Pelican's Perch #22:
Short- and Soft-Field Takeoffs — FAA vs. Reality
What's the best technique for taking off from a short or soft field? According to John Deakin, there's the FAA way (as documented in the POH and the FAA's Flight Training Handbook), and then there's the right way. The problem with the FAA way, says Deakin, is that it's predicated on certification requirements which are totally unrealistic. As usual, AVweb's resident pelican shreds the conventional wisdom, then explains how it's really done.
I hate what seems to be the official FAA-approved way of handling short and soft fields, and it's past time I yelled about it. There seems to be a good deal of confusion in the pilot ranks, and even among some CFIs, over the differences between short-field and soft-field technique, and I'd like to comment on that, too.
A major problem is that these techniques are almost always taught from very long, very wide hard-surface runways. This isn't very realistic or meaningful. It may not be necessary to go find a critically short runway for this training, but we need a lot less than a vast stretch of concrete with no reference points to properly train for this maneuver.
The current training is all but useless, in my opinion. It might as well be discontinued for all the good it does. A pilot at any level can get entirely through the course, take the final FAA checkride, demonstrate the FAA-approved way of doing it, and still not have the faintest idea how to really do one properly.
Beyond that, the lessons learned are the wrong ones, and do not serve pilots well as they move into larger aircraft. The misconceptions taught early are widely shared today among the professional crews operating jet transports. At least the FAA has the theory of "Primacy" right — these pilots were taught wrong from the start, and they remember it well!
It would be far better to have a real soft field (nice deep MUD) and to have a row of trees alongside the runway for reference, so the different techniques might be compared from one takeoff to the next. CFIs would also do their students a favor if they found a runway 30 feet wide for some training, but that is another story.
Tearing Up the FTH
Here's the hot poop from the head shed, right out of the FAA's Flight Training Handbook (FTH), from 1965. No, you didn't misread that — the poor old thing hasn't been updated in 35 years, yet this is "The Bible" as far as the FAA is concerned! Now to be fair, airplanes still fly pretty much like they did 35 years ago, and there's a lot of really good stuff in there. But it's sadly out of date in many things, and flat out wrong in a few.
Let me show the text from the FTH here in italics, and comment on it as we go. Then I'll tell you what I really think.
|Short-Field Takeoff and Climb
Taking off and climbing from fields where the takeoff area is short or the available takeoff area is restricted by obstructions, requires that the pilot operate the airplane at the limit of its takeoff performance capabilities.
Sure, but there are two entirely different scenarios here! One is a short runway, and the other is nearby obstructions. These call for entirely different and conflicting techniques. I cannot begin to imagine why the FAA chose to lump them into one catchall procedure, which serves neither purpose very well.
Furthermore, this is a true statement only if the actual field is the absolute minimum-length field from the beginning of the runway to the assumed 50-foot obstacle. Frankly, it's very unlikely that any pilot in all aviation history has ever needed this very peculiar combination.
|To depart such an area safely, the pilot must exercise positive and precise control of airplane attitude and airspeed so that takeoff and climb performance results in the shortest ground roll and the steepest angle of climb.|
Again, there may be two conflicting requirements, and the shortest ground roll does not give the steepest angle of climb, nor does the technique required for the steepest climb give the shortest roll. That statement is clearly incorrect.
|The result achieved should be consistent with the flight performance section of the FAA-approved Airplane Flight Manual or the Pilot's Operating Handbook.|
Here we get the first hint on certification requirements, and the terrible, terrible idea that we should fly the airplane that way in normal operations. The FAA has decided that takeoff performance must be determined. The requirements for establishing this are found in FAR Part 23 — Airworthiness Standards: Normal, Utility, Acrobatic, and Commuter Category Airplanes.
Certification vs. Reality
In that part, you will find detailed instructions to the manufacturers on just how to determine the certification takeoff distance. All are based on the classic scenario of a 50-foot obstacle, and the runway (or clearway) is assumed to run right up to the obstacle. In other words, the test pilot can roll to the trees and do a vertical climb (clearly impossible), or he can haul it off the ground at the slowest speed permitted (120% of the stalling speed) and thereafter climb at that speed or any faster speed he wants.
