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A Couple Of Questions On Thrust Reversers  
User currently offlineATA1011Tristar From United States of America, joined Sep 2005, 92 posts, RR: 0
Posted (5 years 6 months 5 days 20 hours ago) and read 5529 times:

Hello Airliners.net Friends:

I hope this is in the right forum. Anyway, I have a couple of questions today regarding thrust reversers. I remember reading somewhere that pilots rarely use their aircraft's thrust reversers below around 60-70 knots because this relatively low speed does not allow enough fresh air to get to and enter the aircraft's engines. If this is true, how does a DC-9 performing a power back circumnavigate this problem? I would have thought that a turbojet engine would be even more susceptible to this problem compared to a turbofan engine seeing as turbojets derive most of their thrust from the combustion of fuel in their cores.

I was also wondering about the use of thrust reversers on long/rear mounted engines/T-tailed aircraft during landing. Is it theoretically possible that a combination of large amounts of reverse thrust and a nose pitched up for the landing flare cause an airplane to tilt backwards, resulting in a tail strike? It seems like it would be possible, considering how far behind the main wheels the engines of, say, a MD-90 are.

Thanks in advance!

7 replies: All unread, jump to last
 
User currently offline113312 From United States of America, joined Apr 2005, 572 posts, RR: 1
Reply 1, posted (5 years 6 months 5 days 19 hours ago) and read 5518 times:

Thrust reverse is normally not used below 60-80 knots for two reasons: Most effective at higher speed and ingestion of debris from surface for wing mount engines. At low speeds there is also a degree of ingestion of exhaust gases which isn't too desirable. However, it is better to not go off the end of the runway either. So for any aircraft, you should use maximum reverse on icy runways or if the aircraft might not stop on the available surface!

Some airlines adopted a practice of powerback from the gate, using reverse thrust (tail mount engines) rather than pushback using a tug and towbar. At the time, mechanics were almost always used for pushback to assure that the steering link was reconnected and no damage to the gear prior to taxi. The cost of the fuel for powerback was considered less than the cost of paying for the mechanic.

Today, many airlines taxi with one engine and the pushback and taxi with one engine is less expensive than the powerback routine.

On aircraft with tail mount engines, some of the reverse thrust flow can reduce effectiveness of the rudder. Generally, use of engine reverse does not cause a pitch up or pitch down. If it did, reverse would be prohibited or locked out prior to nose gear touchdown.


User currently offlinePhxpilot From United States of America, joined Jul 2005, 80 posts, RR: 2
Reply 2, posted (5 years 6 months 5 days 19 hours ago) and read 5509 times:

In the CRJ-900 use of thrust reverse created a noticable pitch up moment on landing, especially at aft centers of gravity. It was easily mitigated with slight forward yoke pressure. This was most evident on landing with the nose wheel still off the ground. My airline's flight manual carried a small advisory so the pilot would not be caught unaware.

User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 3, posted (5 years 6 months 5 days 18 hours ago) and read 5478 times:



Quoting Phxpilot (Reply 2):
This was most evident on landing with the nose wheel still off the ground

The 727-200 and the MD-11 both can pitch up and therefore we don't (aren't supposed) to go past idle reverse until the nose wheel is on the ground.


User currently offlineWing From Turkey, joined Oct 2000, 1571 posts, RR: 23
Reply 4, posted (5 years 6 months 5 days 18 hours ago) and read 5476 times:



Quoting CosmicCruiser (Reply 3):
The 727-200 and the MD-11 both can pitch up and therefore we don't (aren't supposed) to go past idle reverse until the nose wheel is on the ground.

Even the CFM56 equipped A321's has this tendency of pitch up ıf you are too quick to deploy reversers before you lower the nose.70 kts is the limit of max reverse usage,you can still use it on İDLE until taxi speed .

Some captains used to deploy thrust revers to slow down after long straight taxis to slow down during the good old times when brake wear costs were higher than use of reversers and airplanes are not equipped with brake fans.

Now with the fuel saving awareness crews noticed that fuel flows jump to very high level of fuel consumption during max reverse deployment and avoiding unnecessary usage saves fuel.



Widen your world
User currently offlineAirbuster From Netherlands, joined Mar 2007, 443 posts, RR: 0
Reply 5, posted (5 years 6 months 5 days 18 hours ago) and read 5462 times:

Ze little fokkers do not have that problem really, idle reverse below 60 kts, but because of the clamshell design and tail mounted, they are less prone to FOD and exhaust gas ingestion so you can keep them out whilst taxiing.

We often use one reverser out on long straight taxiways as to not need to use the brakes.
Every brake application costs about 40 euro's in maintenance, parts, spares, ware, etc...at least that's what the company told us. So it's better and more economical to use the reverser.

rgds

AB



FLY FOKKER JET LINE!
User currently offlinePilotpip From United States of America, joined Sep 2003, 3150 posts, RR: 10
Reply 6, posted (5 years 6 months 5 days 15 hours ago) and read 5429 times:

The 170 has a pitch up too. However the spoilers do a good job offsetting this. It's pretty hard to get the latches up and the thrust levers back before the nose is on the ground. The reversers won't deploy until you have weight on wheels and the spoilers deploy faster than they do. When the boards go up the nose goes down fast.


DMI
User currently offlineDashTrash From United States of America, joined Aug 2006, 1528 posts, RR: 2
Reply 7, posted (5 years 6 months 5 days 12 hours ago) and read 5396 times:

The Citation X has extreme pitch up when in reverse. Normal procedure is all three on the ground, push the yoke full forward, then open the buckets and spool. Idle reverse by 70 kts.

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