yyzala
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Slowing Down Only With Thrust Reversal?

Sun Nov 29, 2009 10:42 pm

I am wondering if it is possible to slow down a light airplane with few passengers on a long runway such as Almaty (4500m) using only thrust reversal. How about an airplane that is right at Maximum Landing Weight? Or would this be against company SOPs where they require auto-braking selected prior to landing?
 
tdscanuck
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RE: Slowing Down Only With Thrust Reversal?

Sun Nov 29, 2009 11:00 pm



Quoting YYZALA (Thread starter):
I am wondering if it is possible to slow down a light airplane with few passengers on a long runway such as Almaty (4500m) using only thrust reversal.

Yes, although it's not a great idea on many airplanes since many of them recommend stowing reversers at 60-80 knots. If you keep running them at low speed, you run the risk of the engine re-ingesting its own exhaust.

Quoting YYZALA (Thread starter):
How about an airplane that is right at Maximum Landing Weight?

Unlikely, unless it's a really small plane on a really long runway.

Quoting YYZALA (Thread starter):
Or would this be against company SOPs where they require auto-braking selected prior to landing?

If the company had that SOP, then it would be against it.

Tom.
 
Stealthz
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RE: Slowing Down Only With Thrust Reversal?

Sun Nov 29, 2009 11:20 pm

Funny how things turn full circle.
My father's cousin was very high up in engineering at QF, I recall him saying one of the reasons they moved to RR engines on the 747* was that they could use the reversers down to stopped, and save on brake wear. Nowadays with carbon-carbon brakes it is better to use the brakes rather than the engines.

Cheers

* I want to say on their 2nd buy of 742 but may have been 743
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tb727
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RE: Slowing Down Only With Thrust Reversal?

Sun Nov 29, 2009 11:21 pm

We can typically get the airplane pretty slow using just reversers. On the 727 you can leave the engines in idle reverse right down to taxi speed but nothing above 1.1 EPR once you are below about 80 KIAS. The trick with the JT8D's is not letting the engine spool down to idle for too long in the landing for it to be most effective when you go into reverse.

I'm just now getting comfortable with the feel of the throttles to the point I am pulling them much past reverse idle so I am using less brakes. If you have about 8000' of runway, you should only have to touch the brakes at the end to make the turn off.
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mrskyguy
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RE: Slowing Down Only With Thrust Reversal?

Mon Nov 30, 2009 2:25 am



Quoting Tdscanuck (Reply 1):
Yes, although it's not a great idea on many airplanes since many of them recommend stowing reversers at 60-80 knots. If you keep running them at low speed, you run the risk of the engine re-ingesting its own exhaust.

I'm not an engine man, so forgive me if this is ignorant.. but what's the problem with the engine re-ingesting its own exhaust? Flame-out due to asphyxiation? I know NWA used to (not sure if this is still common practice) use reverse thrust to "push back" their aircraft from the gate from a dead stop.
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tb727
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RE: Slowing Down Only With Thrust Reversal?

Mon Nov 30, 2009 2:46 am



Quoting MrSkyGuy (Reply 4):

I'm not an engine man, so forgive me if this is ignorant.. but what's the problem with the engine re-ingesting its own exhaust? Flame-out due to asphyxiation? I know NWA used to (not sure if this is still common practice) use reverse thrust to "push back" their aircraft from the gate from a dead stop.

No the problem is with FOD damaging the engine. At lower speeds you have a better chance at kicking stuff forward and ingesting it. NW used to do powerbacks but I don't think they do them anymore because of fuel costs.
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n901wa
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RE: Slowing Down Only With Thrust Reversal?

Mon Nov 30, 2009 2:54 am

I really don't know about the landing and rollout and how much TR is used. It might depend on the carrier and on the weather, But as a Mechanic everytime I land on a Long Runway and the crew uses little Reverse I think about changing another Brake  Smile At least it not as bad as it was on a 727 but Brake changes suck. Im glad we don't have snow and I really feel for the Mechanics in snow stations,with a iced over tarmac. I don't know about the Pilots use of TR's on the gnd, But we use to back up 727 using TR's and Heavy MD-11 and L-1011's in the rain to help the push crew pushback off the gate. If I remember right I think it was AA in ATL that use to pushback with TR's off the gate (Might have been Eastern with 9's) with MD-80's. I remember one thing. When Backing up using TR's Don't hit the Brakes  Smile stow the Buckets and push the throttles fwd. I think it depends on what type of aircraft, and the crew.  Smile
 
pilotpip
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RE: Slowing Down Only With Thrust Reversal?