These certification requirements drive virtually all test pilots, in all cases, to leave the airplane on the ground to just under 120% of stall, then pull it off briskly to achieve and maintain that 120% to 50 feet.
To illustrate, the book says my airplane stalls at 64 knots at max gross (that's also wrong, but let it go for now). 120% of that is 77 knots. If we check the fine print on the takeoff chart, we see:
Once the test pilots do that a number of times, they get proficient enough to do it with utter precision, even on a bad hair day. On one of the good days, under perfect conditions, someone measures that point on the runway at which the airplane lifts off, and the point over the runway at which it reaches 50', and those measurements become the "Ground Roll" and "Takeoff Distance" in the POH.
|In all cases the power setting, flap setting, airspeed, and procedures prescribed by the airplane's manufacturer should be observed.|
Maybe, if you have a situation where reality is the same as the certification standard. Find me one case, please? Anyone? A 50-foot obstacle, exactly in your way, exactly at the minimum distance for your takeoff, "today"?
Folks, the stuff in the POH is certification stuff, and is not the most effective data for the real world.
|In order to accomplish maximum performance takeoff safely, the pilot must be well indoctrinated in the use and effectiveness of the best angle of climb speed (Vx) and best rate of climb speed (Vy) for the specific make and model of airplane being flown.|
I don't like this. Again, you will probably never face a takeoff involving both minimum runway, and obstacles at the minimum distance. If you are, you need to stop, do some very careful figuring, and maybe get a professional bush pilot to fly it out for you. Better yet, rent a heavy-lift helicopter.
What Good Are Vx and Vy?
If you will allow me to separate the takeoff maneuver itself from the climb well after the takeoff maneuver is complete, then Vx and Vy have nothing to do with the takeoff. Vx and Vy are steady-state airspeeds that are useful for steady-state conditions. The takeoff is not such a steady-state condition, and these speeds are not useful below 50' (except for test pilots during certification).
|The speed for best angle of climb (Vx) is that which will result in the greatest gain in altitude for a given distance over the ground.|
Correct, and very useful for climbing over yonder mountain ridge if you're climbing up a canyon that won't allow a turn. But if you're down in the gully, trying to get out of tough strip and you're worried about Vx, you really shouldn't have been there in the first place, and it's time for that heavy-lift helo again.
|It is usually slightly less than the speed for best rate of climb (Vy) which provides the greatest gain in altitude per unit of time. The specific speeds to be used for a given airplane are stated in the FAA approved Airplane Flight Manual or the Pilot's Operating Handbook. It will be found that in some airplanes, a deviation of 5 knots from the recommended speed will result in a significant reduction of climb performance. That being the case, precise control of airspeed has an important bearing on the safety of the operation.|
All true. Vy is good for "Gosh, it's rough down here today, I want to get up to a smooth altitude as quickly as possible so my spouse doesn't do a technicolor yawn." It's also good for the above "yonder mountain ridge," because you can usually do a turn or two, and take advantage of Vy to clear that ridge in less time.
In most high-performance airplanes with air-cooled engines, either Vx or Vy is going to run your engine temperatures way up, and I don't like that, even if they do remain within factory limits. On the other hand, if it's not my engine and it is my tummy, please do climb as rapidly as possible.
Additionally, forward visibility will be so poor in most airplanes that a Vy climb is clearly unsafe, and should only be used when absolutely necessary. Vx is worse, almost an emergency speed.
