TOPIC: AVOIDING THE GROUND WHILE KEEPING THE WINGS ATTACHED
Written by: Karl “Schlimmer” Schlimm
APS Emergency Maneuver Training
CFI, ATP – Multi IFR, IAC, ICAS
FAA – Aviation Safety Counselor
Former USAF F-16 Fighter Pilot & Instructor
RECOVERING FROM THE DIVE
Avoiding the Ground while Keeping the Wings Attached!
Airplanes are truly free to maneuver unrestrained in three dimensions. We know this of course, but we may try to deny the fact that we really mean unrestrained in roll, pitch and yaw – all 360 degrees worth! For the sake of our safety and for that of our passengers, we usually fly within narrow tolerances as we takeoff, climb, level off, turn, cruise, descend and finally land. But if we depart from the relative safety of controlled flight, whether due to pilot input, mechanical failure, or an external influence (such as turbulence), gravity usually wins out, and we find ourselves with a windscreen full of rapidly approaching ground. As an instructor at Aviation Performance Solutions (APS) teaching pilots how to recover from unusual attitudes, I notice that the dive recovery, typically the last step in many unusual attitude recoveries, sometimes does not get the attention it deserves. The same mistakes are made – mistakes, which, if not corrected through a training program such as ours, could be costly in aircraft not as structurally capable as our Extra 300Ls. So, let’s discuss the dive recovery as a separate beast in this month’s article!
The dive recovery procedure that I recommend is as follows:
Power: Definitely pull power to idle if above VA (maneuvering speed) or if about to exceed VA. If below VA, what to do with throttle depends on dive angle, airspeed, and acceleration. At shallower dive angles, at speeds close to VS, and/or if acceleration above VA is not imminent, it may be best to increase power to full.
Push: Move the yoke or stick towards neutral if “loaded” (pulling G’s). This will make ailerons more effective in rolling the airplane upright, and will reduce likelihood of structural failure that could result from rolling and pulling at the same time at high load factors. If beyond 90° of bank, pushing forward will minimize altitude loss until a roll upright can be accomplished.
Roll: Find the nearest horizon (shortest direction to roll upright) and roll to level the wings with respect to it. This will orient the lift in the vertical to expedite the dive recovery. If IMC, the attitude indicator may have tumbled. You can roll towards the high wing indicated on the turn coordinator.
If Spatially Disoriented: You must trust your eyes! After leveling the wings, your vestibular system (inner ear) may make you feel as though you are still rolling or spinning. Attempt to suppress that feeling in favor of visual confirmation of level flight by referencing the horizon reference (VMC) or instruments (IMC).
Neutralize Rudder: Neutralizing the rudder following a spin will reduce the likelihood of a secondary spin during the dive recovery, and will make the recovery more efficient.
Recover from the Dive:
Initiate a smooth pull without delay: If below VA, pull to the aerodynamic limit of the airplane. Do not stall! If above VA, do not exceed the limit load factor.
Terminate the pull when the flight path is reversed: This is especially important if altitude is critical. Pull until the nose is above the horizon – enough to reverse the flight path from a descent to level flight or even a climb (as verified by altimeter and/or VSI).
To understand the forces at work in a dive recovery, reference will be made to Figure 1. This is a V-n diagram (sometimes called a V-G diagram) for a typical general aviation aircraft in the normal category. I’ll discuss two hypothetical recoveries. The first will be a recovery beginning at fairly slow speed, close to VS at point A, and ending near VA at point C. The second will be a fairly high-speed recovery beginning at or above VA at point C and ending close to VNE at point E.
FIGURE 1. V-n DIAGRAM
To understand altitude loss in a dive recovery, it is helpful to understand the factors that actually affect an aircraft’s turn radius. Obviously, the smallest turn radius possible is desired to minimize altitude loss. Figure 2 depicts an equation for turn radius. Note that radius increases with the square of velocity, but decreases with radial G (GR). Radial G is the force that actually turns the aircraft, and it, in turn, is a function of load factor, n, and the dive angle, at least for the purpose of this discussion. Since lift potential (and hence, the ability to pull to a higher load factor) increases rapidly while accelerating along the aerodynamic curve from VS to VA, turn radius can be kept small by pulling harder as airspeed increases below VA. Turn radius typically decreases somewhat as airspeed increases toward VA, but how much depends on dive angle.
