The recent loss of Alaska Airlines 261 waslike an unexpected hard punch in the gut for me, and for everyone in the airline business,because we know that Alaska is one of the very finest airlines, with excellent attitudesprevailing in the cockpits and cabins, good equipment, good maintenance, good training,and highly skilled pilots who operate in somewhat more difficult weather and terrainconditions than most.
I can personally attest to this. I often have the privilege of riding in their jumpseats as I commute between my home in Seattle and my crew base in Los Angeles. I have cometo know some of the crews, and I can only say Alaska Air is a place I’d like to work. Mostof us know that if this tragedy can happen at Alaska, it can happen anywhere.
The loss to family and friends is immeasurably greater, and most of the people in theindustry are acutely aware of that, too.
Along with the rest of the world, I watched TV and newspapers a lot over the daysfollowing the crash, and as always, I was struck by the abominable job they do with”breaking news.” With rare exceptions, there seems to be a compulsive need tofill the airwaves with senseless chatter, and fill column inches with meaningless prattle.The rare times they do bother to get genuine experts like Barry Schiff or John Nance onTV, they seem to go after them for the taped “sound bites” while not giving themmuch chance to make any intelligent comments. At the same time, they’ll give endlessminutes of airtime to some “talking head” that knows nothing except how to lookhis or her best on camera. A three-second sound bite is not enough for a real expert tosay much. Astonishingly, network newscasts presented some general aviation pilots as”experts” when it was clear they had not the slightest idea how any jettransport systems work. Their calm, assured comments were flat wrong, and seriously so. Iguess to the TV networks, “a pilot is a pilot, and airplanes are all alike.”
While I’m slamming the media, let me mention another pet peeve. I feel like taking aroundhouse swing at my TV set with a baseball bat every time some fool with a mike shovesit in the face of a distraught relative, and asks the inevitable inane question “Howdo you feel about losing your whole family?” Have these morons no decency? Are thepeople running the TV news shows really that stupid and heartless? No one I’ve ever talkedto will admit such “coverage” is a good thing, or worthwhile. This is carrying”if it bleeds, it leads” much too far!
On the other hand, there was some tasteful coverage on the Seattle stations of the manyservices and memorial gatherings that took place, although I thought those should havebeen more private, too. Seattle was hard hit by this one, as many of the passengers werefrom that area.
But enough of my soapbox.
The other thing that struck me was the profound ignorance of pitch control systems, notonly in the media (where I expect it), but also among the aviation fraternity. I thoughtit might be useful to do a column on this very complex subject, if only to point out howmany different systems and variations there really are … and probably reveal my ownignorance in the process!
No, I am NOT trying to solve the AS261 crash – that is the job of the NTSB, who seemsto be doing this one right, in my opinion. However, I hope that what follows will help youto better understand the investigation into the horizontal stabilizer.
Just like the feathers on an arrow, the basic tail feathers on an airplane help to keepthe airplane going forward in a straight line. But there’s a lot more to the story when westart talking about controlling and trimming pitch.
C.G. vs. C.L.
Briefly, most pilots know that the Center of Gravity (C.G.) is that point on or in theairplane where it “balances” in all three axes. Put an eyebolt there, and hangit from the roof of the hangar, and it should remain in any attitude you place it. This isthe point where all the “down” forces (gravity) are centered in level flight. Onmost common airplanes in level flight, it’s somewhere above or below roughly the one-thirdpoint on the wing’s chord line. The Center of Lift (C.L.) is exactly the same idea, it’sthe point in or on the airplane where all the “upwards” lift forces arecentered, a “balance of lift forces.” For fun, let’s mentally put anothereyebolt at this point. (Don’t confuse “C.L.” with “CL“which is used for “Coefficient of Lift,” another matter entirely.)
New pilots (and a few old ones!) are always amazed that not only do the C.G. and C.L.NOT meet at the same point; we don’t even want them to. Instead, the designers arrange thelocation and shape of the wing and the weight distribution of the airplane so that theC.L. is behind the C.G. This means that if you hang the airplane from the fictitiouseyebolt at the center of lift, the airplane will promptly point its nose at the floor.
We avoid this distressing condition in flight by putting a horizontal tail surface onthe airplane with some special characteristics to hold the tail down. This is nothing morethan a small wing, mounted upside down. The curved part (if any) will be on the bottom,and the flat (or less curved) part will be on top. This is to counterbalance the nose-downsituation described above.
