
Joined: Fri Jan 27, 2012 10:30 am

Re: Jet engine run-up

Post by 1000islander »

Back in the day.. some US carriers [dc9-727]would power back rather than use a tug -quicker and less manpower/equip req'd - very noisy though and not fuel efficient to say the least.
---------- ADS -----------
 
nightguy
Rank 2
Rank 2
Posts: 72
Joined: Fri Jan 07, 2005 2:13 am
Location: yyc

Re: Jet engine run-up

Post by nightguy »

FADEC deals with run ups on a modern engine. The only time you might see what seems like a run up is during icing conditions. My airplanes AOM calls for ice sheading after 30 min of taxi. Long story short we will ask to stay in position for 30 sec or for a place to go to apply power. We bring the engines one at a time up to about 75% n1 then back to idle. This gets rid of the ice on the first stage fan and as an added bonus it sound cool too.

Oh and btw FADEC = Full Authority Digital Engine Control
---------- ADS -----------
 
Eric Janson
Rank (9)
Rank (9)
Posts: 1362
Joined: Thu Feb 08, 2007 10:44 am

Re: Jet engine run-up

Post by Eric Janson »

FenderManDan wrote:I haven noticed this in a while. Except the b757 doing the run-up in St. Marteen. Just courious why not anymore. As a kid flying often the dc9 and b727 to vacations, I always noticed the crew did the full static run-up after the push back. Is this something to do with the particular airline SOP or something that needed to be done on the older jets with the tail mounted engines?
Full static run up on a jet will overpower the parking brake. Normally if an engine needs to be run at maximum thrust only 1 engine will be at maximum thrust. The aircraft will also need to be chocked in case the parking brake gets overpowered. These procedures are written in the maintenance manual.

What you may be referring to is the engine power being advanced prior to take-off. This is to ensure the engines are at a power setting that will ensure symmetrical acceleration.

Example:- A340 N1 set to 50%. When all engines stabilise at this setting you increase thrust to Flex/TOGA. Normally we do a rolling take-off.
---------- ADS -----------
 
W5
Rank (9)
Rank (9)
Posts: 1004
Joined: Wed Feb 18, 2004 4:44 pm
Location: Edmonton,AB

Re: Jet engine run-up

Post by W5 »

After pushback? Could have been a crossbleed start. They used to be a lot more common.
---------- ADS -----------
 
User avatar
Pugster
Rank 6
Rank 6
Posts: 469
Joined: Mon Aug 09, 2004 9:46 am
Location: B.C. Again!

Re: Jet engine run-up

Post by Pugster »

W5 wrote:After pushback? Could have been a crossbleed start. They used to be a lot more common.
X2...I'm almost positive that's what was happening. FYI if you're not a jet guy, a crossbleed start is when you use bleed air from one engine to start the rest. If the APU is broken and you're hooked up to an external air start cart, sometimes (although I'm not sure why to be honest) crews will elect to start one engine off the cart and the remaining engine(s) via crossbleed. They require the operating engine to be brought up to a fairly high thrust setting (70% is pretty common) in order to get enough duct pressure to start the other engines. My guess is that back in the day when fuel was cheap they'd do it to save time after the pushback, as the ground crews could be disconnecting the air cart while crews were starting the remaining engines...

These days even with a broken APU - the cost of fuel, the noise of run-up, and the liabilities associated with doing a high thrust run on a ramp mean that a cross bleed start is very rare.
I've done only one over the last 6 years, and it was because the APU departed the fix just after we started engine #1. No biggie, crossbleed and you're good to go!
---------- ADS -----------
 
bobcaygeon
Rank 7
Rank 7
Posts: 711
Joined: Sat Mar 12, 2005 8:03 am

Re: Jet engine run-up

Post by bobcaygeon »

At small airports it is common now for a classic "airstart" init/truck to not be available. These airports now use a tank system similar to your compressor at home where a small compressor fills a tank where the compressed air is stored.

It is only good for one start usually. This is why we did crossbleed starts at out stations. If the you don't get a successful start on the first try then you have to wait about 30 minutes for the compressor to fill the tank. It never happened to me.

It is a much cheaper overall system, is effective at airports that aren't that busy, and something as simple as that could have likely prevented Dryden from happening.
---------- ADS -----------
 
Post Reply

Return to “General Comments”