FIGURE 2. TURN RADIUS EQUATION
“Low” Speed Dive Recovery: Refer to Figure 3. (The points A, B, and C in this figure correspond to the respective points in the V-n Diagram in Figure 1.) At point A, the aircraft is in a vertical dive, at an airspeed just above VS. In this example a smooth pull is initiated to a load factor of 1.0 G (which is the G force that the pilot feels). All of that lift force is available to turn the aircraft, since gravity (at 1 “G”) acts perpendicular to lift, and does not detract from it. Thus, radial G (the force turning the airplane) also equals 1.0. Full throttle could be applied at this point to accelerate away from VS, and increase maneuvering potential, but there are exceptions to be discussed later.
FIGURE 3. LOW SPEED DIVE RECOVERY
As the recovery progresses to point B, a portion of gravity now counteracts lift’s ability to turn the airplane. Although the load factor has been increased to 2.5 G’s (n = 2.5), radial G is only 1.8 G. As the dive shallows, gravity increasingly detracts from the lift force’s ability to turn the airplane. This penalty is more than offset, however, by pulling harder, thus increasing lift, as airspeed increases. By the time, the dive recovery is just about complete, at point C, the aircraft has accelerated to VA, and a pull to 3.8 Gs, the limit load factor of the aircraft, is achieved. Since gravity now acts parallel to lift, and in the opposite direction at this point, radial G is 2.8 (3.8 G’s minus 1.0 G due to gravity). This is somewhat of an ideal recovery, since airspeed was kept below VA. In reality, this most likely won’t be the case if starting from a very steep dive angle.
High Speed Dive Recovery: Refer to Figure 4 below. (The points C, D, and E correspond with the respective points in the V-n Diagram in Figure 1.) In this example, the dive recovery begins at VA. A smooth pull is initiated to the limit load factor of 3.8 G’s. For the remainder of the recovery, we are limited by the positive limit load factor of 3.8 G’s. As airspeed increases, our turn radius also increases rapidly. Throttle should be immediately retarded to idle. At point C, radial G is 3.8, the same as load factor. At point D, not only has airspeed most likely increased dramatically, thus increasing turn radius, but also, radial G has decreased to 3.1 due to gravity’s influence. This further increases turn radius. By the time the dive recovery is nearly complete at point E, airspeed, in this example, has increased to VNE, and radial G has decreased to 2.8. Therefore, turn radius, fairly small at the start of the recovery, increased dramatically by the time the recovery neared completion. This is why it is absolutely critical to reduce the throttle to idle if above VA. In reality, how fast the airspeed winds up depends on starting airspeed and dive angle. Speeds above VNE could be achieved.
FIGURE 4. HIGH SPEED DIVE RECOVERY
How to prevent pulling too hard and over-stressing the aircraft: Most aircraft are not equipped with G-meters. Therefore, there is no direct indication of the load on the aircraft. At APS, we teach all recoveries to the limit load factor of the aircraft that our students typically fly. A kinesthetic feel for 2.5 or 3.8 G’s is gained through experience.
When pulling on the yoke or stick in a dive recovery, aft pressure required to produce the desired load factor may be light to begin with. The controls are sensitive, G-onset rate is high, and aircraft trim is also trying to pull the nose up. Being aware of this fact is especially important in aircraft with lower limit load factors, say of 2.0-2.5 G’s. Pulling abruptly on the yoke or stick could cause the structural limit of the aircraft to be exceeded. On the other hand, pulling smoothly with a progressive and moderate increase in G’s allows the pilot to “feel” the G-forces increase, giving him or her time to limit the increase in G before limit load factor is exceeded. Most pilots are routinely exposed to G-loads of less than two, and find 3-3.5 G’s very noticeable and sometimes uncomfortable (which is good!). For pilots who routinely fly aircraft with low limit load factors of 2.0-2.5 G’s, it might be beneficial to practice a few 60° banked turns in a suitable aircraft to get a feel for at least 2 G’s.