We don’t want too much of a good thing, because the more down force that tail has toproduce, the more lift the wing has to produce to support the total weight. Picture a4,000-pound airplane, with 100 pounds of “download” on the tail, and you’ll seethat the rest of the airplane must produce 4,100 pounds of lift. The drag from producingthat extra lift can be considerable. Some cargo airlines take the extra trouble to loadcargo so the aircraft is very close to the aft C.G. limit. This reduces the nose-downpitching moment, which reduces the “down force” needed to counteract it, all ofwhich reduces the drag and can make a very noticeable difference in fuel burned.
A Balancing Act
But the most important result from all this is stability. Any increase in airspeed willproduce more lift at the wing and more “down force” at the tail, making the nosecome up, which will tend to reduce the speed again. Decrease the airspeed and you’ll getless lift on the wing, less “down force” on the tail, and the nose will want topitch down to regain the speed.
If the horizontal tail should happen to fall off in flight, the normal airplane willviolently pitch nose down to approximately vertical, and stay there to impact. The goodnews is that tails rarely fall off.
If the designer didn’t make use of this balance of forces, a little backpressure mightpull the nose up, then the natural forces would make the nose come up even more, and thepilot would be in a constant battle to keep the nose pointed where he wanted the airplaneto go. Some of the very early aircraft had this problem, and were quite difficult tohandle. This is also why an aircraft gets a good deal more pitch-sensitive with an aftC.G., when the C.G. moves back closer to the C.L. and very little or no down force isrequired at the tail.
To this point, virtually all aircraft are the same, with exceptions being aircraft likethe Beech Starship, the Wright Brothers “Flyer,” Mignet’s “FlyingFlea” and other oddball types. Let’s not go there, this time.
Three Different Trim Systems … and Some Variations
However, even on “mainstream” airplanes, the devices to accomplish this basicpurpose can be very different from each other, with some really strange ones used over theyears.
Stabilizer and Elevator
Perhaps the first among what I’ll call “conventional” systems is the simplefixed horizontal stabilizer with a movable elevator, as found on the Caproni N’20 or thedeHaviland DH4 pictured here. Note the primitive external cables, simple controlhorns and complete lack of any device for trimming. Also see the Curtis Robin withinternal cabling, and an external pushrod and bellcrank system to move the elevators. Ifyou preflighted one of these, you got to see it all!
(My thanks to the Seattle Museum of Flight at Boeing Field for allowing me close accessto the museum aircraft pictured here.)
Why were there no trim tabs? Most of these aircraft were very slow, so they had a verylimited range of speed between stalling speed and redline speed, even in a dive. Thepilots usually sat in the rear cockpit, well away from the C.G., but the airplanes weredesigned for an average pilot’s weight back there, and any fuel, payload, or passengerseating was roughly on the C.G., so that anything from “no passenger” to a realheavyweight didn’t matter very much, at least for C.G. purposes. These aircraft alsodidn’t fly straight and level much, so no one cared if the pilot had to hold a littlepressure on the stick to correct the balance during the rare straight and level portion ofthe flight.
Stretching the Point
But pilots are a lazy bunch, and often clever, so I’ll bet it didn’t take too longbefore some of them figured out how to use a few strips of rubber from a discarded innertube tied between the stick and some handy point in the cockpit to relieve the load, andsliding the rubber bands up and down the stick to get it “just right.” How do Iknow this? Because I’ve used this little trick for aileron and rudder trim!
Later, some designers would use a variation of this for trimming, building in some sortof bungee system connected to the cables, with an adjustable lever in the cockpit to varythe tension on the cables. The bungee created a new “center point” while stillallowing elevator inputs.
It didn’t take too long for the early designers to respond to the whines of the poor,overworked pilots, and one method of alleviating pressure on the stick was to add a smallfixed tab at the back of the elevator, as on the 1928 Boeing P-12. By flying the aircraft,then bending the tab a little, it didn’t take long to set the aircraft up for somespecific pilot weight, configuration and speed. We see these little tabs on rudders andailerons even today, correcting for minor flaws in the shape or balance of forces in rolland yaw.
