cba
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Power Back

Thu Aug 08, 2002 1:58 pm

I've heard the term "Power back" used before. I know it's associated with pushback, but how exactly does it work?
 
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RE: Power Back

Thu Aug 08, 2002 2:09 pm

Basically...

The airplane uses its thrust reversers to "power-back" the aircraft. Since the aircraft is doing so under its own power, the engines are started at the gate, the aircraft moves slowly ahead a couple feet (to move off the flat spots on the tires) and then deploys its thrust reversers. It's an experence to see and feel. After the aircraft is away from the gate, forwart thrust is applied to slow, and stop the aircraft. This is important. Using the brakes to stop, during reverse motion, would shift the C of G aft the main landing gear, causing the aircraft to pivot around the main landing gear (acting as a fulcrum), and strike its tail against the ground.

After that, the aircraft can taxi out for departure.

This procedure is used as least often as possible, however, it is sometimes required due to lack of a tug. It may also be used if the aircraft cannot sit out on the ramp for the time it’ll take to start the engines (such as in a high ground traffic density area).

Hope I’ve explained it!

Cameron
 
AJ
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RE: Power Back

Thu Aug 08, 2002 2:32 pm

Photographic evidence:

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Photo © Thomas Millard

 
bio15
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RE: Power Back

Fri Aug 09, 2002 5:48 am

Something not mentioned is that powerbacks are only performed on high engine aircraft such as the DC-9, B727, MD-80/90s, B717s. Since the engine is higher, there is a low risk of debris ingestion by the engine. With wing mounted engines, there is a lot of ground debris blown toward the front when the reverser is activated, thus the risk of ingestion.

-bio
 
flight152
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RE: Power Back

Fri Aug 09, 2002 5:52 am

Not always Bio15. The 737-200 was certifited for powerbacks.
 
AJ
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RE: Power Back

Fri Aug 09, 2002 11:11 am

Also the ATP and Metros.
 
PPGMD
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RE: Power Back

Fri Aug 09, 2002 11:19 am

Certified to power back? Is that something that has to be entered in the aircrafts Operations Handbook or what?

I know that the Rice Rocket (MU-2) can be powered back, but I wasn't sure about any type of certfication.
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goboeing
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RE: Power Back

Fri Aug 09, 2002 11:26 am

I know that the Air Florida 737-200 that crashed into the Potomac River on departure from DCA did a powerback before takeoff. I think this was being considered during the investigation. There's a risk of foreign object damage (FOD), like rocks, little stuff like that laying on the ramp, can get sucked in. I just looked, but could not find, a funny picture of a baggage cart that got sucked into a Delta L-1011 engine. If someone finds it, please post it!

Nick
 
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RE: Power Back

Fri Aug 09, 2002 11:43 am

Yeah, I should have mentiond that F.O.D. is a very serious power-back issue.

Generally (company policy with most airlines) the marshallers who guide the aircraft with batons are required to wear saftey goggles during the power-back.

Cameron
 
airplay
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RE: Power Back

Fri Aug 09, 2002 11:54 am

"Certification" for "power-backs" is generally a subject of operating procedures rather than aircraft certification.

Although some airplane flight manuals have a "limitation" that restricts power back operations, it's usually up to the individual airline SOPs and the particular airport rules. Many airports prohibit power-backs.

In my opinion, "power backs" should only be used in special circumstances. They introduce unnecessary risk to personnel at the gates and accelerate the wear and tear of the structures in and around the gate. All that extra soot isn't great either.
 
ngr
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RE: Power Back

Mon Aug 12, 2002 9:00 am

http://www.boeing.com/commercial/aeromagazine/aero_06/textonly/s02txt.html

About engine thrust hazards. (diagrams at the bottom)

I'm trying to find the baggage cart in the L-1011 engine!!!!!!  Smile/happy/getting dizzy

ngr
 
Guest

RE: Power Back

Sun Aug 18, 2002 1:30 am

The only airplanes that use the powerback feature have the bucket type of reverses, I.E. the DC-9s and their variants, and the 737-200, all other aircraft use a different style of thrust reversers. Sorry if I missed an aircraft not a big fan of the smaller ones.
 
shaun3000
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RE: Power Back

Sun Aug 18, 2002 4:25 am

What about the C-17? It doesn't have buckets, does it?
 
concorde1518
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RE: Power Back

Sun Aug 18, 2002 5:32 am

Didn't Eastern power back their 757's also? I thought that the 757 had the sliding collar (sorry, I dont know that exact word) type reversers?