The Takeoff Roll
|Taking off from short fields requires that the takeoff be started from the very beginning of the takeoff area and the airplane accelerated as rapidly as possible. At the field threshold, the airplane is aligned with the intended takeoff path and maximum allowable power applied promptly while releasing the brakes.|
This is much too simplistic, but perhaps good enough for a manual at the primary level. Tests have shown there is very little difference between stopping, then going to full power with brake release, or rolling onto the runway at a normal taxi speed, and turning onto the takeoff heading while going to full power. If anything, the latter will produce slightly better performance, and will certainly save your prop from dings and nicks from the gravel and dirt that often goes with the short runway.
|If the use of flaps is recommended by the airplane manufacturer, they should be extended the proper amount before starting the takeoff roll. This permits the pilot to give full attention to the proper technique and the airplane's performance throughout the takeoff. There is no significant advantage to extending flaps just prior to liftoff.|
Again, this is probably appropriate at the primary level, and with many modern aircraft with flaps that cannot be extended quickly. But it is clearly untrue with some airplanes, under some conditions, and veteran bush pilots will snicker at the thought of throwing away the useful technique of "popping" the flaps at the last minute on a Super Cub or an older 180 or 182 with the lovely big "Johnson bar" flap lever.
|Takeoff power should be applied smoothly and continuously — there can be no hesitation — to accelerate the airplane as rapidly as possible. As the takeoff roll progresses, the airplane's pitch attitude and angle of attack should be adjusted to that which results in the minimum amount of drag and the quickest acceleration (Fig. 8-4). (Emphasis by author.) In nosewheel type airplanes this will involve very little use of the elevator control, since the airplane is already in a low drag attitude. In tailwheel type airplanes, the tail should be allowed to rise off the ground slightly, then held in this tail low flight attitude until the proper liftoff or rotation airspeed is attained.|
All good, except that final sentence is not very clear to me. It seems to suggest a tail-low attitude for the taildragger, and not for the nosedragger. For a short-field takeoff on a hard, smooth surface, we want as little aerodynamic drag as possible, and this implies a level attitude for minimum (parasite) drag, and minimum lift (with attendant induced drag) being produced during the acceleration phase of the takeoff. It makes no difference which end has the little wheel — the attitude during the takeoff roll should be the same. Ideally, no lift should be produced when using a hard-surfaced runway.
|For the steepest climbout and best obstacle clearance, the airplane should be allowed to roll with its full weight [i.e., no lift — author] on the main wheels and accelerated to the liftoff speed.|
Yes! The FAA got one right, this time! But note this is only true on hard smooth runways.
|The airplane should be smoothly and firmly lifted off, or rotated, by applying back pressure on the elevator control as the best angle of climb speed (Vx) is attained. Since the airplane will accelerate more rapidly after liftoff, additional back pressure becomes necessary to hold a constant airspeed. After becoming airborne, a straight climb should be maintained at the best angle of climb speed (Vx) until the obstacles have been cleared or, if no obstacles are involved, until an altitude of at least 50 feet above the takeoff surface is attained.|
Ahh, they were doing so well until that "if no obstacles are involved." What the heck does 50 feet have to do with anything, if there are no obstacles? Why the 50 feet? As we shall see later, if you are at Vx in most high-performance singles at 50 feet and the engine quits, you're dead.
Also, and very important, note they don't mention the end of the runway. This dreadful paragraph clearly assumes certification runway length, which you will probably never see! If no obstacles are involved, why not just leave the airplane on the ground until nearing the end of the runway, taking the additional speed gained? Why force it to be a "short-field takeoff" when there is perhaps enough runway to make it a "normal" one? Far too many pilots learn to fly off a 10,000-foot-long, 200-foot-wide runway, then when faced with a 1,500-footer, their heart rate accelerates, and everything they've been taught is screaming at them "short field, short field," and they are suckered into using a clearly inappropriate technique!
|Thereafter the pitch attitude may be lowered slightly, and the climb continued at the best rate of climb speed (Vy) until reaching a safe maneuvering altitude.|
What is a "safe maneuvering altitude," please? Anyone? There is a terrible tendency to overdo this, with some even saying it's the altitude from which you can execute an engine-out landing back on the runway. At 99.9% of the airports we use, a "safe" altitude is anything from about ten feet above the runway on up.