Adjusting these fixed tabs is non-intuitive, for they must be bent in what most thinkis the wrong direction. This Boeing P-12 obviously had a nose up tendency that distressedthe pilot assigned to it, forcing him to push the stick forward, holding some”down” pressure on the elevator. The little tab is bent UP, creating a smalllocal amount of camber or curve at the trailing edge, and this creates a tiny bit of”lift” at that point, tugging at the trailing edge of the elevator (down in thiscase.) The advantage here is that this trimming force occurs without placing any stress onthe control system. This can be important, because the control stick is a long lever,which means it is very powerful, and the elevator takes a lot of force to move it againstthe airflow. But the little bell cranks and lever arms in the system are not very long atall, which means they must be very, very strong.
Fixed tabs work great for fixed conditions, but as airplanes got faster, and as theycould be loaded at different C.G. locations, something more was needed. Two common devicescame into being: the adjustable trim tab, and the movable stabilizer.
A good example of the adjustable trim tab can be seen on the Grumman-American Cheetah.This one is set for considerable nose up trim, perhaps after the last landing. With thissystem, if the stabilizer is not at the correct angle for the loading, the elevator willbe unfaired, creating drag, and if the pilot has to trim to hold it there, the trim tabwill be unfaired (with the elevator) as well, creating more drag. This is not a problem onslower aircraft, but it becomes important at higher speeds.
The earliest example of an adjustable stabilizer that I know of is on the Taylor(later Piper) Cub,and it remained a favorite method on Pipers for many years, as on the Piper Tri-Pacerpictured here. The leading edge is simply cranked up and down by a system of cables andpulleys. By setting the stabilizer angle to remove all stick pressure, all variations inspeed and C.G. can be corrected, and the elevator will be faired with the stabilizer innormal hands-off flight, reducing drag. This didn’t help the early Pipers much, for theywere no speed demons, but this is exactly the same arrangement found on almost all modernjet transports! Another neat feature of the movable stabilizer is that full elevatorauthority is always available when trimmed.
For example, without the moving stabilizer, and with an elevator that moves 10 degreesup and down, if the pilot must hold five degrees of elevator position for level flight,then there is only five more degrees of elevator available in that direction. With themovable stabilizer, the elevator should always be faired when properly trimmed, so fulltravel is available in either direction.
Another variation is the all-moving horizontal tail. The entire surface is hinged atroughly the center point. When the pilot moves the stick or yoke, the entire horizontalsurface (“stabilator,” as in “stabilizer+elevator”) moves as one unitas on the Cessna Cardinal or the Piper PA-28R-200. But variations in C.G. and speed stillaffect the airplane; so again, some form of trim is needed. On this Cardinal, there willbe some heavy-duty cables moving the stabilator in direct response to the pilot’s input onthe yoke, and smaller cables from the trim tab control in the cockpit to the trim tabs onthe back of the stabilator.
Note the very interesting “slots” in the leading edge of the Cardinalstabilator. Cessna added these when they discovered that the early Cardinal stabilatorshad the nasty habit of stalling abruptly during the landing roll, causing the nosewheel tocrunch abruptly and unceremoniously onto the runway. To remedy this stabilator stall, thedesigners were forced to add the slots to enhance airflow over the “top of thewing” (the bottom of the tail). These slots were an afterthought, added well afterthe Cardinal was first introduced, and retrofitted to the pre-existing fleet.
When you see slots like this on a wing, the air enters below the leading edge, andexits on top of the wing. Slots like this are often used on a wing just forward of theailerons to improve roll rates. You may also see them used more extensively on STOL(“Short TakeOff and Landing”) aircraft like the Helio Courier, or the DornierDO-28, where they are installed for the length of the leading edge of the wing.
Bungees, trim tabs, and adjustable-incidence stabilizers – these are the three simple,basic pitch control systems.
Cheating with Tabs
Control system designers often “cheat” a little, too. They will often add alittle tab that looks just like a trim tab, except there is no control for it in thecockpit. This tab will be connected directly to the main surface (stabilizer orstabilator) by a linkage that will move it with or against the motion of the elevator (orstabilator). It may move more in one direction than the other. If the tab moves oppositeto the surface it’s hooked to, it assists the pilot by lowering the force needed to movethe surface, and is called a “servo tab.” If it moves with the surface, it addsto the force required (and improves the effectiveness), and is called an “anti-servotab.” Both will sometimes be called “geared tabs.” These will be added andadjusted as needed to produce the desirable stick forces and control harmony. In somecases, they will be separate tabs, as on this Goodyear FG-1D Corsair. Here the two inboardtabs are trim tabs, and the two outboard tabs are servo tabs, according to Chris Avery,who flies one of the few remaining ones in existence.