 Smile
 
mirrodie
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RE: Power Back

Sun Aug 18, 2002 5:52 am

CLL777-

The 72S also powers back regularly from DTW terminals.
I have video of that and it is non-bucket type.


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737doctor
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RE: Power Back

Sun Aug 18, 2002 6:35 am

Goboeing, you are correct about the Air Florida crash. After the tug failed to get enough traction to push the plane, the pilots tried unsuccessfully to power it back. While the snow continued to fall, the plane sat and waited while another tug capable of doing the job was found.
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LMP737
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RE: Power Back

Sun Aug 18, 2002 9:34 am

The coolest power back I've ever seen was an Air Force C-17 at an airshow.
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Guest

RE: Power Back

Sun Aug 18, 2002 9:54 am

Essentially ANY airplane that has reversers could be "powered back"...
First it has to be "approved" and requires proper training...
Location of the engines.. for FOD has to be considered...
I was trained to do power back in 727s...
One time had to do a power back with another type, illegally... the only airport tug available had a dead engine... it was either that or put 180 passengers in hotels for the night, bitching about how bad our airline is...
xxx
Recently, I had to do a "partial" power back with a... 747-200... yes...
Same circumstances, full passenger load, the only tug capable of pushing us back had a failed engine... So they did locate a "small tug" (just heavy and powerful enough to push back a C-150)... they provided "steering" with the tow bar... and with our engines in reverse, we provided the "power"...
xxx
A tug has to do quite a job... in normal circumstances, for any airplanes, not only pushing a heavy aircraft, but also pushing "against" idle thrust, if the engines are started before push back...
(s) Skipper
 
delta-flyer
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RE: Power Back

Sun Aug 18, 2002 11:55 am

Notice the thrust reversers on DC-9's and successors have their axes of symmetry just off vertical? That's to prevent debris from being sucked back into the engine during reverse thrust.

The original DC-9 was apparently designed with a vertical centerline. One of my college classmates had a summer job at Air Canada in 1967 and he he;ped redesign the thrust reverser to the off-vertical configuration. Then Douglas incorporated their design into subsequent DC-9's.

This change was not meant specifically for power-back, though, it was primarily for normal landing.

Pete
"In God we trust, everyone else bring data"
 
sebolino
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RE: Power Back

Mon Aug 26, 2002 5:20 pm

Please excuse my ignorance, I don't see how a thrust reverser behind an engine could make the plane go backward.
The force on the reverser is at most equal to the force on the engine, which would just make a total force of 0.
Or is there something else, like a vacuum behind the reverser ?
Can somebody give me the physical explaination (I guess it's fluid dynamics related, which I don't know much).
 
AmericanAirFan
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RE: Power Back

Sat Jul 17, 2004 1:52 am

Ok I dug this forum up from the archives looking ofr this subject and to answer your long lost question the bucket type are certified to to do it because their reverse thrusters are high powered and push the thrust forward.
"American 1881 Cleared For Takeoff One Seven Left"
 
aogdesk
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RE: Power Back

Sat Jul 17, 2004 2:08 am

Sebolino,

The engine runs at various thrust settings depending on the stage of flight, from FORWARD IDLE at engine start to FORWARD MAX THRUST (which may exceed actual takeoff thrust). In the same respect, once a reverser is deployed, the engine itself will still operate throughout its various power settings. There is no single "reverse" power setting. As the crew lands, the ground spoilers deploy (automatically, if armed), brakes are applied (automatically in some cases), but the reverse thrust is always controlled and adjusted by the pilot performing the landing. So it is entirely possible that an engine is at 80% or more thrust, but only in the other direction hence providing some impressive stopping distances. All of that air/thrust is simply displaced in the opposite direction. Hope this helps...
 
miamiair
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RE: Power Back

Sat Jul 17, 2004 2:18 am

Eastern also powered back with the 757's.
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Starlionblue
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RE: Power Back

Sat Jul 17, 2004 2:38 am

I talked to a CRJ crew the other day and asked about powerbacks.

