Case Study A
The airplane involved was a four-engine turboprop in airline cargo service.The only occupants were the captain, co-pilot, flight engineer, and a dead heading pilot in the jump seat. The captain and flight engineer were highly experienced and both had many hours in type. The co-pilot was relatively inexperienced but had received all required training and was certified and fully qualified.
The flight was cruising at 22,000 feet at night, between layers, in smooth air. The co-pilot was doing the flying, navigating, communicating — everything— and the flight engineer was attending to normal duties at his panel. (The captain’s activities are relevant to what happened, and will be discussed shortly.) To appreciate what happened to the flight during the next 30 seconds,look at this plot of data from the flight data recorder (FDR) and cockpit voice recorder (CVR):
Times of significant events (in seconds) are read against the horizontal scale at the bottom. The triangular symbols mark the time of relevant CVR comments and other key events. The FDR’s heading trace was inoperative.
The flight was entirely normal up to Time 08 (i.e., eight seconds into the plot above), and had been on autopilot in "altitude hold" mode for over 30 minutes. There were some earlier comments on the CVR about a problem with the co-pilot’s attitude indicator, but after switching its source to the captain’s vertical gyro, operation was normal. Here’s a transcript of the final 22 seconds of the CVR:
08.0 First departure from cruise altitude
13.4 "Altitude" — deviation from normal first noticed Unidentified voice, CAM?
16.0 "What’s happening here" Co-pilot’s voice, CAM2
17.6 Increasing wind noise (airspeed increasing)
17.9 "You got it?" Co-pilot’s voice, CAM2
21.0 "No" Captain’s voice, CAM1
24.0 Landing gear warning sounds begin -power levers pulled to IDLE
25.9 Overspeed warning sound begins
28.2 Sounds of structural breakup
29.8 "We’re dead" Unidentified voice, CAM?
NOTE: In the CVR transcript at left, "CAM" denotes cockpit area microphone. "CAM1" is on the left side of the cockpit, while "CAM2" is on the right side. "CAM?" denotes that the location from which a sound came was unidentified.
The first anomaly begin at Time 08 when the flight first departed from the altitude it had held for over 30 minutes, probably when the autopilot was disconnected. This wasn’t discovered for some six seconds as noted by the CAM?statement "Altitude" at Time 13.4. 1.9 seconds after the co-pilot said"What’s happening here?" he asks "You got it?" — presumably asking the captain if he had taken control of the aircraft. The captain’s"No" was without any particular inflection or concern in his voice.
The descent from Time 18 (Point A on the diagram) to Time 30 (Point B)averaged over 32,000 feet per minute, which is consistent with TDS characteristics.
Most shocking was that it took only 14 seconds to go from normal flight to structural breakup, in good weather and smooth air, with an experienced crew and no evidence of mechanical failure.
The fact that the co-pilot was the sole pilot flying/navigating/communicating was related to the captain’s probable activity during an extended period preceding the initial departure from cruise altitude. Specifically, other pilots this captain had flown with reported that he had a long-standing habit of reading what was euphemistically termed "non-operational material"while flying. Indeed, one could hear pages turning on the CVR recording.
The primary cause of this accident was "nobody watching the store,"a factor that has preceded every TDS incident on which relevant data is available. Complicating matters was a relatively inexperienced co-pilot getting no help from either the captain or the flight engineer.
Case Study B
Another classic incident occurred in the earliest years of U.S. airline jet operations. A four-engine airliner — on a moonless night in smooth air over the ocean — got into TDS, diving from 36,000 feet to 6,000 feet before recovering.Recovery load factor exceeded 6g, and despite a wing spar that was permanently bent, the crew managed to land the aircraft successfully. This near-catastrophe started while the captain was in the passenger cabin, the co-pilot was doing paperwork, and the flight engineer and navigator were working behind an anti-glare curtain between themselves and the pilot stations. The captain (in the passenger cabin) was the first to realize something was amiss. The loss of control began shortly after the autopilot disconnected unexpectedly.
Another no-one-watching-the-store case.
