BigGiraffe
Topic Author
Posts: 256
Joined: Sat Jan 29, 2000 7:58 am

Why Pull Forward To Reverse?

Wed Feb 16, 2000 11:20 am

There has been a good discussion over the last few days on reverse thrust. American Airlines routinely uses reverse thrust to back away from the gate. When they do this, why do they move the acft forward with the engines prior to reversing, rather than simply reversing from a standing position?
 
VC-10
Posts: 3546
Joined: Tue Oct 26, 1999 11:34 am

RE: Why Pull Forward To Reverse?

Wed Feb 16, 2000 11:46 am

Hello again - I would guess it is easier to break the "sticktion". i.e While the a/c has been sat on the gate the a/c would have settled in postion, to break that "sticktion it is easier to roll fwd as the thrust vector is greater for a given power setting in fwd thrust than reverse

Rgds

VC-10
 
AAR90
Posts: 3140
Joined: Fri Jan 21, 2000 11:51 am

RE: Why Pull Forward To Reverse?

Wed Feb 16, 2000 2:06 pm

Reverse thrust powerback limit is 1.3EPR.
Forward thrust ground ops limit is 1.2EPR.

As noted elsewhere, all the forward 1.2EPR is available to move the acft while only a small portion of the 1.3EPR reverse thrust is actually used in the intended direction of travel.

After sitting a short period of time, the tires become "flat-spotted." Rolling forward moves the acft off the flat spot and makes for easier (less power required) powerbacks. Less power means less chance of foreign object being sucked into the intakes.

Haven't looked at the MD90 powerback procedures, but I'm sure I'll see plenty of them during this week's training. ;-)
*NO CARRIER* -- A Naval Aviator's worst nightmare!
 
Buff
Posts: 1066
Joined: Fri Mar 16, 2007 1:29 pm

RE: Why Pull Forward To Reverse?

Wed Feb 16, 2000 2:36 pm

Two good answers to another good question. Many types of airplanes taxi in reverse - lots of turboprops. At many airports though as far as jets go, "backing" out of the gate is prohibited, usually for noise reasons, but also local safety regulations.

Another reason to taxi forward briefly is to ensure the nose wheel is straight and the chocks are removed!

Stopping an airplane that is moving backwards can be exciting if not done properly: don't touch those brakes!

Best Regards,

Buff
 
seb146
Posts: 13794
Joined: Wed Dec 01, 1999 7:19 am

RE: Why Pull Forward To Reverse?

Wed Feb 16, 2000 4:08 pm

Buff, you make a good point about the safety of using reverse thrust to back out of the gate. I have only seen it once--Contenintal DC-9 at PDX. I guess my question is: Where did BigGiraffe see it?
Life in the wall is a drag.
 
BigGiraffe
Topic Author
Posts: 256
Joined: Sat Jan 29, 2000 7:58 am

RE: Why Pull Forward To Reverse?

Wed Feb 16, 2000 8:56 pm

Thank you for your answers on this. I never would have thought the tires take a set so quickly, but that and your reminder about the difference in power between fwd and rev make sense. And it is easy to see why you wouldn't want to use more power than required.

I routinely ride between ATL and DFW and this procedure is used on both ends by American. Aircraft are 727, MD-80, F100; can't speak for the 757 and up. Delta backs out using tugs.

Next time I'll try to discern whether they are using brakes are fwd thrust to stop the backwards travel. Is idle thrust strong enough that you require brakes to hold position?
 
Buff
Posts: 1066
Joined: Fri Mar 16, 2007 1:29 pm

RE: Why Pull Forward To Reverse?

Wed Feb 16, 2000 11:00 pm

Using brakes to stop yourself while in reverse taxi can very easily set you on your tailskid. It depends on how fast you're going, your loading (aft cg) and how hard the brakes are applied.

The procedure to stop reverse taxiing is to stow the reverser mechanism. And yes, the aircraft will more than likely start to move forward at idle thrust. Again, depending on loading, pavement integrity (hot asphalt can cause an aircraft to "settle in"), individual aircraft type.

Modern turboprops are rarely operated at Idle on the ground. They have so much power that it is almost possible to take off with it. Now before the guns come out, I used to demonstrate why power MUST be at idle for landing the Dash 8: with a lightly loaded airplane and a long backtrack (back taxi in some books), I would leave the power levers at flight idle and watch the indicated airspeed climb to nearly takeoff speed. Not an indelicate exercise, but demonstrated when needed!

Although I stated earlier that reverse taxiing of jets is prohibited at a great many airports, obviously it is not prohibited at all! I've seen DC-9 type a/c reverse taxi at many Canadian airports, some US airports and a very few European.

On the topic of "how fast" do 757's or other large a/c backup? No faster than the marshaller can walk. Backing up without marshallers is like unprotected you-know-what: you might get away with it some of the time, but...

Best Regards,

Buff
 
AAR90
Posts: 3140
Joined: Fri Jan 21, 2000 11:51 am

RE: Why Pull Forward To Reverse?

Wed Feb 16, 2000 11:23 pm

>I routinely ride between ATL and DFW and this
>procedure is used on both ends by American.

Used just about anywhere the airport permits it.

>Aircraft are 727, MD-80, F100; can't speak for the
>757 and up.

When AA first got the 757 we had powerback procedures. After the first couple of FODed engines, they were eliminated.

>Next time I'll try to discern whether they are using
>brakes are fwd thrust to stop the backwards travel.

Better be fwd thrust per procedure. ;-)

>Is idle thrust strong enough that you require brakes
>to hold position?

Depends upon acft and its weight. F100--DEFINITELY!
MD80--up to about 1/2 max gross weight. Lots of variables involved.
*NO CARRIER* -- A Naval Aviator's worst nightmare!
 
BigGiraffe
Topic Author
Posts: 256
Joined: Sat Jan 29, 2000 7:58 am

RE: Why Pull Forward To Reverse?

Thu Feb 17, 2000 7:10 am

Well, I've learned some neat things through this question. Didn't know tires took a set so quickly; didn't know you aren't supposed to use brakes when traveling backwards; didn't know reverse thrust was limited...

AAR90 - Sorry I hadn't realized you are an AA captain prior to now. Otherwise I wouldn't have wrote about waiting until my next flight. I'll blame my cold.

Thanks to everybody,
BigGiraffe
 
AAR90
Posts: 3140
Joined: Fri Jan 21, 2000 11:51 am

RE: Why Pull Forward To Reverse?

Thu Feb 17, 2000 3:24 pm

>Sorry I hadn't realized you are an AA captain prior
>to now. Otherwise I wouldn't have wrote about
>waiting until my next flight.

If done properly, you won't feel the engines coming out of reverse... you'll hear it only.

I still laugh at the big deal AA makes about not touching the brakes during powerbacks. Never was a problem for me. Of course I'm used to backing an aircraft up, making 90+ degree turns, parking within 1-2 inches of other aircraft/equipment and all using reverse thrust while the "ground" is tilting and moving at 30 or more knots.

Oh yeah, when you park your "pax" are hanging out over nothing but water (turn right out the door or you get wet). Touch the brakes and the 80 million dollar aircraft and its occupants go swimming... assuming one survives the fall. Naw, never had a problem with brakes during powerbacks. ;-)

ex-Hummer Driver CVN-65
*NO CARRIER* -- A Naval Aviator's worst nightmare!

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