Mon Nov 30, 2009 5:27 am

Flying something with buckets like the 145 I used the reverse a lot. You could feel the drag even at idle. It's much more effective than cascades in the 170. You really feel them above 100kts but they quickly diminish in effectiveness. By the time the engines spin up you don't feel much unless you're heavy/have icing speeds. We're prohibited by SOP from using any more than idle below 60.

Modern Carbon brakes and antiskid are much more effective as has been discussed before. Next time I'm in the hangar I'll try and find out how often our mechanics are replacing brakes.
DMI
 
ex52tech
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RE: Slowing Down Only With Thrust Reversal?

Mon Nov 30, 2009 3:19 pm

One of the limitations of the larger fan engines is N1 rpm in reverse, some are limited to a max reverse N1..........JT9's for instance were 80% in reverse. Something to do with the amount of pressure behind the fan causing the fan blades to drag the case outside the abraidable rub strip.

NWA went through a program where they placed metal wear strips about one inch wide and three inches long on the abraidable material to see how far the blades were flexing, never saw any of them come loose. It was due to the CAT III landing system on the DC-10 being reactivated and the system was causing the airplanes to land long (analog system) and the pilots would then need more reverse thrust in order to stop.

In maintenance we used to power back 727s, MD80s, and DC9s all the time, take them to the hangar. One would not want to touch the brakes on an empty 727 or an MD80 while rolling backwards, you can get a good look at the sky.

Quoting Pilotpip (Reply 7):
Modern Carbon brakes
and antiskid are much more effective as has been discussed before. Next time I'm in the hangar I'll try and find out how often our mechanics are replacing brakes.

Well the general understanding with carbon brakes is that they last twice as long as a standard brake and cost twice as much.
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tb727
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RE: Slowing Down Only With Thrust Reversal?

Mon Nov 30, 2009 3:40 pm



Quoting N901WA (Reply 6):
At least it not as bad as it was on a 727 but Brake changes suck

That's why I like TR's. I still remember when I was a kid and my dad was a NW mech. We landed in CLE and the pilots got on the brakes hard and didn't use hardly any reverse. On the way off the airplane he had a couple words for the pilots and asked if they ever changed the brakes on an airplane outside at night in the winter time, lol. They said they hadn't thought of that.
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747classic
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RE: Slowing Down Only With Thrust Reversal?

Mon Nov 30, 2009 4:00 pm

On the early large turbofan engines (PW JT9D and CF-50) we had on the 747 both fan and turbine reversers. They were very effective and were used between touchdown and 50 kts due reduced effectiveness below this speed and increased FOD injection risk at low speed, to reduce the brake wear of the standard (non carbon) brakes. At 50 kts the RPM had to be at or below 50 %N1.

Later-on the turbine reversers were deleted due high maintenance costs and low reliability.
With only fan reversers installed the effectiveness of the reverser system was halved, but still available, if needed.( runways with reduced friction, due weather.)

After the introduction of carbon brakes on the 747-400 the need for engine reverse (fan only) diminished. It can still be used to save the brakes but is often limited to idle reverse due noise limitations. Despite all said, it is still a very nice (backup) system in adverse weather landings/rejected Take Off's, with reduced runway friction.
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rwessel
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RE: Slowing Down Only With Thrust Reversal?

Mon Nov 30, 2009 11:07 pm



Quoting YYZALA (Thread starter):
I am wondering if it is possible to slow down a light airplane with few passengers on a long runway such as Almaty (4500m) using only thrust reversal. How about an airplane that is right at Maximum Landing Weight? Or would this be against company SOPs where they require auto-braking selected prior to landing?

Assuming the operating limitations allowed it, theoretically yes, but you'd generally need a lot of runway, especially for a heavy aircraft. Remember that the brakes typically generate several times more braking force than do the reversers (at least if the runway is not slippery).
 
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n901wa
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RE: Slowing Down Only With Thrust Reversal?

