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Fcnse Region On 737  
User currently offlinesmartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0
Posted (1 year 8 months 6 days 13 hours ago) and read 4140 times:

Hoping someone clever out there can clarify this for me. Came across this whilst trying to brush up on 737 tech knowledge. What I can't get is that. How does putting the elevator in a more nose down position enable you to move stab trim to a more nose up. The limits of stab trim movement don't change under these circumstances do they? Is it just a simple case of you getting more benefit from keeping it all faired i.e. the elevator and stab?



"the mach trim actuator also causes the elevator to be in a more nose down position during takeoff which allows the pilots to move the stabilizer to a more nose up position
- this allows a more nose up attitude if there is an engine failure during takeoff and is called the FCC controlled neutral shift enable (FCNSE) region"


Any info would be greatly appreciated

Thanks

7 replies: All unread, jump to last
 
User currently offlinejetmech From Australia, joined Mar 2006, 2699 posts, RR: 53
Reply 1, posted (1 year 8 months 5 days 21 hours ago) and read 4044 times:

Quoting smartt1982 (Thread starter):

I think the following links may help;

http://www.pprune.org/tech-log/20912...draulic-flight-instruments-qs.html
http://www.737ng.co.uk/B_NG-Flight_Controls.pdf

The pdf link can also be found on www.smartcockpit.com, where info on the flight controls of the 737 classics should also be available.

According to pprune, the movement of the stabiliser and elevators is interconnected due to the Q feel unit, which I believe, operates on the stabiliser. Mach trim is applied to the elevators.

Regards, JetMech

[Edited 2013-04-18 19:35:35]


JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlinejetmech From Australia, joined Mar 2006, 2699 posts, RR: 53
Reply 2, posted (1 year 8 months 4 days 20 hours ago) and read 3963 times:

Quoting smartt1982 (Thread starter):

You may want to try these direct links to smartcockpit which are the flight control descriptions for various generations of the 737.

http://www.smartcockpit.com/aircraft...ources/B737CL-Flight_Controls.html
http://www.smartcockpit.com/aircraft...sources/B737E-Flight_Controls.html
http://www.smartcockpit.com/aircraft...ssources/B_NG-Flight_Controls.html

I will have a read myself and see what information can be revealed.

Regards, JetMech



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlinesmartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0
Reply 3, posted (1 year 7 months 4 weeks 1 day 6 hours ago) and read 3807 times:

Thanks for that JetMech, apologies for the late reply. Other than the moving of the Stab trim automatically I order to avoid any Mach tuck issues I still cannot understand how moving the elevator to a more nose down position enables more up stab trim!!

Steve


User currently offlinejetmech From Australia, joined Mar 2006, 2699 posts, RR: 53
Reply 4, posted (1 year 7 months 4 weeks 19 hours ago) and read 3748 times:

Quoting smartt1982 (Reply 3):
I still cannot understand how moving the elevator to a more nose down position enables more up stab trim!!

I just happened to find the operation and description sections of the flight control chapters I printed out many years ago for the 767 and 744. The Q-feel operation is broadly similar for both aircraft, however, the 767 being a twin is more relevant to the discussion, and interestingly, neutral shift is mentioned as something that is only fitted to the 762. Anyway, the basic idea is the following.

Just below the cockpit floor, is a spring loaded cam wheel riding upon a heart shaped (cardioid) cam, with neutral being in the valley of the two lobes of the cardioid. This provides a base level of resistance to the control column. Column movements are mechanically transmitted to the tail section where they act upon aft quadrants, the aft quadrants in turn transmitting mechanical inputs to the elevator power control actuators.

The elevator feel and centring units are connected to the aft quadrants and modify the forces upon them, which in turn, alters the feel of the control column as experienced by the pilots. The feel and centring unit hydraulically adds resistance to the aft quadrant for increasing airspeed, and increasing nose down trim (stab leading edge up). The elevator feel and centring unit also adjusts elevator neutral position according to a direct mechanical input of the stab position.