Another interesting tab is the “spring tab” or “flying tab.” Theyoke is directly hooked to a cartridge with a heavy-duty spring in it, and that cartridgeis hooked to the tab. Initial motion of the yoke will move only the spring tab, which will”fly” the elevator (or stabilator) into a new position. Once the springcartridge runs out of travel, it will then move the elevator directly, as on the C-46. Orthere may be no springs at all as on the DC-9, which runs all direct pitch control fromthe yoke by tabs alone. This is a very effective device, making relatively light controls,while providing immense control power.
Various combinations of all these may be found on some airplanes, and they are notlimited to just pitch control. All of them have been used for roll and yaw, as well. Onthe C-46 you will find both spring tabs and conventional trim tabs on the elevators andthe rudder. The first picture here shows the left elevator with the trim tab neutral, andwith someone in the cockpit pulling back on the yoke, deflecting the spring tab. Note thecontrol blocks are in place, preventing elevator motion, but there is a considerableamount of yoke movement to produce this tab deflection. The next picture shows theopposite yoke deflection for airplane nose down. Old-time check pilots liked to stump newhires with “Which tabs are which?” The ancient acronym is “OTIS” (asin elevators) for “Outer Trim, Inner Spring,” but of course this may not applyto other aircraft! The outer trim tabs are also unusual on this airplane, being set sothat the zero point has one side up 15 degrees, the other side down 15 degrees. This is aquick and dirty fix to control flutter. Wartime needs did not allow elegant fixes, andthis artifact remains to this day.
The 1952 Martin 404 really gets carried away – the deceptively simple-looking rudder tabon this one is a combination of servo, trim, and spring tab! Setting it up is a nightmare,and the designer is rumored to have spent the rest of his days cutting out paper dolls inthe loony bin.
Using all these tricks and more, some very large airplanes have been flown only withcable systems. Most notable in this regard was the Hughes HR-1, the largest airplane everbuilt. (Howard Hughes hated the widely-used nickname “Spruce Goose.”)
Big Airplanes, Weak Pilots
But larger airplanes get really heavy on the controls, and even the strongest pilotsneed some help. Most large aircraft today use various versions of hydraulic”boost” where the first motion of the cable system moves a hydraulic valve tosimply assist the cable motion – or as on the 747, where all surfaces are entirelyhydraulic, with no connection whatsoever between the controls and the surfaces.
Hydraulic controls add a whole new layer of problems, and the solutions to those addmore complexity, and more chances for failures. Fully hydraulic control surfaces are”irreversible,” so the pilot has no feeling for speed, as the controls alwaysmove with complete ease. Certification requires that controls should get stiffer withincreasing speed, so entirely separate “artificial feel” systems must bedeveloped, added and tested, with possible failures and procedures for dealing with thosefailures.
As jet aircraft entered service, some additional problems were encountered, and solvedin ingenious ways. All the jet transports operate over a very large range of speeds (100to 500 knots). Most will double their empty weight when loaded, and some will nearlytriple it. Designers must provide for much greater pitch control. For example, it isnecessary to correct for the C.G. shift caused by flight attendants walking up and downthe length of the airplane. Of course, some FAs have more effect than others. I hasten toadd that the Alaska Airlines FAs are all slim, trim, smart and beautiful, so this doesn’tapply at that airline. (I don’t want hot coffee dumped in my lap the next time I ride onAlaska!)
On jets, the differences between IAS and TAS are far greater, Mach effects come intoplay, the aircraft must operate over a large range of temperatures (-54C to +54C in the747, for example) and pressures (from 15 PSI at sea level to 2-3 PSI at altitude). It’s awhole different world, and the design challenges are formidable. For one thing, the 747changes its length by a couple inches with the extremes in temperatures. Cables can getvery sloppy when it is cold, without special devices to keep them taut all the time.