Mon Nov 30, 2009 11:28 pm

For Tb727. Great story. Im glad your Dad did that  Smile I landed on a 737-800 in atl and they didn't use any T/R and I later checked and both were servicable with no history and they just put on the Brakes. I wonder if they knew how fast 737-800's go thru brakes and They are made of Steel  Smile I hope the -800 get the Carbon Brakes like the -700 we have, But Steel Brakes are nasty. The other thing is Tire Wear  Smile
 
fr8mech
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RE: Slowing Down Only With Thrust Reversal?

Tue Dec 01, 2009 1:58 am



Quoting MrSkyGuy (Reply 4):
I'm not an engine man, so forgive me if this is ignorant.. but what's the problem with the engine re-ingesting its own exhaust? Flame-out due to asphyxiation?

Along with the FOD issue, some engine were/are very sensitive to airflow disturbance at the inlet. The aircraft is slowing down and is getting less 'ram' effect at the inlet. At the same time, when an engine goes into reverse, bleed valves move around to provide optimum airflow through the engine and try to prevent stalls. The air thrown forward can diturb the inlet air and cause stalls.
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tb727
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RE: Slowing Down Only With Thrust Reversal?

Tue Dec 01, 2009 2:37 am



Quoting Fr8mech (Reply 13):
The air thrown forward can diturb the inlet air and cause stalls.

The sim is super sensitive to this and they pop all the time. I have found the real plane doesn't do it at the same point, but by that point I am definitely stowing them. It is in the back of my mind as we slow so I guess the sim did it's job.
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tdscanuck
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RE: Slowing Down Only With Thrust Reversal?

Tue Dec 01, 2009 3:04 am



Quoting MrSkyGuy (Reply 4):
I'm not an engine man, so forgive me if this is ignorant.. but what's the problem with the engine re-ingesting its own exhaust?

There's three biggies...two were mentioned already:

Quoting Tb727 (Reply 5):

No the problem is with FOD damaging the engine.



Quoting Fr8mech (Reply 13):
Along with the FOD issue, some engine were/are very sensitive to airflow disturbance at the inlet. The aircraft is slowing down and is getting less 'ram' effect at the inlet.

The third has the same symptom as Fr8mech noted (engine stall) but a different cause...fan air is considerably hotter than ambient, and if the engine inhales that hot low-density air it can screw up the compressor because the engine is expect air about 50-100 degF cooler, leading to a stall.

Tom.
 
ex52tech
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RE: Slowing Down Only With Thrust Reversal?

Tue Dec 01, 2009 5:20 am



Quoting Fr8mech (Reply 13):
At the same time, when an engine goes into reverse, bleed valves move around to provide optimum airflow through the engine and try to prevent stalls. The air thrown forward can diturb the inlet air and cause stalls.

JT9s had the RABS solenoid that would open one of the 3.5 bleed valves in reverse, and you could still get a pretty good stall.

Quoting 747classic (Reply 10):
Later-on the turbine reversers were deleted due high maintenance costs and low reliability.

I enjoyed de-activating those turbine reversers, hard to believe they were seemingly more effective than the fan reverser, but I just fixed them. You can't imagine how fast mechanics would scatter when a turbine reverser was called in at the gate, that had to be one of the dirtiest jobs there was.
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bond007
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RE: Slowing Down Only With Thrust Reversal?

Tue Dec 01, 2009 12:30 pm



Quoting N901WA (Reply 12):
I wonder if they knew how fast 737-800's go thru brakes and They are made of Steel

Yeah, but on a dry runway, the effectiveness of T/Rs is minimal, especially when many SOPs call for idle reverse only.

Remember too, the maintenance of T/Rs is not insignificant. There was a study done by NASA many years ago, the result being (if I remember) that T/R maintenance costs were 4 times the saving on brakes (perhaps T/Rs are less costly to maintain now). Now, I don't know whether increased T/R use necessarily equates to increased maintenance, but regardless, there is a cost that is often ignored.

Another factor is that AFAIK, carbon brakes wear more evenly when 'hot', so using them sparingly is probably more detrimental than regular recommended use.