The exact amount of feel force added is determined by the feel computer, which receives two inputs, a direct mechanical input of the stab position, and a pitot pressure (Q) input. These inputs allow the feel computer to supply a regulated hydraulic feel pressure to the feel and centring units. This regulated hydraulic feel pressure is also fed to the stab trim control modules.

The Q input is applied to bellows which expand with increasing airspeed. This acts upon hydraulic valving in the feel computer such that regulated hydraulic feel pressure rises rapidly with airspeed. This regulated hydraulic feel pressure is fed to the feel and centring units, the end result of course, being added control column resistance as experienced by the pilots.

The stab position input acts upon a spring loaded, movable stop in the feel computer. Once the bellows have expanded sufficiently to contact the spring loaded stop, further increases in bellows expansion meets added resistance. The end result is a reduction in the rate of increase of metered hydraulic feel pressure.

The 767-200 feel and centring unit adjusts the neutral position of the elevator system according to a direct mechanical position input from the stabiliser. For stab positions of +2 to -6.4 degrees, there is no effect on neutral elevator position. From -6.4 to -12.5 degrees stab position, the neutral position of the elevators moves from streamlined to 2 degrees trailing edge up. This is done in order to increase the effective nose up trim of the aircraft as a whole.

There is no mention of Mach tuck compensation in the 767MM - either that or I didn’t print it off - but I’d say that the 737 elevator / stabiliser system is broadly similar. Anyway, according to the 737 classic FCOM available from smartcockpit;

Mach trim is automatically accomplished above Mach 0.615 by adjusting the elevators with respect to the stabiliser as speed increases. The flight control computers use Mach information from the flight data computers to calculate a Mach trim actuator position. The Mach trim actuator repositions the elevator feel and centring unit which adjusts the control column neutral position

Mach tuck causes nose down pitch, thus I assume elevator trailing edge up is used initially to counteract this, followed up by stabiliser movement in order to return the elevators to the streamlined position. Obviously, the Mach number during take-off is too low for any of this to happen.

Anyway, assuming the 737 elevator / stab control and operation system is similar to the 767, it appears the whole point of FCNSE is to get the stabiliser out of the neutral shift region, which theoretically allows the pilots to use the full elevator movement range from streamlined to trailing edge up, which is obviously beneficial for engine out situations on take-off.

The thing I don’t understand is they need to forgo some of the elevator trailing edge up movement range in order to allow the stabiliser to move out of the neutral shift range in the first place!

Time for a 737 driver / engineer to chime in!

Regards, JetMech



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineyeelep From United States of America, joined Apr 2011, 670 posts, RR: 0
Reply 5, posted (1 year 7 months 4 weeks 4 hours ago) and read 3691 times:

According to Boeing, the primary function of the flight control computer (FCC) controlled neutral shift is to reduce the pilot column force necessary to trim the airplane during initial climbout after takeoff with a forward CG and both engines operating.

Quoting smartt1982 (Thread starter):
How does putting the elevator in a more nose down position enable you to move stab trim to a more nose up.

Say you have the stab and elevator faired so the plane will fly level. By shifting the elevator neutral point a/c nose down, you are able to run the stab a/c nose up and still fly level. That is what they mean by enabling more nose up trim, you maintain the same pitch up force.

Quoting smartt1982 (Thread starter):
The limits of stab trim movement don't change under these circumstances do they?

No

Quoting smartt1982 (Thread starter):
Is it just a simple case of you getting more benefit from keeping it all faired i.e. the elevator and stab?

No, at stab neutral (4 units on the NG) the elevators are down rigged 4°. To fair the stab and elevator you would have to trim to 15 units nose up, or 13 units nose up with the fcc neutral shift active. Maximum nose up trim is 17 units. We are only talking about a 5° misfair at its worse which I guess is of little concern at low speeds.

Quoting jetmech (Reply 4):
Mach tuck causes nose down pitch, thus I assume elevator trailing edge up is used initially to counteract this, followed up by stabiliser movement in order to return the elevators to the streamlined position. Obviously, the Mach number during take-off is too low for any of this to happen.

