A beginner's doubt on jet engines

[Ramapriya]

I know that a car's engine needs to be mated to the transmission for
propulsion. This I know isn't the case with an aircraft with a jet
engine; if the engine is turned on without the brakes applied, the jet
of air leaving the engine will hurl the craft forward.

My doubt is, why does this forward motion effect not occur during
push-back, when the engines are normally turned on? Is it because at
low revs the engine would lack the punch to initiate the forward motion
of a heavy aircraft? If not, I'd imagine the push-back becomes a bit of
a tiresome affair by the time it's over...

Sorry if this is infuriatingly elementary, but I need to ask somewhere
:)

Ramapriya

[Mike Rapoport]

There is not enough thrust at idle to be a problem.

Mike
MU-2

"Ramapriya" > wrote in message
oups.com...
>I know that a car's engine needs to be mated to the transmission for
> propulsion. This I know isn't the case with an aircraft with a jet
> engine; if the engine is turned on without the brakes applied, the jet
> of air leaving the engine will hurl the craft forward.
>
> My doubt is, why does this forward motion effect not occur during
> push-back, when the engines are normally turned on? Is it because at
> low revs the engine would lack the punch to initiate the forward motion
> of a heavy aircraft? If not, I'd imagine the push-back becomes a bit of
> a tiresome affair by the time it's over...
>
> Sorry if this is infuriatingly elementary, but I need to ask somewhere
> :)
>
> Ramapriya
>
>

[Morgans]

"Ramapriya" > wrote in message
oups.com...
> I know that a car's engine needs to be mated to the transmission for
> propulsion. This I know isn't the case with an aircraft with a jet
> engine; if the engine is turned on without the brakes applied, the jet
> of air leaving the engine will hurl the craft forward.
>
> My doubt is, why does this forward motion effect not occur during
> push-back, when the engines are normally turned on? Is it because at
> low revs the engine would lack the punch to initiate the forward motion
> of a heavy aircraft? If not, I'd imagine the push-back becomes a bit of
> a tiresome affair by the time it's over...
>
> Sorry if this is infuriatingly elementary, but I need to ask somewhere
> :)
>
> Ramapriya
>
>
At idle, turbo fan engines, or turbo jet engines do not make a tremendous
amount of thrust. The push-back tugs are very powerful, with very low
gearing. They simply push harder than the engines are pushing.

Sometimes, the engines are not started until after push-back, or while the
push-back is taking place.

Some of the heavy metal pilots can tell you better than me, but it is my
understanding that it takes well over 50% RPM to get 50% thrust, so it
goes to follow that 10% RPM is way less than 10% thrust.

Where have you been? Kinda' lonely around here, without your constant
questions! <g>
--
Jim in NC

[Jay Beckman]

"Ramapriya" > wrote in message
oups.com...
>I know that a car's engine needs to be mated to the transmission for
> propulsion. This I know isn't the case with an aircraft with a jet
> engine; if the engine is turned on without the brakes applied, the jet
> of air leaving the engine will hurl the craft forward.
>
> My doubt is, why does this forward motion effect not occur during
> push-back, when the engines are normally turned on? Is it because at
> low revs the engine would lack the punch to initiate the forward motion
> of a heavy aircraft? If not, I'd imagine the push-back becomes a bit of
> a tiresome affair by the time it's over...
>
> Sorry if this is infuriatingly elementary, but I need to ask somewhere
> :)
>
> Ramapriya
>
>

Sometimes they don't even need a tug to get away from the gate...

I've been on American Airlines MD80s that have pushed back under their own
power via their reversers.

Jay Beckman
PP-ASEL
Chandler, AZ

[Ron Parsons]

In article et>,
"Mike Rapoport" > wrote:

>There is not enough thrust at idle to be a problem.
>
>Mike
>MU-2
>
>"Ramapriya" > wrote in message
oups.com...
>>I know that a car's engine needs to be mated to the transmission for
>> propulsion. This I know isn't the case with an aircraft with a jet
>> engine; if the engine is turned on without the brakes applied, the jet
>> of air leaving the engine will hurl the craft forward.
>>
>> My doubt is, why does this forward motion effect not occur during
>> push-back, when the engines are normally turned on? Is it because at
>> low revs the engine would lack the punch to initiate the forward motion
>> of a heavy aircraft? If not, I'd imagine the push-back becomes a bit of
>> a tiresome affair by the time it's over...
>>
>> Sorry if this is infuriatingly elementary, but I need to ask somewhere
>> :)
>>
>> Ramapriya
>>
>>

I've seen numerous times on an icy ramp when it was.

