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Md-11 Lsas

Old 23rd Feb 2010, 14:04
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Md-11 Lsas

I think, I'm very near to understanding the MD-11 LSAS (from book reading), but how does it effect real life hand flying? Do you, respected -11 guys, use pitch trim or is it automatic? What is the feel of the airplane with operating LSAS? (Similar to FBW Airbus "centering" side stick?)

Many thanks,
b
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Old 24th Feb 2010, 13:22
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LSAS

I'm rather curious because for the third time in three days someone has asked me about LSAS. I'm not the ultimate authority so bear with me kindly. I did not even know that there was a "book". At the time that Douglas transitioned from the DC10 to the MD11 there was an air of decay hanging all around their enterprise. As 411A can tell you the better engineered big trijet was the TriStar but the demise of R-R didn't help! Douglas took what seemed to be the simpler aerodynamic option for the "straight-thru" engine no 2 but at a structural penalty and some would say a thrust line penalty too. Be those as they may they had a few problems with the boat-tail drag and that cost them development time and money. So you may say that the straight through design was a "bought bunny" when it came to MD11. The DC10 was I believe a "peaky airfoil" and the MD11 I think is first generation supercritical. Yet mindful of their own parlous financial affairs and how nearly the B747 had cost Boeing the farm they put their money on the trijet. They could squeeze more out of the 10 and the longed for 12 might not be a pipedream if only the 11 could sell. What they had to do was cut down on drag, the wing airfoil was I believe pretty good, they were wedded to three nacelles, the fuselage length was a pretty much given so all they could do was look at the tail. Not much could they do with the fin as it had the dual job of being part of the engine support structure. So they cut down on elevator area and hence elevator authority. Now you could do that then but I seriously wonder that FAA would be as accommodating now. That is a personal view so no takers please. To win the FAA round they used two approaches: firstly LSAS which is not a nice to have, whatever will they think of next device but a necessity to counteract the loss of elevator authority. Next along the line they could argue that having a hot engine right there was handy for ice prevention and that therefore the traditional ideas on elevator area could be relaxed. It is interesting to note that when Boeing acquired the marque they quickly saw to it that MD11 operators saw attractive incentives to move on to a twinjet of their own making. Without going into the reason why many aircraft become converted to freighters the MD11 if it really was that goldarn good was unfairly discontinued. I'd say the MD11 requires respect, its tendency to PIO is noted and specific traing is given. It is not a bland machine and many have argued that its delights, and it has many, do not outweight its vices. What interests me is the sudden appearance of interest in an aircraft that will bite hard if mishandled, is a bit controversial accident wise in landing when elevator authority, pitch and PIO can rebder the aircraft geography in a quick space of time. When it hits hard, it does seem to self dismantle but I think that all the mory gory pictures are of aircraft with "history" before the ensuing "geography". In nmy view LSAS was made necessary by Douglas having to make more fuel efficient a design that already had enough shortcomings. Personally I have no great gripes about the MD11 but I take the point expressed so often that whenever extra special care and training is necessary it is a pointer to underlying design issues. Regarding Airbus similarity, there is a certain nodding similarity in the Honeywell but that is about it. This ain't a fly-by-wire aircraft or an earl Airbus either. Really intrigued by the upsurge in interest though. Three times in as many days and I know of no-one hiring. Coincidence?
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Old 24th Feb 2010, 20:32
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1. Many thanks mathy for this little essay on the MD-11, really great overwiev, I printed it out for my "archive".
2. Under "book" I meant AOM.
3. I searched this forum but couldn't find more discussion directly related to LSAS.
4. As far as the "surge" of interest towards the MD-11: coincidence perhaps - as far as I'm concerned it is a very vague/faint and rather unique "project". Most probably it'll go down the pipe sooner or later.
But I don't regret reading the MD-11 stuff, it is a really special bird ...especially with all that history.

best rgds
b
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Old 25th Feb 2010, 18:51
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LSAS

If you could watch the elevators of an MD-11 in flight at airspeed and altitude you would see what looks like an insainly fast fultter of pitch imputs.

Due to the length of the plane LSAS corrects pitch inputs quicker than the human is capable of to correct "PIO's" that would not just make the pax in the aft and the pilots queezy but could get quite violent. Similar to the actions of a yaw dampner in preventing "dutch roll" in intent but more active.

It was designed on the platform of latter DC-10's but not worth the cost to deem necessary as I have been told by some Long Beach instructors.

Must make the clear distinction that LSAS is an FCC but not autoflight function. The autoflight function has it's own longitudinal stabilzation.

Pitch trim becomes automatic with the autopilot enguaged and I believe it is also an LSAS function after certain degree of elevator input while several inputs of manual trim are of cource available while hand flying. The extent of pitch trim and LSAS's relationship I am not 100% sure on, just vague memories of tech training.

Last edited by muduckace; 25th Feb 2010 at 19:02.
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Old 25th Feb 2010, 19:24
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LSAS D&O From old AMM (ref Only)

LONGITUDINAL STABILITY AUGMENTATION -- DESCRIPTION AND OPERATION


1. General

A. The longitudinal stability augmentation system (LSAS) supplies stability in the pitch axis via series
control of the elevators. LSAS is a function of the flight control computers (FCCs) that isactive
whenever the autopilot is not engaged and the LSAS pushbuttonson the auto flight system control
panel (AFSCP) are not OFF. LSAS receives inputs from the inertial reference units (IRUs), air data
computers (ADCs), and control wheel sensors. The IRUs supply pitch angle and pitch rate, the ADCs
supply airspeed, and the control wheel sensors supply the pitch force applied to the control columns.
B. The FFC sums the Captainís and First Officerís control wheel inputs. When there is no force on the
control columns, LSAS holds the current pitch attitude. When the force exceeds two pounds, LSAS
operation is suspended and the elevators are in manual control. When the force on the control
column is reduced below two pounds, LSAS holds the newly acquired pitch attitude. The limits on the
attitude that LSAS can hold are +30 and -10 degrees. The maximum elevator deflection that LSAS
can command is five degrees.
C. LSAS also supplies overspeed and stall protection. If the airspeed exceeds the LSAS maximum
speed reference, LSAS overspeed protection will limit the airspeed to the maximum takeoff or
landing speed (Vmo). The LSAS maximum speed reference is equal to Vmo plus an increment which
is a function of throttle position. If the pitch attitude exceeds the LSAS alpha reference, LSAS stall
protection will limit the pitch attitude to the alpha reference. LSAS alpha reference is defined to be an
angle that is less than stick shaker alpha.
D. Each FCC contains two LSAS channels. This permits each FCC to have a self monitor function and
the system supplies a total of four redundant LSAS channels. Each channel drives a separate
elevator. The power for each elevator is supplied by a separate circuit breaker as follows:

(did not include diagram)

2. System Operation
A. If a failure is detected, both LSAS channels in that FCC shut down and the related LSAS FAIL lights
on the AFSCP come on. An LSAS CHAN FAIL alert will come into view on the engine and alert
display (EAD). When this occurs, the flight crew must turn off the failed LSAS channels. The flight
crew will push in the related LSAS OFF pushbuttons on the AFSCP. This will permit the other FCC to
go to single channel operation if a failure is found that can be isolated to one channel in the FCC. If
this cccurs, the related LSAS FAIL light will come on, which the flight crew must shut off. If the second
FCC can not find which FCC channel is serviceable, the two other LSAS FAIL lights will come on. The
flight crew will then have to push each LSAS OFF pushbutton, one at a time, in order to set a single
channel for control.
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