I believe on the 737 the Mach trim actuator is all that is used to control Mach tuck. Its probably why the elevators are rigged 4° airplane nose down with the stab in the neutral position.

Quoting smartt1982 (Thread starter):
FCC controlled neutral shift enable (FCNSE) region

This is any time the flaps are not zero and N1 is over 18%.

Quoting jetmech (Reply 4):
it appears the whole point of FCNSE is to get the stabiliser out of the neutral shift region,

Whenever the stab is trimmed there will be a neutral shift, although it appears between 3 and 4 units there is hardly any. The FCNSE just changes the amount, which varies relative to what the neutral shift rods do on their own throughout the travel of the stab. In fact, past 12 units the fcc neutral shift uprigs the elevator instead of downrigging.


User currently offlinejetmech From Australia, joined Mar 2006, 2699 posts, RR: 53
Reply 6, posted (1 year 7 months 3 weeks 6 days 19 hours ago) and read 3644 times:

Quoting yeelep (Reply 5):

Thanks for the input yeelep! Great to finally get some concrete information on the issue!

Quoting yeelep (Reply 5):
Say you have the stab and elevator faired so the plane will fly level. By shifting the elevator neutral point a/c nose down, you are able to run the stab a/c nose up and still fly level. That is what they mean by enabling more nose up trim, you maintain the same pitch up force.
Quoting yeelep (Reply 5):
I believe on the 737 the Mach trim actuator is all that is used to control Mach tuck. Its probably why the elevators are rigged 4° airplane nose down with the stab in the neutral position.

I think I'm beginning to get a grasp of the Mach trim situation. From the info you have provided, and re-reading the 737 FCOMs for flight controls, it appears that Mach tuck is indeed compensated for via a movement of the elevators. Boeing appears to achieve this nose up trim, by moving the elevators from a slightly trailing edge down position to the streamlined position.

My confusion on the situation arose from the assumption that the nose up trim required to compensate for Mach tuck was achieved by an initial elevator trailing edge up movement, followed by stab trim movement to return the elevators to streamlined. It actually makes sense the way Boeing has done it, as it would minimise the amount of stab movements.

Quoting yeelep (Reply 5):
In fact, past 12 units the fcc neutral shift uprigs the elevator instead of downrigging.

Similar to the 767 then. I assume this is done to preclude the need for an excessively larger stabiliser or one requiring a greater amount of leading edge down movement. Low-speed operations (take off/landing) at a forward C of G would be the only situations requiring a large amount of nose up trim, so it makes sense that Boeing would use some elevator trailing edge up to provide the additional force required for these relatively short phases of the overall flight.

Regards, JetMech



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlinesmartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0
Reply 7, posted (1 year 7 months 3 weeks 6 days 9 hours ago) and read 3593 times:

Thanks a million for the helpful info.

Quoting yeelep (Reply 5):
Whenever the stab is trimmed there will be a neutral shift, although it appears between 3 and 4 units there is hardly any. The FCNSE just changes the amount, which varies relative to what the neutral shift rods do on their own throughout the travel of the stab. In fact, past 12 units the fcc neutral shift uprigs the elevator instead of downrigging.

I can understand the reason in relation to moving the elevator (the FCNSE that is) in order to correct for the Mach tuck info. But In relation to the take off etc, I cannot seem to understand the reason behind it.

Quoting yeelep (Reply 5):
Say you have the stab and elevator faired so the plane will fly level. By shifting the elevator neutral point a/c nose down, you are able to run the stab a/c nose up and still fly level. That is what they mean by enabling more nose up trim, you maintain the same pitch up force.

If I get the same pitch force etc, what is the benefit of doing it?

Any ideas were I could get access to the 737 NG Maintenance manual? Not talking particularly about this topic but I saw one briefly once and noticed that it has some really good info that you cannot get in the FCOMS
Cheers
Steve


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