--
Ron Parsons

[Ron Parsons]

In article >,
"Morgans" > wrote:

>"Ramapriya" > wrote in message
oups.com...
>> I know that a car's engine needs to be mated to the transmission for
>> propulsion. This I know isn't the case with an aircraft with a jet
>> engine; if the engine is turned on without the brakes applied, the jet
>> of air leaving the engine will hurl the craft forward.
>>
>> My doubt is, why does this forward motion effect not occur during
>> push-back, when the engines are normally turned on? Is it because at
>> low revs the engine would lack the punch to initiate the forward motion
>> of a heavy aircraft? If not, I'd imagine the push-back becomes a bit of
>> a tiresome affair by the time it's over...
>>
>> Sorry if this is infuriatingly elementary, but I need to ask somewhere
>> :)
>>
>> Ramapriya
>>
>>
>At idle, turbo fan engines, or turbo jet engines do not make a tremendous
>amount of thrust. The push-back tugs are very powerful, with very low
>gearing. They simply push harder than the engines are pushing.
>
>Sometimes, the engines are not started until after push-back, or while the
>push-back is taking place.
>
>Some of the heavy metal pilots can tell you better than me, but it is my
>understanding that it takes well over 50% RPM to get 50% thrust, so it
>goes to follow that 10% RPM is way less than 10% thrust.
>
>Where have you been? Kinda' lonely around here, without your constant
>questions! <g>

Thrust and RPM do not have a linear relationship. Idle can be as high as
60%.

--
Ron Parsons

[Corky Scott]

On 22 Dec 2004 22:26:08 -0800, "Ramapriya" > wrote:

>I know that a car's engine needs to be mated to the transmission for
>propulsion. This I know isn't the case with an aircraft with a jet
>engine; if the engine is turned on without the brakes applied, the jet
>of air leaving the engine will hurl the craft forward.
>
>My doubt is, why does this forward motion effect not occur during
>push-back, when the engines are normally turned on? Is it because at
>low revs the engine would lack the punch to initiate the forward motion
>of a heavy aircraft? If not, I'd imagine the push-back becomes a bit of
>a tiresome affair by the time it's over...
>
>Sorry if this is infuriatingly elementary, but I need to ask somewhere

As others have mentioned, there is not so much thrust generated by the
jet engines at idle that the tugs cannot overcome it to push it back.
Take a look at the tugs sometime, they are massively constructed with
extra weight added on so that they have sufficient mass to push and
pull the big jets.

In addition, next time you are take a flight in a jet airliner, notice
how much power the pilot has to make the engines produce in order to
get the airliner rolling.

Corky Scott

[Ramapriya]

Jay Beckman wrote:
>
> I've been on American Airlines MD80s that have pushed back under
their own
> power via their reversers.
>

Oops! Do they have rear-view mirrors? Prima facie, reversing blind
doesn't seem overly safe...

Ramapriya

[Ramapriya]

Hi there Jim, been in the wilderness of northeastern Sudan, laying a
gas pipeline, hence the bit of silence :)
Returned a couple of hours ago (sneaked a visit to the A320 cockpit
until TOD), and will be returning back there in about 3 days for
another month or so, but my laptop works in the deserts too, so the Qs
might just keep comin :p
Cheers,
Ramapriya

PS: Don't know if they trolled me, but both the pilots today didn't
seem to know much of the CFM56's internals. Possibly they didn't want
to bother answering, but I'll have to look elsewhere to figure the
compressor's and ignition chamber's construction...

[Garner Miller]

In article . com>,
Ramapriya > wrote:

> Jay Beckman wrote:
> >
> > I've been on American Airlines MD80s that have pushed back under
> > their own power via their reversers.
>
> Oops! Do they have rear-view mirrors? Prima facie, reversing blind
> doesn't seem overly safe...

They have wing-walkers to direct the reversing, just as they direct
them into the gate. They aren't doing it blind at all. I've done it
in a turboprop; it's a very strange sensation. AA seems to do it more
than any other airline; not sure why.

--
Garner R. Miller
ATP/CFII/MEI
Clifton Park, NY =USA=

[Robert M. Gary]

Ramapriya wrote:
> I know that a car's engine needs to be mated to the transmission for
> propulsion. This I know isn't the case with an aircraft with a jet
> engine; if the engine is turned on without the brakes applied, the
jet
> of air leaving the engine will hurl the craft forward.
>
> My doubt is, why does this forward motion effect not occur during
> push-back, when the engines are normally turned on? Is it because at
> low revs the engine would lack the punch to initiate the forward
motion
> of a heavy aircraft? If not, I'd imagine the push-back becomes a bit
of
> a tiresome affair by the time it's over...

As others have said, the tug can certainly over power it. However, I
have read accounts of pilots from the 727 days saying that at idle, the
engines did put out a lot of thrust and that on landings it added a lot
of float. Will most jets roll on the idle engines if the brake is not
set?