The simplest and most common pitch system on jets is probably the trimmable stabilizerand moving elevator, just like the old Piper Cub. Due to much larger aircraft, much fasterspeeds, and much greater control forces, the stabilizer’s leading edge is moved byheavy-duty electric motors (DC-9) or by hydraulic motors (most Boeings.) In all cases Iknow of, these motors drive a jackscrew arrangement, where one end of a long and verystrong threaded rod is attached to the leading edge of the horizontal stabilizer, whilethe aft end of the horizontal stabilizer is hinged to allow roughly 15 or 20 degrees ofmovement. This jackscrew is exactly like a huge bolt, and the bottom end runs in acoupling exactly like a huge nut. In some cases the jackscrew itself spins inside a fixed”nut,” while in others the jackscrew is stationary and the “nut”turns. Either way, the jackscrew mechanism runs the leading edge of the stabilizer up anddown as needed. Generally, the pilots will have electrical toggle switches under theirthumbs to run the stabilizer trim up and down at a high rate for maneuvering at loweraltitudes and speeds. Most pilots turn on the autopilot for high-altitude, high-speedflight, as these airplanes are fairly miserable to hand-fly in that regime. Sometimesthere is another control lever to run the stab trim, perhaps at a reduced rate, in casethe electrically powered toggle switch fails. The autoflight systems will trim at theslowest trim rate.
Two Types of Brakes
Once all this machinery is set in motion to move the stabilizer, inertia comes intoplay and there would be considerable “coasting” if the designers didn’t installsome sort of system to “brake” the stab trim to a stop when the pilot (orautopilot) releases the trim switch. For electric motors, the circuit will often power thewindings in both directions at once, bringing everything to a quick stop. For hydraulicsystems, the usual control valves will do the trick with closed valves blocking all fluidflow. The result is trim motion only when wanted, and “instant stop” when done.This braking system is usually engaged all the time when trim is not being used, and mustbe released prior to trimming. When you see dual switches under the pilot’s thumb, one isusually to release this brake; the other is to run the trim. As a safety measure, bothmust be moved to get the desired trim change.
The Jackscrew and Trim Motor Stall
By its mechanical design, a jackscrew is almost “irreversible.” Just as youcannot normally turn a nut by pushing on the bolt (unless the threads are extremelycoarse), the immense forces on the stabilizer cannot move the jackscrew arrangement.
Early jackscrew systems on Boeing 707s and Douglas DC-8s were badlyflawed, as designers did not realize how much force might be needed inone unusual case. In one early accident, one of these aircraft flew intoa massive updraft, pitched sharply down (weathervane effect) whilegaining altitude in the updraft. Instinctively, the pilots responded tothe altitude gain and began trimming nose down (leading edge of thestabilizer full up), arriving at that unfortunate stabilizer positionjust as the airplane ran out of updraft and began to respond to thenose-down attitude and trim with a vengeance! The airplane nose pitcheddown hard and the speed built up very rapidly. The pilots promptlypulled back on the yoke to get the nose up, and when they ran out ofelevator travel, they began frantically trimming as well. But therapidly increasing speed placed such a load on the out-of-positionhorizontal stabilizer that the trim motors could not move it from thefull-stop position. The elevators were not sufficient to overcome thebadly mis-set stabilizer, and the airplane was lost.
The solution to this was bigger, more powerful trim motors on laterairplanes, so that no combination of factors will “stall” thestabilizer trim drive motor. Also, trim limits for the thumb switcheswere reduced, with full trim available only by “manual”handles, or the like. New prevention and recovery techniques were alsodeveloped – notably, leaving the autopilot on (but with altitude holdoff), and also flying by straight pitch attitude without chasing theairspeed or altitude with control inputs or power/thrust changes. Sincethese procedures were discovered and implemented, no further upsetaccidents have occurred unless there were other factors involved.Northwest’s legendary Paul Soderlind was an early pioneer in theresearch into high-altitude upsets, and developed the techniques westill use today.
There were also malfunctions in the electrical control systems that ran the trimmotors, so “stabilizer brakes” were added to some. If the trim is in motion, andthe yoke is moved in the opposite direction, the trim is blocked from further movement byvarious means, physical or electrical. On the 727, it was a heavy-duty pin that acts justlike the “Park” position on an automobile’s automatic transmission. Whenactivated, it would slam into the mechanism with a huge “klunk,” bringing it toan instant and violent stop. I hated testing that puppy, it just wasn’t a good sound, andit had to be hard on that pin!
