DC-9 Thrust Reverse from Gate

[Steveair]

Just a question for you guys who have the priviledge of not having to use a tug to back up...

Why do you roll forward at first before you start backing up? Is it just what happens when you take the parking brake off?

 

[typhoonpilot]

Why do you roll forward at first before you start backing up? Is it just what happens when you take the parking brake off?

No, it's a question of momentum or lack there of. It would take a significant amount of thrust reverse to start the airplane rolling backwards from a standstill, kind of an unsticking effect, if you will. By rolling forward first you actually require a smaller amount of reverse thrust to reverse direction and start rolling backwards.

The other big thing is not to use the brakes when rolling backwards .


TP

 

[PowerCurve]

As the aircraft sits at the gate, the tires get flat spots where they make contact with the ramp. The roll-forward part of the initial power-back is for the mains to move off of the flat spot.

If you look at the powerback coordinator at the nose you'll see 1)the chocks out signal, 2)the come forward signal, 3)the stop signal, 4)the powerback signal, 5)the come ahead signal once again at the end of the powerback. Forward thrust is used to stop the powerback, never the brakes.

Hope that helps.

 

[Jeff Helgeson]

Typhoonpilot and powercurve...

are both correct. Their combined response is my response.

Jeff

 

[340drvr]

Unsticking and getting off tire flat spots, yes; Momentum? No. Go back to freshman physics, conservation of momentum, there's no way it takes less steam to reverse direction of travel and start moving the other way.

 

[Spinn555]

Unsticking and getting off tire flat spots, yes; Momentum? No. Go back to freshman physics, conservation of momentum, there's no way it takes less steam to reverse direction of travel and start moving the other way.

I'd have to disagree with 340. Live in a cold climate? Ever try and get a car unstuck out of snow? What do you do? You rock the car back and forth, and gain some momentum.

I suppose the same goes for stuck in mud items also.

 

[421Driver]

You don't gain any momentum for backing up by first going forward. That is like saying sprinters should step backwards before they run forward to "gain some momentum". Look up static versus kinetic friction. Flat spots on the tires and not having to overcome static friction are the reasons for rolling forward.

 

[146guy]

yeah, using the brakes while rolling backwards equals an airplane sitting on its tail. Not a good way to start a flight.

 

[340drvr]

Thanks, 421Driver, I was starting to think I was the only one thinking about this properly. The snow-mudhole routine is all about rocking up and out the far side of a divot hole, not reversing momentum.

 

[Propsync]

I thought they moved forward first because you have to spool the engines before you can send the buckets. Didn't think you could put the reversers in at idle. Just my thinking on it. Have never done it, but seen it often. Also, if I'm not mistaken, Nortwest also still does this in their 9's.

 

[Stifler's Mom]

I thought the rolling forward part was just to scare the $hit out of the remaining passengers in the terminal.

 

[CWASaab]

As mentioned the forward movement prior to the powerback is to get the aircraft off of the flat spots in the tires. The reason this is done is that engines are designed to move an airplane forward, and are considerably more efficient at moving an airplane forward than backward. In theory you could go backwards straight off, but to generate the required (reverse) thrust to breakaway from the parking spot you could very well exceed engine limitaions and cause damage.

 

[TWA Dude]

I thought they moved forward first because you have to spool the engines before you can send the buckets.

The buckets are hydraulically actuated and could even deploy with the engines shut down.

 

[Jeff Helgeson]

A valide point!

The buckets are hydraulically actuated and could even deploy with the engines shut down.

Accumulator pressure will deploy the buckets while sitting at the gate, engines off.

In addition, the MX manual and GOM (POH) states to start the engines with the buckets deployed if there is a tail wind greater than 20 knots. This is to avoid a hot start (475 egt). I was in this situation in IAH for a fuel tech stop in high winds on the tail. I spoke to MX control and decided to have the tug put the nose in to the wind before start.

Okay FLX757, put off that intercompany memo and break out your manuals, but don't spank me if the limitatoin was 10kts!

Jeff

 

[flx757]

Okay FLX757, put off that intercompany memo and break out your manuals, but don't spank me if the limitatoin was 10kts!

Jeff

No spanking necessary Jeff, my man. The book says...and I quote...

"When the airplane is exposed to exceptionally strong tailwinds (in excess of approximately 20 kts) and gate layout prevents an alternate position for starting...", etc, etc...

I was going to bring up the fact that you don't even need the engines running to deploy the thrust reversers earlier, but I let it slide figuring someone else would.

And on a completely different subject, since I'm typing and don't want to start a separate thread...so those reading here get the scoop before anyone...Alaska Airlines to start SEA-DFW service with 2 daily round trips beginning in July. Ya heard it here first...

 

[LarryinTN]

In addition, the MX manual and GOM (POH) states to start the engines with the buckets deployed if there is a tail wind greater than 20 knots.

No such limit on the DC9s (-10 through -40). I guess that's on the bigger engines on the -80 and later?

I've started with JT8D (-7, -9, -11, -15) with significant tailwinds without any tendancy to hot start. I had not previously heard of the start with the T/R deployed procedure. Makes sense, though.

 

[AA717driver]

I believe it was TWA's policy to push the aircraft with the tail into the wind whenever possible.

TC
(Witness to numerous RB211 hot starts with a 20 kt tail wind... )