Of course, many of the options here are dictated by SOPs, and I'm sure few involve maximum reverse thrust followed by minimum braking ... in fact quite the contrary  Wink

Jimbo
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tdscanuck
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RE: Slowing Down Only With Thrust Reversal?

Wed Dec 02, 2009 12:23 am



Quoting Bond007 (Reply 17):
Another factor is that AFAIK, carbon brakes wear more evenly when 'hot', so using them sparingly is probably more detrimental than regular recommended use.

They brake better when hot, but the wear is proportional to number of cycles. That's why you use carbon brakes more...you "pay" the cost of using them just by applying them, but the amount of wear isn't closely tied to how hard you use them. So, since you need to use them anyway, you might as well use them really hard, save other systems, and not be any worse off than if you'd used them lightly.

This is quite different from steel brakes, which wear proportional to the amount of energy absorbed...in that case, the cost of use is directly related to how hard you use the brake.

Tom.
 
bond007
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RE: Slowing Down Only With Thrust Reversal?

Wed Dec 02, 2009 3:07 am



Quoting Tdscanuck (Reply 18):
and not be any worse off than if you'd used them lightly.

Lightly, and usually more times, which is also why different taxi techniques are recommended for carbon brakes. Wait a little longer until you brake, and brake heavier less frequently, instead of many lighter taps.


Jimbo
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tinpusher007
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RE: Slowing Down Only With Thrust Reversal?

Thu Dec 24, 2009 8:43 pm

On the CRJ-900 that I fly, I typically peak at the GS just prior to touchdown to determine how much reverse thrust Im going to use. That and the length of the rwy. For instance, in DTW, the landing rwys are 10,000ft each which is more than enough to stop. Typically it helps traffic flow if you roll close to the end of the rwys before turning off. If we are light enough or Im only showing a GS of say 120kts or less I will use just reverse and no brakes at all...sometimes just idle reverse with no brakes until the capt takes the a/c at 80kts to exit the runway and it slows just fine. Obiously if Im going to someplace like MDW or DCA I'll use max reverse and moderate braking.
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2H4
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RE: Slowing Down Only With Thrust Reversal?

Thu Dec 24, 2009 9:14 pm



Quoting Tb727 (Reply 3):
If you have about 8000' of runway, you should only have to touch the brakes at the end to make the turn off.

Reminds me of an interesting article I once read about a 727 pilot who attempted a turnoff on packed snow/ice. Despite rolling very slowly, the nosewheel lost traction. The jet started to turn, but then just plowed straight forward toward the grass. He saved it at the last second, but then vowed to forever alter his turnoff technique on slippery runways...from then on, his rule was to slow so much that he must add power to turn off of the runway.

Not directly related to the discussion at hand, but for some reason, that story has stuck with me. As light as the 727s nosewheel can get, his rule seems to be a good one.

Quoting Pilotpip (Reply 7):
Modern Carbon brakes and antiskid are much more effective as has been discussed before. Next time I'm in the hangar I'll try and find out how often our mechanics are replacing brakes.

A spent carbon brake rotor might make a good garage wall decoration.

Mechanics - what (if any) special precautions are required when changing/handling used carbon brakes? I imagine the dust must be a concern.

2H4
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ex52tech
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RE: Slowing Down Only With Thrust Reversal?

Fri Dec 25, 2009 3:23 am



Quoting 2H4 (Reply 21):

Mechanics - what (if any) special precautions are required when changing/handling used carbon brakes? I imagine the dust must be a concern.

Wear a respirator if you want, but they are not to bad as far as dust, but every operator has it's own rules so I could not speak for everyone. You can see how dusty they make the wheel, but they are not too bad.
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brons2
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RE: Slowing Down Only With Thrust Reversal?

Fri Dec 25, 2009 8:15 am

With all this carbon and steel brake talk, did any airplane ever use asbestos brakes?
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tdscanuck
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RE: Slowing Down Only With Thrust Reversal?

Sun Dec 27, 2009 9:41 pm



Quoting Brons2 (Reply 23):
With all this carbon and steel brake talk, did any airplane ever use asbestos brakes?

I've got a chunk of steel brake on my desk...the friction surfaces are some kind of non-steel stuff that rides on the steel rotors. I suspect it had asbestos in it once-upon-a-time, just like car brakes.

Tom.

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