Several different failures can cause unwanted trim, so most airplanes have some sort of”trim-in-motion” warning. Boeing aircraft tend to have little wheels that goclickety-clack when the trim is in motion. The 727 has one that is very large and noisy,but with the added benefit of the pilot being able to actually crank that wheel to movethe jackscrew directly (slowly and painfully). It takes a lot of turns and a strong arm,but it works. The 747 has a similar wheel, but it is driven by very light-duty cables offthe jackscrew assembly, and serves as an indicator only. Early Douglas DC-8s had nowarning at all, until one was nearly lost at JFK and only quick action by the crew savedthe airplane. The trim ran away to full nose-up, but the quick-thinking captain rolledinto a bank steep enough to help counter it until they could sort it out. Ever since then,Douglas products have a loud, very intrusive horn, or even a voice warning that seems toyell at the pilots, often just when someone keys a mike to talk to ATC. You can probablytell which warning I prefer!
Move the Tail
Finally, some airplanes have “all moving” tail surfaces, like the F-4. Theseare most often seen on transonic and supersonic airplanes, due to the very strong controlforces needed in those regimes, and some very bizarre effects from shock waves thatrequire major inputs. These are universally driven by hydraulics, and the systems must bevery carefully designed to permit moving the “center” point for trim, and forsynthetic “feel” to the pilot in all speed ranges from stall to maximum speeds.
Some Unusual Variations
Concorde (please don’t put “the” in front of “Concorde”) has a mostinteresting pitch system, as there is no tail as such. Most of the trailing edge of thedelta-winged bird is taken up with moving surfaces called “elevons”(elevators+ailerons) most of which serve multiple purposes. There are no flaps at all -every landing is a “zero-flapper.” According to James Bedforth, a captain onthat magnificent airplane, this control system is one of the most daunting systems tolearn during the transition training. Not only are there multiple functions of thesurfaces, it is the world’s first “fly by wire” system (with cable backupcontrolling the hydraulic actuators). Trimming simply resets the center point of controltravel, and there is a system to provide artificial feel to the pilots. Further pitchcontrol comes from moving fuel around among 13 tanks to improve the C.G. as needed for thevarious stages of flight. The fuel system is the second most daunting system to learn,according to James.
Lockheed, never content with “simple” when “complex” will do, putanother oddball on the L-1011. This looks like a moving stabilator with trim tabs orflying tabs, but the hydraulics actually drive the stabilator, with the “tabs”being moved by linkages to assist! There is no direct connection between the”tabs” and the control yoke, according to Chuck Tully, a longtime captain onthis machine.
Lockheed also did a complex design on the Connies, where pitch control is by aconventional hydraulically boosted elevator with the usual tabs, on a fixed stabilizer.But with the hydraulic system out, elevator forces on this very large old aircraft wouldbe so high that pilots could not control pitch. So Lockheed put a shift mechanism in tochange the leverage when hydraulics fail. This reduced the available elevator travel by60%, but reduced the force needed by the same amount. You can bet that system gets checkedon every flight!
One of the most bizarre pitch control systems was installed on the unfortunate GrummanF10F “Jaguar,” a little-known and even less-respected effort by a manufacturerof many fine airplanes. Famed test pilot Corky Meyer was the only person to ever fly thisabomination, and he did a fascinating article in the April 2000 issue of FlightJournal.(Highest praise for this publication, it never fails to delight me.)
Picture a long, skinnytorpedo-like thingie mounted right up on the very tip of the vertical stabilizer, with astabilizer fixed to that. That’s funny enough, but now picture that whole torpedo as beingfully free to tilt up and down, no connection to anything, except at the hinge point.Finally, a very small controllable wing was mounted to the very front of that, and thiswas hooked to the stick for pilot control. I get the distinct impression that Corky didn’tlike it much. I get the feeling that somewhere early in the design, someone made a betthat he could do something very different. He did, but I doubt he tried to claim anywinnings on this turkey.
But enough of the mechanics; let’s move on to some failure modes.