"It's been done in the simulator... But it's not an accepted procedure".

Also, they used to taxi on one engine so as not to hit the brakes all the time. Maintenance said no. Then they used to taxi on two, but with one of the engines on idle reverse. Maintenance said no again.

"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
FinnWings
Posts: 633
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RE: Power Back

Sat Jul 17, 2004 5:03 am

I haven't ever seen jet aircraft powerback here at Helsinki or any other airport where I have been... However, Finnair ATRs does powerbacks all the time and I have seen some Saab 2000s of Blue 1 doing those as well.

Best Regards,
FinnWings
 
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longhauler
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RE: Power Back

Sat Jul 17, 2004 6:32 am

We used to use a power back procedure in the B737-200C/F at Canadi>n. These aircraft were equipped with a gravel kit. As the nose gear differs in the gravel kit equipped aircraft from the regular B737, quite often we would arrive at American stations and they would not have the proper tow bar available.
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AmericanAirFan
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RE: Power Back

Sat Jul 17, 2004 7:16 am

Ive seen several power backs and experienced them as a passenger. It was cool moving slight forward then rolling backwards.

-AmericanAirFan
"American 1881 Cleared For Takeoff One Seven Left"
 
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Starlionblue
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RE: Power Back

Mon Jul 19, 2004 2:13 am

Sebolino, one of the best metaphors for a jet engine (or any reaction engine including a rocket) is a balloon full of air (like a children's balloon). When the balloon is sealed, the air inside exerts an equal pressure on all the inside surfaces. Therefore, the balloon tends to stay put if you ignore gravity and external airflow (wind).

If you open the nozzle (opening) of the balloon, pressure is released at that location. At the opposite side, the pressure is still exerting a force. The result is an inequality of forces acting on the inside of the balloon. The resultant force propels the balloon away from the opening. You have probably tried this as a kid.

Reaction engines work the same way. The pressure of burning, expanding gases is released at the back (exhaust). At the front of the combustion chamber, the pressure is contained by airflow entering the engine. The inequality of forces means the engine wants to move forward.

When reversing, the pressure is released in a forward direction instead of backward (by redirecting the exhaust). The result is that the engine wants to move backwards since the net pressure on the engine is greater in that direction.

However, engines are designed to move forward, so they are more efficient in normal thrust regimes than while reversing, meaning you get more power in the correct direction.


Simplistic explanation, and maybe someone can help me out. Anyway it's the best I can come up with.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
FL1TPA
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RE: Power Back

Wed Jul 21, 2004 8:08 am

Not to sound too neive, but...

Concerning Eastern's supposed power backing of 757s - don't the thrust reversers on those engines only reverse the bypass air and not the trust-generating turbine air? It would seem to me that there wouldn't be enough thrust generated by only reversing the bypass air. Also, the more you increase power to gain more reverse thrust from the bypass air, the more forward thrust you get from the turbine air.

I'm confused. Can anyone confirm or refute any of the above?

Tim.

<>
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Starlionblue
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RE: Power Back

Wed Jul 21, 2004 8:16 am

FL1TPA, I'm no expert on reversers but the fan typically generates in excess of 80% of the thrust for a modern turbofan. So without considering other issues, the bypass air would be enough.

I'm sure a more knowledgeable A.nutter can shed light on this.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
cancidas
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RE: Power Back

Wed Jul 21, 2004 12:22 pm

A tug has to do quite a job... in normal circumstances, for any airplanes, not only pushing a heavy aircraft, but also pushing "against" idle thrust, if the engines are started before push back...


good point skipper. this is why, at least we at USX at LGA do not allow aircraft to spool engines while pushing them back. our tugs just can't push them if they were to spool.
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