After a detailed inspection by the manufacturer’s engineers, the airplane was declared structurally safe, and it went on to fly out the rest of its years without serious incident.
Aerodynamic and Control Aspects of TDS
The basic problem is an airplane’s spiral mode, which is inherent in the airplane’s shape. Virtually all airplanes have weak spiral stability and "want"to start turning, however slowly. The typical airplane, if left unattended, will simply not go straight for long. To believe that it will implies that it has heading stability — i.e., once headed west, it will keep heading west. But it won’t. There’s no such thing as inherent heading stability.
Assume for the moment that:
- the airplane is perfectly shaped
- it is perfectly rigged
- it is perfectly trimmed for straight-and-level cruise
- fuel is perfectly balanced between left and right wings
- if multi-engined, power is perfectly balanced between left and right sides
- there is not a hint of the slightest wind shear
- the air is smoother than a mouse’s tummy
You will never find such perfection in the real world, but let’s pretend it’s all true for purposes of the discussion that follows.
With a perfect airplane in perfect conditions and nobody attending to the controls, one could conceivably continue straight and level for several minutes,but more likely the time is measured in seconds. In any case, sooner or later a wing will drop — it may be either left or right, the direction being entirely random. When the wing drops, the nose will go down and the airspeed will increase — just a little if the bank angle is small. But with one wing down,the airplane will start to turn. The higher wing being on the outside of the turn is moving faster than the lower wing, producing more lift, causing bank angle to increase, the nose to drop further, airspeed increase even more … and on and on, the situation feeding on itself … a "vicious spiral" in more ways that one.
Once the turn starts, one of two things will happen if the turn is not stopped.
Case 1. The Stable Spiral
When a wing drops, the airplane will begin to turn, the nose will go down and the airspeed will increase. After a relatively short time, airspeed will stop increasing and remain a few knots above the original trim speed, and bank angle will remain constant at 20 to 30 or so. The spiral has reached a stable mode and the airplane will continue in a descending turn as long as altitude remains.
Case 2. The Unstable Spiral
Once the turn starts, airspeed and bank angle will continue to increase, a stable state will never be reached, and the spiral ultimately will develop into a near-vertical dive at airspeed and bank angle far beyond all normal limits. Our "perfect" airplane — perfectly rigged, trimmed, etc. — will usually go "all the way" no matter which wing drops initially. (No two airplanes, even if they are of exactly the same type, will react exactly the same.)
Now so far, we’ve been discussing a "perfect" airplane flying in"perfect" conditions. Neither of these ever occur in the real world,where secondary effects make things worse.
Consider the airplane with only a "bendable" rudder tab adjusted to counter "torque" in cruise. (Never mind that the turning tendency that pilots often refer to as "torque" is not torque at all … that’s a discussion for another time.) Though the tab has been adjusted to offset the left-turning tendency attributable to "torque," it will do so only for one particular combination of altitude, power, indicated airspeed, etc. With any other condition, the tab’s anti-turning force will no longer balance the"torque" and a turn will start. The same is true with cockpit-adjustable rudder trim. Anything that unbalances the airplane — a tiny fuel or power imbalance, for example — will require the trim to be readjusted.Such a small imbalance almost always will go unnoticed by the crew until the airplane reacts by starting to turn, at which point the trim is readjusted.
But what if it’s not readjusted? Will the airplane enter a stable spiral or an unstable spiral? Let’s take another look.
When our airplane is cruising serenely along in smooth air, two opposing turning forces are at work — "torque" tending to turn the airplane left, and the rudder tab tending to turn it right. What happens then depends on which of the opposing turning forces is dominant. If, for example, power increases (a temperature decrease can cause that) and if nothing else is changed, the increased torque becomes dominant and the airplane wants to turn left. But if power is reduced, the effect of rudder trim becomes dominant and the airplane wants to turn right. If the right wing drops first, the increased airspeed strengthens the right-turning effect of the tab and the airplane goes "all the way." If the left wing drops first, the tab’s right-turning forces will dominate when speed increases, and the airplane will reach the stable mode.