|Unlike a short-field approach where the power can be varied to maintain the desired approach airspeed, the short-field takeoff requires the use of full takeoff power. Since the power setting is thereby fixed, airspeed must be controlled by adjusting the pitch attitude, which in turn will also vary the climb angle.|
Badly said, I think. Better might be "At full power, the steady-state climb angle will vary with airspeed, with the steepest angle occurring at Vx. Since the power is fixed (full takeoff power), the airspeed must be controlled with pitch in this case."
|Remember that an attempt to pull the airplane off the ground prematurely, or to climb too steeply, may cause the airplane to settle back to the runway or into the obstacles.|
Not exactly. This is true only if the performance is marginal, as at high elevations and density altitudes, or with very underpowered airplanes, which is pretty much the same thing. For example, take a 172 to Crede, Colorado, 9,000 feet MSL and 6,500 feet long, and you might need to accelerate in a level attitude to Vy (yes, Vy, not Vx), then gingerly lift it off, and accept a ten-foot-per-minute rate of climb. If you pulled the nose up to the point where the tail was almost dragging (soft-field technique), the extra drag might well prevent acceleration above a very fast taxi, and you'd roll right off the end, still too slow to fly.
The most (in)famous example of this was a North American F-86 "Sabrejet" being flown by a pilot not very familiar with the breed. The aircraft, like most early jets, is rather underpowered at low speeds, and only really becomes a real airplane above about 200 knots. The new pilot lifted the nose a bit too early on the takeoff roll, and too much, causing too much drag. This reduced the acceleration (or stopped it entirely), and the airplane rolled (or flew in ground effect, I forget) right off the end of the runway in that nose-high attitude with less than flying speed, slamming into an ice cream parlor with a bunch of kids in it. This nearly killed the "vintage" and "warbird" aircraft community, and the FAA instituted a ton of regulations and changes as a result of "The Sacramento Ice Cream Parlor Crash."
While the runway was none too generous, it was well within the capability of the aircraft that day, under those conditions. Had the pilot simply left the nose on the ground, in a level, low-drag attitude and gained a bit more speed first, the airplane would have flown out nicely. The kids would have been startled at the "low pass," then delighted, instead of killed.
|On short-field takeoffs, the flaps and landing gear should remain in takeoff position until well clear of obstacles (or as recommended by the manufacturer) and the best rate of climb speed (Vy) has been established. It is generally unwise for the pilot to be looking in the cockpit or reaching for flap and landing gear controls, until obstacle clearance is assured.|
Well, yes, but again, this is appropriate to the primary level, and as a general rule only. With more experience, and in some conditions, it may be better to be able to retract the gear without looking for the switch (with practice), and retract it early to get over some distant obstacle, or even to get out of ground effect. With that 172 at Crede, you might end up being able to get airborne into ground effect, but never be able to accelerate or climb out of it. You'll wish you could retract the gear for that little bit of extra performance!
|When the best rate of climb speed has stabilized, retraction of the flaps may be started. It is usually advisable to raise the flaps in increments to avoid sudden loss of lift and settling of the airplane. After the flaps are fully retracted, the landing gear should be retracted (if so equipped) and the power reduced to the normal climb setting.|
BAD advice! Bad, bad dog! Go to the doghouse, without dinner! Someone's nose ought to be rubbed in that mess, then they should be thrown through the window (No, I don't do that to dogs, just bad FAA people). I know of no airplane for which this advice is valid on takeoff, and a whole bunch of airplanes where it is clearly incorrect. That's bad initial training, because it will require breaking a bad habit later.
Remember that the FTH is talking about takeoffs here! The flaps should be set for "high lift" and "low drag." The airplane should fly just fine with that, and the gear may be retracted normally, improving performance.
(There is always the caveat about the airplanes that open gear doors first in the retraction cycle, momentarily adding to the drag. Some of these may need additional speed or altitude before retracting the gear.)
On the other hand, missed approaches and go-arounds generally involve the use of "high lift/high drag" flap settings for the landing, and it is the almost universal practice to retract the "drag" portion of the flaps first, then the gear, then the "lift" flaps. This is good procedure in everything I know of, from a 150 to a 747. But we are ripping up takeoff procedures here, not approaches and landings.
