the effect of reversers on landing distance question

[torrence]

I remember some of those discussions from PS1 days also, John

The "$6M man" sounds a bit like an avi-urban legend, but clearly using rev thrust for pushing back has likely never been used regularly.  But if it ever was mentioned in AOM's as even a 'pushback assist' it suggests that some real negative thrust is generated somehow.

Cheers,
Torrence

[John H Watson]

But if it ever was mentioned in AOM's as even a 'pushback assist' it suggests that some real negative thrust is generated somehow.

Of course, we have to distinguish between negative and neutral thrust. At idle forward thrust, there is still thrust opposing the pushback tug. Using reverersers to reduce or negate that forward thrust will assist the tug.

[torrence]

Ah, yes.  I tend to think about these cases in pure physics terms (i.e. not real life). So pos/neg thrust for me relates to the behavior of a fictional aircraft on a level surface, no wind, no tug etc.  With engines on, definitely neutral thrust with reversers on would be easier to move.  I was assuming pushback before engine start since at  least in my experience in the US as a passenger, most pushbacks at crowded airports are with engines off, with engine start after tug disconnected with plane pointed toward taxiway. So I assume what is involved in an 'assisted pushback'  depends on what circumstances would require it - close to max wt., under-powered tugs, poor runway conditions, adverse slopes, Menehunes pushing against the main gear, etc. Are there specific procedures for what you do if the tug can't get the Queen moving?

Cheers
Torrence

[John H Watson]

Are there specific procedures for what you do if the tug can't get the Queen moving?

You might have to ask those engineers working in more inclement regions. I haven't heard of any issues at my local airport in the last 20 years. In that time, the tugs have become heavier and more powerful, the tarmac has been grooved and some of the tugs lift the aircraft nosewheel off the ground (using the weight of the aircraft to get better traction). Also, extremely wet conditions in my area were usually associated with thunder and lightning. In which case, OH&S rules wouldn't let us push the aircraft.

Yes, if the aircraft was particularly heavy or the tow involved a significant period of forward movement, we might delay engine start until after the tug had disconnected. Otherwise, the engines were started during pushback. Some airline crews were particularly impatient and would want start permission before the aircraft had travelled half it's own length. The vehicle roadways on the tarmac which run behind the tail of aircraft at the gates are often littered with foreign objects, so it's usually best to wait until the aircraft have cleared these during push.

[emerydc8]

I just wanted to follow up on this thread with some new info. I can verify that the 744 reversers with the CF6 engines can actually cause the aircraft to taxi backwards. At my airline, a captain was recently invited to the room with the long table (and no ash trays at his end) for doing just this. In his defense, it was at a military base and he missed the taxiway he was planning to take and realized too late that the taxiway he took was unsuitable. There was no tug available at the time. He was able to back the aircraft a significant distance.

Jon D.

[IefCooreman]

Scientifically it is possible as long as the reversers are not exactly perpendicular but the airflow is pointed slightly forward (which is the case).
Reality check includes dangers of FOD, extreme loads on the engine depending on the amount of thrust, dangers for ground crew as the jetblast area is completely different and nobody on the ground knows about it,...

In other words, trying it to push back a 747 in todays world of aviation is plain stupid :-)

[Britjet]

Further to my previous post that the BA sims don't go backwards with reverse selected...they do..
I have had numerous opportunities to see this recently when demonstrating reverser function when stationary.
I was initially surprised. (You wouldn't be able to test it for real due to engine surge, and of course it is prohibited anyway)
Apologies, Hardy.

Peter.

[emerydc8]

Thanks, Peter.

Did you happen to notice if the spoilers extended when you powered back in the sim (#2 or #4 in reverse)?

Jon

[Britjet]

The spoilers would have been out anyway, (after landing) but I'm sure they would come out..
Peter..

[Hardy Heinlin]

There is a mechanical link between the speedbrake lever and the #2 & #4 reverser levers. They lift the speedbrake lever to the ARM position mechanically. There is no electrical logic involved. This effect can also be seen in the lever animation in PSX: just pull a little on #2 or #4 and watch the speedbrake lever moving a few millimeters off the DN position.

On the ground, whenever the lever is in the ARM position (moved by that mechanical link or directly by the pilot's hand), the electrical logic engages the servo that drives speedbrake lever to the UP position.


|-|ardy

[John H Watson]

Did you happen to notice if the spoilers extended when you powered back in the sim (#2 or #4 in reverse)?

All the autospeedbrake logic is satisfied if:

2 or 4 reverser pulled
aircraft on ground
hydraulics 1 & 4 pressurised
elec power to relevant components

I recall that on certain aircraft, you can pull the Hydim CB's to simulate hydraulic pressure even with the hydraulics off, but, of course, only the speedbrake handle will move.

[John H Watson]

They lift the speedbrake lever to the ARM position mechanically.

Strictly speaking, the reverse levers don't really move the lever to the ARM position, they just lift the lever out of the detent. Pulling on the reversers activates a bypass switch & electrical circuit which overrides the normal ARM electrical switch. The mechanical lifting of the speedbrake lever (with reverser application), just gets it out of the slot (so that when the servomotor activates, it can actually move the lever).

Bypass circuit

Rgds
JHW

[Hardy Heinlin]

It gets it out of the slot, that's what I mean.


|-|ardy

[Avi]

All the autospeedbrake logic is satisfied if:

2 or 4 reverser pulled
aircraft on ground
hydraulics 1 & 4 pressurised
elec power to relevant components

One thing is missing here: throttles 1 and 3 should be at IDLE (or very close to it).

Cheers,

[cagarini]

It moves back, but never accelerates past 4nm GS backwards :-)

I can't takeoff this way :-(

[Jeroen Hoppenbrouwers]

It's not a DAF 33.      :-)