-Robert

[Hilton]

Robert M. Gary wrote:
> Will most jets roll on the idle engines if the brake is not set?

I spoke with an F-15 pilot at Nellis AFB and he said the F-15 will reach
about 80 knots at idle.

Hilton

[BTIZ]

> PS: Don't know if they trolled me, but both the pilots today didn't
> seem to know much of the CFM56's internals. Possibly they didn't want
> to bother answering, but I'll have to look elsewhere to figure the
> compressor's and ignition chamber's construction...

too many questions.. to the wrong person or persons.. may label you a
suspected terrorist..
and why would they give information like that.. to someone they don't know..
and information that is readily available in a public library?
some one may come knocking on your door to check you out...

BT

[Cockpit Colin]

For what it's worth, a 757 at ground idle produces around 3000 pounds thrust
per engine - you can back them using the reversers, but you can't go full
power as the re-ingest the exhaust gasses.

And when you're backing you must always use forward thrust to stop, and not
the brakes - anyone guess why?


"Ramapriya" > wrote in message
oups.com...
> I know that a car's engine needs to be mated to the transmission for
> propulsion. This I know isn't the case with an aircraft with a jet
> engine; if the engine is turned on without the brakes applied, the jet
> of air leaving the engine will hurl the craft forward.
>
> My doubt is, why does this forward motion effect not occur during
> push-back, when the engines are normally turned on? Is it because at
> low revs the engine would lack the punch to initiate the forward motion
> of a heavy aircraft? If not, I'd imagine the push-back becomes a bit of
> a tiresome affair by the time it's over...
>
> Sorry if this is infuriatingly elementary, but I need to ask somewhere
> :)
>
> Ramapriya
>
>

[Morgans]

"Cockpit Colin" > wrote in message
...
> For what it's worth, a 757 at ground idle produces around 3000 pounds
thrust
> per engine - you can back them using the reversers, but you can't go full
> power as the re-ingest the exhaust gasses.
>
> And when you're backing you must always use forward thrust to stop, and
not
> the brakes - anyone guess why?

The disk brakes would roll our of the calipers?
--
Jim in NC

[Morgans]

"Morgans" > wrote

> The disk brakes would roll our of the calipers?
> --
> Jim in NC
>
Make that "out" of the calipers.

[Bob Moore]

"Cockpit Colin" > wrote

> And when you're backing you must always use forward thrust to stop,
> and not the brakes - anyone guess why?

Sure...brakes will set it on its tail. I've backed-up
B-727s frequently.

Bob Moore

[Michelle P]

10% is usually about half way to light off for a turbine engine. Starter
usually cuts out around 50% and stabilize at 60-70% core speed.
Michelle

Morgans wrote:

>"Ramapriya" > wrote in message
oups.com...
>
>
>>I know that a car's engine needs to be mated to the transmission for
>>propulsion. This I know isn't the case with an aircraft with a jet
>>engine; if the engine is turned on without the brakes applied, the jet
>>of air leaving the engine will hurl the craft forward.
>>
>>My doubt is, why does this forward motion effect not occur during
>>push-back, when the engines are normally turned on? Is it because at
>>low revs the engine would lack the punch to initiate the forward motion
>>of a heavy aircraft? If not, I'd imagine the push-back becomes a bit of
>>a tiresome affair by the time it's over...
>>
>>Sorry if this is infuriatingly elementary, but I need to ask somewhere
>>:)
>>
>>Ramapriya

>>
>>
>>
>At idle, turbo fan engines, or turbo jet engines do not make a tremendous
>amount of thrust. The push-back tugs are very powerful, with very low
>gearing. They simply push harder than the engines are pushing.
>
>Sometimes, the engines are not started until after push-back, or while the
>push-back is taking place.
>
>Some of the heavy metal pilots can tell you better than me, but it is my
>understanding that it takes well over 50% RPM to get 50% thrust, so it
>goes to follow that 10% RPM is way less than 10% thrust.
>
>Where have you been? Kinda' lonely around here, without your constant
>questions! <g>
>
>

[Michelle P]

Tail strike! Seen it happen. Brakes grab more in reverse leading to to
quick a stop and rotation over the mains.
Michelle

Cockpit Colin wrote:

>For what it's worth, a 757 at ground idle produces around 3000 pounds thrust
>per engine - you can back them using the reversers, but you can't go full
>power as the re-ingest the exhaust gasses.
>
>And when you're backing you must always use forward thrust to stop, and not
>the brakes - anyone guess why?
>
>
>"Ramapriya" > wrote in message
oups.com...
>
>
>>I know that a car's engine needs to be mated to the transmission for
>>propulsion. This I know isn't the case with an aircraft with a jet
>>engine; if the engine is turned on without the brakes applied, the jet
>>of air leaving the engine will hurl the craft forward.
>>
>>My doubt is, why does this forward motion effect not occur during
>>push-back, when the engines are normally turned on? Is it because at
>>low revs the engine would lack the punch to initiate the forward motion
>>of a heavy aircraft? If not, I'd imagine the push-back becomes a bit of
>>a tiresome affair by the time it's over...
>>
>>Sorry if this is infuriatingly elementary, but I need to ask somewhere
>>:)
>>
>>Ramapriya

>>
>>
>>
>
>
>
>

[G.R. Patterson III]

Ramapriya wrote:
>
> PS: Don't know if they trolled me, but both the pilots today didn't
> seem to know much of the CFM56's internals.

Perhaps they don't know that much about the internals. Ernest Gann said he never
understood why American Airlines felt it necessary that he be able to answer
detailed questions about the workings of the radial engines he flew behind. He
couldn't imagine climbing out on the wing to affect repairs in flight, though he
said "there would be times when I fervently wished it were possible."

George Patterson
The desire for safety stands against every great and noble enterprise.

[Cockpit Colin]

10 Points to Bob, who got it first :)

Merry Christmas everyone.


"Cockpit Colin" > wrote in message
...
> For what it's worth, a 757 at ground idle produces around 3000 pounds
thrust
> per engine - you can back them using the reversers, but you can't go full
> power as the re-ingest the exhaust gasses.
>
> And when you're backing you must always use forward thrust to stop, and
not
> the brakes - anyone guess why?
>
>
> "Ramapriya" > wrote in message
> oups.com...
> > I know that a car's engine needs to be mated to the transmission for
> > propulsion. This I know isn't the case with an aircraft with a jet
> > engine; if the engine is turned on without the brakes applied, the jet
> > of air leaving the engine will hurl the craft forward.
> >
> > My doubt is, why does this forward motion effect not occur during
> > push-back, when the engines are normally turned on? Is it because at
> > low revs the engine would lack the punch to initiate the forward motion
> > of a heavy aircraft? If not, I'd imagine the push-back becomes a bit of
> > a tiresome affair by the time it's over...
> >
> > Sorry if this is infuriatingly elementary, but I need to ask somewhere
> > :)
> >
> > Ramapriya
> >
> >
>
>

[Marty]

"Cockpit Colin" > wrote in message
...
> 10 Points to Bob, who got it first :)
>
> Merry Christmas everyone.
>
>
Tail strike was my first thought only because I did it in my TriPacer when a
friend got over zealous pushing me back into a tiedown. We had just got back
from OSH and the back seat & cargo was full of gear tho. ;-)

Merry X-Mas to all,

Marty

[Greg Faris]

Never seen jets do this. I have seen turboprops do it at one airport
(Southhampton UK). I would think airport ops would be worried about loose
tools, articles debris (like ramp agents) getting thrown through the
terminal's picture windows if heavier aircraft start doing this!

G Faris

"Jay Beckman" > wrote in message
news:3buyd.350$yW5.320@fed1read02...
> "Ramapriya" > wrote in message
> oups.com...
> >I know that a car's engine needs to be mated to the transmission for
> > propulsion. This I know isn't the case with an aircraft with a jet
> > engine; if the engine is turned on without the brakes applied, the jet
> > of air leaving the engine will hurl the craft forward.
> >
> > My doubt is, why does this forward motion effect not occur during
> > push-back, when the engines are normally turned on? Is it because at
> > low revs the engine would lack the punch to initiate the forward motion
> > of a heavy aircraft? If not, I'd imagine the push-back becomes a bit of
> > a tiresome affair by the time it's over...
> >
> > Sorry if this is infuriatingly elementary, but I need to ask somewhere
> > :)
> >
> > Ramapriya
> >
> >
>
> Sometimes they don't even need a tug to get away from the gate...
>
> I've been on American Airlines MD80s that have pushed back under their own
> power via their reversers.
>
> Jay Beckman
> PP-ASEL
> Chandler, AZ
>
>

[Bob Moore]

"Greg Faris" > wrote

> Never seen jets do this. I have seen turboprops do it at one airport
> (Southhampton UK). I would think airport ops would be worried about
> loose tools, articles debris (like ramp agents) getting thrown through
> the terminal's picture windows if heavier aircraft start doing this!

Yes...here in the US, the FAA grants permission for "Power-Back"
operations at specific gates and specific aircraft. Pilots
receive special training before they are allowed to conduct
Power-Back operations. I did Power-Back operations in B-727 a/c.

Bob Moore
ATP B-707 B-727
PanAm (retired)