What if a stall occurs during the recovery? Stall speed increases with increasing load on the wings, whether it is due to pulling G’s or adding weight to the aircraft. Remember, critical angle-of-attack can be exceeded regardless of the aircraft’s pitch angle. Refer to Figure 5. Angle-of-attack (AOA) is the angle between the wing’s chord line and the relative wind, and at any point in a dive recovery, relative wind is tangent to our flight path. Although the aircraft’s pitch angle (typically the angle between the longitudinal axis of the aircraft and the horizon) could be very steep, AOA could easily approach or
FIGURE 5. ANGLE-OF-ATTACK IN THE DIVE
exceed critical AOA if the pilot pulls too hard for the airspeed available. Upon consideration, this may seem obvious, but I often see students of our Emergency Maneuver Training Course surprised to find themselves in a stall while recovering from a dive with the nose well below the horizon and with seemingly plenty of airspeed. If a stall does occur, it is critical to move the yoke or stick forward enough to break the stall before resuming the recovery. This is difficult to do with a windscreen full of rapidly approaching ground, and is a valuable scenario to experience in a training environment, albeit with plenty of altitude.
How can a pull right below the aerodynamic limit be accomplished? It is not desirable to have the stall be the first indication that the pull was too hard! The answer is – it depends, on the aircraft and how it warns the pilot of approaching critical angle-of-attack. A pull just hard enough to activate the stall warning horn could be performed. The stall warning horn is essentially a crude angle of attack indicator. If it typically gives 5-10 knots of warning as the pilot slow towards VS in more normal flight regimes, it will also give warning that critical angle-of-attack is near as a pull towards, but below the aerodynamic limit is accomplished, increasing load factor as airspeed allows. Some corporate jets and airliners have stick shakers that provide an artificial buffet or vibration to the yoke or stick when approaching critical angle-of-attack. One could pull smoothly until the stall warning horn or stick shaker activates then back off slightly, repeating the process as load factor is increased with increasing airspeed.
What are some considerations for adding power in a dive recovery at airspeeds below VA? The low speed dive recovery discussed earlier is a hypothetical example only. Certainly, if the aircraft winds up in a vertical dive, airspeed increase will be swift throughout most of the recovery. Even if the throttle is kept in idle, it is very likely that VA will be exceeded before recovery is complete. And although applying power may seem appropriate initially, the much higher airspeeds, well above VA, that may result during the later stage of the dive recovery (even if throttle was subsequently retarded) and accompanying high turn radius might just negate any advantage of increasing power. At the risk of being too long-winded, I want to caution both students and instructors of unusual attitude training. Most of the guidance on dive recoveries pertains to military or aerobatic aircraft with high limit load factors and a much wider airspeed spread between VS and VA. Some military aircraft even have speed brakes that can slow airspeed increase. In aerobatic aircraft, just pulling to 6 G’s produces a tremendous amount of drag, which can curb acceleration. In some of these aircraft, it may be appropriate, if below VA, to just throw the throttle forward and “pull to the buffet.” Such guidance could be detrimental if applied indiscriminately to general aviation, corporate and airline type aircraft. Whether to increase power or not, depends on how steep the dive is and how close we are to VA to begin with. For instance, following some spin recoveries, the dive is near vertical. In this case, it may be best to leave the throttle in idle until the dive recovery is nearing completion. There is one time that I will always increase power, and that is if the flight regime is “low and slow,” as it is on final approach. Quick reactions are necessary to minimize dive angle and power should be increased to minimize altitude loss and increase maneuvering potential (airspeed). In any dive recovery with shallow to moderate dive angles that begin well below VA, increasing power will allow the pilot to increase airspeed and maneuvering potential while minimizing altitude loss. Otherwise, as the recovery is initiated, and the airspeed increase stagnates close to VS, the pilot may repeatedly find him- or herself in the stall buffet with the commensurate sink rate.