Real Short Fields
What's the best technique for really getting off short? As usual, "it all depends." To get the absolute minimum ground roll, you need to get it airborne at the slowest speed possible, just above the stall, with the flaps set to the position that gives the greatest lift, but without so much drag the airplane won't accelerate in level flight, in ground effect. This can vary from airplane to airplane.
For many of the common GA airplanes, at sea level, that implies full flaps, and if you can't "pop" them at the last minute, they should be set before starting the takeoff run. We could get really fancy and time them, starting the extension at just the right time so that they reach full down precisely at the point of liftoff, but that's hardly practical!
The airplane should be accelerated in a level (no lift) attitude to the minimum flying speed, then the airplane should be aggressively lifted off into ground effect, and no higher, because if any ground effect is lost, the airplane will stall. This is not good.
To those who yell "What about 1.3 times the stalling speed?" I ask "What about it when you're making a full-stall landing? If you can stall it out on landing from six inches up, what's wrong with lifting it six inches off the ground just above the stall on takeoff?
What's this aberrant technique do to climb performance? You guessed it, it kills it, at least until some speed is gained, the flaps can be eased up to a "high lift, low drag" position, and the gear retracted.
Using this technique, a fully grossed Bonanza, at sea level standard conditions, with no wind, can be hauled off the ground in about 500 feet, and kept off, with a gradual acceleration, and a return to normal flight. But at the point of liftoff, there's nothing left for a turn, or any climb at all. Not for the neophyte, but it's not that difficult or dangerous to practice, either.
If there is, say, 700 feet available, then leave the airplane on the ground for 200 more feet, there will be more speed to play with, a quicker acceleration, and an earlier opportunity to clean up.
Nearby obstructions call for a careful review of the published data as a guide, and a lot of judgment that can come only with practice and experience. You know the saying, "Good judgment comes from experience, and experience comes from bad judgment." But the book performance can be improved upon.
Let us assume that you have the certification case, "for real." There is a fifty-foot concrete wall ahead of you, at exactly the distance the POH says you can clear it. Would any of us go? Of course not, at least not without a loaded gun at our heads! But assume we must go (a hot date with a tall blonde) and our desire overrides our common sense — a trait not unknown among pilots, at least those I know.
How can we improve on the book performance? Simple. Leave it on the ground until the last possible moment (judgment, again), which should give us some speed above Vx, haul it off the ground and climb with decreasing airspeed, reaching the fifty-foot top of the wall at stalling speed (out of ground effect). Squeak over the wall, lower the nose to gain speed again, and you're on your way.
This is a thought exercise only! I am not suggesting that you do this! I am merely trying to illustrate the difference between certification performance, and "real world" performance.
These are among the most challenging of all conditions faced by bush pilots. The "softness" of the runway is very seldom constant along the runway, there will be "soft spots" and "hard spots," and speed gained on the "hard spots" may be lost again in the "soft spots." There is simply no book data that will help, and no reliable rules. Soft fields simply cannot be simulated properly, and few pilots will ever have the opportunity, or the desire to play with them. Few wish to expose expensive machinery to deep mud or snow for fear of damage, and even deep wet grass can make an awful mess.
But if you ever get the chance to wallow around in the mud with something like a Super Cub, don't miss the opportunity — it's a ton of fun!
The highest possible angle of attack is desirable, to lift the airplane out of the mud and reduce the drag on the landing gear. The more weight supported by the wings, the better, for that means the less weight the wheels must support. The shallower the ruts, the less drag, and the sooner the airplane will reach flying speed. But don't drag the tie-down ring on the tail, that will add drag, too.
In Short (So to Speak) …
The book data assumes certain very rigid, very standard conditions. This may be helpful as a guide, and it certainly allows comparing different airplanes under identical conditions. But you will probably never see those conditions in the real world. What you do see will call for some understanding of how the book numbers are established, and may allow you to gain additional performance from your machine by altering the "standard" techniques to suit the actual conditions.
Be careful up there!