Every airplane has a built-in turning tendency — even brand new ones —usually due to miss rig or miss trim in roll or yaw or both. The initial turning tendency is usually small, usually difficult to detect at first, and extremely difficult to isolate the specific underlying cause. The onset is usually insidious, beginning very slowly, usually with little or no seat-of-the-pants clues strong enough to alert an inattentive pilot that something’s awry. An airplane cannot suddenly "snap" into a spiral unless it’s grossly out-of-trim in the yaw or roll axis. Nevertheless, a well-developed spiral often develops with astonishing rapidity, as we saw in our first case study where it took only 14 seconds for a large transport aircraft to go from controlled flight to structural breakup.
The only true "cure" for TDS is avoidance, avoidance, avoidance.Someone must be watching continuously what’s going on, and be prepared to initiate recovery from an incipient spiral without delay.
Recovering from TDS
One widely published recovery procedure involves
seven steps, several of which are either unnecessary or can actually be detrimental. Considering the time-critical circumstances under which it might be needed, a seven-step procedure is far too complicated. There is a better one, and it involves only a single step:
Level the wings with slow, gentle rudder pressure, keeping hands off the controls.
Applying rudder produces yaw, which produces roll, and the airplane will unbank. Relax rudder pressure as the wings approach level, then continue to hold them level with the rudder only. Keep hands off the yoke or stick. Do not fret that aileron and rudder are not coordinated in the recovery — coordination is unnecessary.
As the wings begin to unbank, the nose will come up. There is no need to apply back pressure to recover from the dive. The nose will come up by itself with no great increase in load factor (i.e., g force). If the airplane was intrim at a reasonable airspeed before the spiral began, it will return to the same airspeed by itself …
provided the pilot doesn’t interfere by applying pitch inputs!
When the nose comes up, it will momentarily overshoot the original attitude,then pitch down again. This pitch-up-pitch-down cycle will continue, the pitch excursions decreasing with each cycle (engineers call this a "phugoid oscillation") until the airspeed settles down at or within a few knots of the original trim speed. (This assumes, of course, that the airplane was trimmed to a reasonable airspeed before the spiral began, and that the C.G. is within established limits.)
The airplane will "take care of itself" in pitch. It"wants" to seek and hold the airspeed (actually, angle of attack) that it had been trimmed to. Its natural speed-keeping stability will return it to that speed. Let it do so on its own.
Every pilot should get an appropriate demonstration from a knowledgeable instructor that:
- the airplane will begin a turn if allowed to fly hands-off
- the bank angle and airspeed will increase once the turn starts
- the rudder-only, hands-off-the-yoke procedure will work admirably to recover
A Caution to Pilots and Instructors
For a realistic, conservative and safe demonstration, allow the airplane only to
begin a spiral dive.
Don’t let the bank angle increase beyond 25to 30 and don’t let the airspeed get anywhere near redline. Begin at least5,000 feet above the terrain, since the demonstration will take you both below and above the initial altitude. Do the demonstration in smooth air — other wise any gustiness or shear may hide the true effects of the spiral mode. If a spiral dive is allowed to get too steep — meaning with excessive airspeed — the recovery pitch oscillations would be quite large at first, and in the first pitch-up the airspeed may drop to near the stall. Not to worry, the airplane will not stall, though you might get a temporary beep or two from the stall warning system.
Excessive airspeed must be avoided, and the demonstration can be accomplished effectively without getting to a high airspeed.
As with all simulated emergency procedures, caution is the word.
Summary
- An airplane left on its own will sooner or later begin to turn, and airspeed and bank angle will increase. All other factors being reasonably normal, a spiral cannot develop to a dangerous degree suddenly, but it can do so more rapidly than many pilots might imagine … in a matter of seconds.
- Avoidance is the best medicine. A dangerous spiral cannot develop if someone is continuously "watching the store." This is the only guaranteed method of TDS avoidance. Do not depend on an "unsupervised" autopilot, since it may disconnect unexpectedly, and the disconnection may go unnoticed until a dangerous spiral has developed.
- If a spiral develops, use the single-step recovery procedure: Level the wings with slow, gentle rudder pressure, and keep hands off the controls.