How to minimize altitude loss? While the pilot can’t see the aircraft’s turn radius, the nose’s track up to the horizon can be monitored. The rate of turn, in this case in the vertical plane, is also a function of airspeed and radial G, just as turn radius is. Although radius is obviously the critical parameter, turn rate dramatically increases when pulling to the aerodynamic limit, while accelerating from VS toward VA. Therefore, a healthy nose track toward the horizon is a good indication of a fairly tight radius. Be careful though, turn rate decreases dramatically as airspeed increases above VA. For instance, if recovering from a dive at very high airspeeds the nose may not be tracking to the horizon as fast as the pilot desires, even when pulling to the limit load factor. But attempting to rush the recovery by pulling harder could cause structural damage or failure. One thing is for sure though, if a stall occurs in a dive recovery, the turn rate will drastically decrease! The nose will stop tracking and the aircraft will continue it’s descent downward. In this case, the pilot must, as stated earlier, push forward enough to break the stall, and then continue the pull. A healthy turn, with the nose of the aircraft tracking upward, will be a good indication that the recovery is working.
What if the airspeed increases above VNE? Don’t panic! A smooth pull with throttle in idle is imperative. Pulling abruptly will only aggravate the problem. While VNE should be respected as a structural limitation, there is a buffer between VNE and design dive speed (the speed above which the test pilots have determined that bad things can start happening to the aircraft), just as there is a buffer between limit and ultimate load factor.
What are the Common Errors when practicing dive recoveries?
Hesitation: When faced with extreme dive angles, or any situation outside of a pilot’s comfort zone and level of training, it is natural to hesitate as we ponder what to do next. But hesitation can greatly magnify altitude lost. Proper training will minimize hesitation.
Not rolling upright quickly: If the aircraft’s wings are not level with respect to the horizon, then the wing’s lifting force is not where it should be to expedite recovery. Moreover, attempting to simultaneously pull while rolling wings level can produce high stresses on the certain parts of the aircraft. I also notice that if students attempt a loaded roll out, they usually do not get the wings level nearly as fast as if they perform the “push-and-roll” technique mentioned at the beginning of this article. None-the-less, it is imperative to get the lift in the vertical quickly.
Pulling too abruptly: Again, this could result in a stall that delays recovery below VA, or even worse, structural damage or failure if above VA.
So there you have it, dive recoveries in a nutshell. We covered quite a few considerations for a flight regime that looks easy and sometimes is viewed as merely the period at the end of the unusual attitude experience. But as you have seen, performing the dive recovery correctly can be critical in ensuring that we neither exceed the structural limits of our aircraft nor hit the ground!
What is your best defense in aircraft unusual attitude or upset conditions?
- First, attempt to avoid conditions that can induce unusual attitudes in the first place! Steer clear of thunderstorms and wake turbulence! Avoid IMC or flight into low visibility conditions if not properly certificated and trained. Avoid distractions.
- Second, get the proper training. According to an article in AW&ST (May 8, 1995 issue): “Training should include flights in aerobatic aircraft to practice recovery techniques because no simulator can model the disorientation of actually being upside down… recurrent training every two years, with time in an actual aircraft, would be a good start.” Regardless of the aircraft that you fly, proper training will enable you to learn to react decisively in a high-pressure environment, and to learn proper recovery techniques to avoid a “panic” response that could worsen the situation.
- Contact a APS – Emergency Maneuver Training representative. Certainly, we would like to take this opportunity to recommend our program at APS which offers three course layouts to choose from. Please give us a call a 1-866-FLY-HARD and ask to speak with a flight training specialist or submit the form below for more information today!
Get this training somewhere. The life you save may be more than just your own.