A “runaway” stabilizer by itself is extremely dangerous because it can be soinsidious at first. If the pilot is maneuvering, it will manifest itself by increasingpressure in one direction, but it moves fairly quickly, and by the time the pilot realizesthat his trim inputs (thumb switch) are not having any effect, the stab trim is well onits way to a very dangerous condition. It takes very quick thinking to realize “hey,we’ve got a runaway,” then reach over and flip the cutout switches to stop the stabmotion. Yes, the “trim-in-motion” warnings will sound, but since they areconstantly sounding during all normal operations, pilots tend to blank them out. For thisreason, most jets have some sort of automatic “stabilizer brake” installed. Thepilot’s yoke moving in opposition to the trim usually actuates this device. In otherwords, if the trim is running away and moving to produce “airplane nose down,”the normal reaction of the pilot will be to pull the yoke back, and that will”brake” the trim motion fully automatically and well before it gets out of hand.It may cut off electrics, or with a hydraulic trim it may electrically close a valve, orotherwise halt the trimming.
With the Autoflight On
Perhaps worst of all is the runaway stab trim when the autopilot is flying theairplane, because the autopilot itself will counter the initial trim change with simpleyoke movement, without the pilot even being aware of it. Yes, most airplanes have warningsthat will sense a mis-trimmed condition, and even sound an alarm or light a warning light,but by the time action is taken, considerable mis-trim may exist.
Punching off the autopilot is a very natural thing to do, but it’s usually the wrongthing to do. That may produce a very abrupt pitch change that could be very hazardous,either to passengers or to the structure of the airplane. Far better to disable the trim,take some time, analyze the problem, get the trim back in line (if possible) first, eitherwith an alternate stab trim system (if installed) or by changing the speed of the airplaneto match the trim, while either leaving the autopilot on or holding the yoke VERY firmlyas the autopilot is disconnected.
Unwanted Nose-Up Trim
Now, let us assume that for some reason that trim signal gets “stuck” andstarts running the leading edge of the stabilizer DOWN, pitching the airplane’s nose UP.Initially, the pilot (or autopilot) can handle it by feeding in elevator against it – butby the time the cutouts are moved, there will be considerable nose-up trim, and the pilotswill be holding a lot of forward force on the yoke. If there is no backup, or if the stabbecomes “jammed” at that point, it will be a real chore to hold the nose down.With two (or more) people, they might take turns, or brace a knee against the yoke.
Solution? Slow the airplane down until the airplane is at the speed where that trimwould be needed. At that speed, the pilots have plenty of control. When it comes time forlanding, putting the flaps down will cause the nose to pitch down, and the pilots may haveto hold considerable back pressure, or restrict flap extension, or even make a no-flaplanding.
Unwanted Nose-Down Trim
Much more dangerous is the runaway trim in the nose-down direction. Now the pilot willhave to exert a constant heavy pulling force, and this gets VERY tiring VERY quickly. Theonly solution may be to speed up, but if you are already at cruise, that will have littleor no benefit. Any attempt to slow down will produce greater and greater nose downpitching forces, and the pilots will have to exert more and more “pull” tocounter it. On the airplanes like the DC-9 with limited elevator authority (tabs only),this will get very hairy very quickly. As speed drops (or the aircraft descends and TASdrops), the pilots will eventually run out of back yoke, and after that the nose willbegin to drop inexorably. That should increase the airspeed, which would help, but thenatural reaction to increasing airspeed would be to pull the power off, making thesituation worse.
As control is lost, the airplane will go into an outside loop, airspeed will quickly gopast redline, and at some point, structural breakup will probably begin. This is almostcertainly what happened to Alaska 261, and this may have been exacerbated by some part ofthe tail breaking away, further reducing control.
These pitch control systems have proven so reliable for so long on so many airplanesthat most airlines have gotten away from training for “runaway trim” and”jammed stab.” We used to do it all the time, every six months. We’d end upmaking an ILS with both pilots holding very heavy pressure to control pitch. That’srealistic, but miserable, and few of us thought it a worthwhile exercise after doing itonce or twice. Or the instructor would fail it with a nose-up runaway, so that changing tolanding configuration would make it a non-event. I’m personally glad I don’t have to do itevery six months, but the unfortunate part of doing away with it in practice is that manypilots have now forgotten the basic principles behind dealing with pitch trim failures,and that’s bad.
One of the basic principles with this kind of failure is to DISABLE the stab trim, andLEAVE IT THAT WAY. There are exceptions to every rule, including this one, but this is onesystem where you’re almost always better off just leaving it alone once the immediatefailure is contained. In fact, that’s often true of most emergencies, and is precisely thereason we are not supposed to do much trouble-shooting on any systems.
Be careful up there!