When transitioning between Earth and sky and back again, we fly at the lower end of the controlled-flight regime—as Goldilocks might say, “Not too fast, not too slow, but just right.” Pilots departing generally spend less time in the bottom range of their aircraft’s airspeed envelope than during arrivals and approaches. Departing, we accelerate into the takeoff roll, lift off and, still accelerating, climb. Arrivals are the opposite. We descend and slow to approach speed, enter the pattern, and decelerate even more when sliding down the final.
By the time we cross the runway threshold, we’re deliberately slowing to just short of losing control, a condition that can and has precipitated stalling at a higher-than-desired altitude above the runway, but without the excess speed that can bring on one form of loss-of-control event. It’s in that few minutes when we’re low, slow and maneuvering that a significant percentage of loss-of-control accidents begin. With practice, we nail the landing, though, thanks to the instruments. It’s what the airspeed (ASI) or angle-of-attack (AoA) indicators show us that counts. That’s what happens when we do everything right. It’s not all that hard.
But slow flight, when performed improperly, can be a significant contributor to the annual tally of loss-of-control (LOC) accidents. Which is kind of surprising, since it’s one of the most fundamental skills we start learning early in our training, and have opportunity to practice and perfect again and again. What’s our problem with slow flight?
A Little Background
The NTSB notes that LOC accidents account for about 40 percent of fixed-wing general aviation accidents, the largest share of any other causative factor. The board goes further in its declarations about LOC accidents, noting that statistically, the deadliest phase of flight for LOC accidents are: Approach to landing, maneuvering and climb. All three of these areas involve flight in the low-airspeed ranges of our aircraft, usually the white arc, which of course denotes the maximum wing flap operating speed at its upper end and the landing-configuration stall speed (Vs0) at the bottom.
It’s no surprise, then, that reducing LOC accidents from all causes remains firmly implanted on the NTSB’s 10 Most Wanted List. The suggestion is that pilots should focus on honing their slow-flight skills. Unless a pilot always likes to land fast and long, using runways thousands of feet longer than the aircraft requires, the airplane simply must get slow on the way to touching down, just as it starts slow as it transitions from taxi speed to climbing during the takeoff. So what should we be doing to avoid LOC accidents stemming from flying slowly? One answer is the same as it’s always been: practice, practice, practice.
A Slow Flight Refresher
Somewhere around the third or fourth instructional flight, we begin to learn slow flight for real. You and the instructor depart the airport, climbing to an altitude that assures at least 1500 feet agl after each practice maneuver. After leveling off and clearing the area of traffic, we reduce power to a setting matching the one we use during a normal final approach.
Along the way, we’ll configure the airplane appropriately. For example, if we want the absolute slowest speed it can do, we’ll deploy full flaps and lower the landing gear if so equipped. (If we want to demonstrate clean-configuration slow flight, we won’t.) While reducing power, add in some back pressure to increase the AoA and slow down further—all while maintaining within 100 feet of the altitude at which you started the demonstration—until reaching the airplane’s all-engines minimum controllable airspeed (VMCA). Before you get there, the airplane will decelerate into the region of reversed command, where a slower airspeed requires greater power.
How close you get to the airplane’s stall speed depends entirely on the demonstration’s purpose: Do you want to stall out of it, or recover to cruise? Maybe some standard-rate turns while you’re slow? Regardless, and unless you’re intentionally turning, keep the wings level. The controls likely will feel mushy and unresponsive, and greater control input will be needed to obtain a reaction since the primary controls lose effectiveness at slow speeds. As your pitch angle increases, so will drag. Use power sufficiently to stay within 100 feet of your starting altitude.
For most of us, the stall warning horn will be blaring by now. Depending on the airplane, we also could be experiencing a slight buffet in the airframe as the airflow increases its separation from the wing and advances forward from the trailing edge.
Once you’ve established the slow-flight speed appropriate for your aircraft and purpose, it’s always fun to practice shallow turns. And shallow means shallow: probably no more than 15 degrees of bank, which easily could be much steeper than standard rate. Turn 90 degrees in one direction, then 90 degrees in the opposite direction; then make two 180-degree turns, one in each direction.
Finish your practice with two 360-degree turns at VMCA. All of these turns should end within 15 degrees of the original heading, at an altitude plus or minus 100 feet of the start, while keeping the airspeed within five knots of your entry indication.
If you want to recover back to cruise flight, you’ll likely need to add power and lower the AoA. Reduce drag by incrementally stowing the flaps and raise the gear until you’ve restored normal airspeed and altitude.
Avoiding Slow-Flight-Related LOC Accidents
The NTSB posed a simple question in addressing slow-flight loss-of-control accidents: What can be done? Here are some of the board’s recommendations:
1. Pilots should avoid conditions that can lead to an aerodynamic stall, especially situations approaching wing-critical angle of attack (AoA) and/or decreasing airspeed. This is particularly true at low altitudes, where pointing the nose of the airplane down—an effective recovery technique at higher altitudes—can be a limited option for recovery.
2. Pilots should seek training to ensure that they fully understand stall phenomena, including AoA concepts, and how elements such as weight, center of gravity, turbulence, maneuvering loads, and other factors affect an airplane’s stall characteristics.
3. Airplane owners should consider installing an AoA indicator, which, coupled with pilot understanding and training on its use, can enhance situational awareness during critical or high-workload phases of flight.
Why Learning Slow Flight is Important
Maneuvering tops the fatal-accident lists, with approach and landing next a close second, according to the 24th and latest AOPA Air Safety Institute Nall Report. In fact and according to the Report, maneuvering accidents—which includes landing and takeoff flight segments, plus things like wildlife spotting, but also can involve unwarranted low flying or plain and simple showing off—lead all other flight segments in both non-fatal and fatal accidents. For calendar year 2012, loss-of-control was the top accident cause overall, dominating the maneuvering category, with 31 fatal accidents out of a total of 53. The Nall Report notes that maneuvering accidents remain one of the two leading causes of pilot-related fatalities.
The most common cause was unintentional stalls at altitudes too low to allow recovery, an outright majority of all maneuvering accidents and corresponding fatalities in 2012. Weather wasn’t a factor; 52 of 53 maneuvering accidents took place in visual conditions, 48 of them during daylight, but three were fatal of the four that occurred at night.
For approach and landing accidents, as seen before, inadvertent stalls were the leading factor, implicated in 18 of the 50 accidents, including half of the 24 fatal accidents. Certificate level proved no hedge against these types of crashes, with barely more than one-third of the accident flights commanded by private pilots; lethality was actually highest among ATPs (86 percent) and on two-pilot flights (88 percent).
Remember the Rudder
Regularly practicing slow flight can help inoculate you against an inadvertent stall by keeping you familiar with the sound and feel of the aircraft when flying at VMCA or just above stall speed. Adding altitude-maintaining power at a high AOA likely will re-familiarize you with the amount of rudder you need to offset engine torque and a propeller’s P-factor, whether flying straight and level, attempting to climb or just practicing slow flight as we’ve discussed.
You can help avoid inadvertent spins during slow flight simply by keeping the slip/skid ball within the cage—the two lines marking the center of the curved glass tube—or at least slightly overlapping one of them. Depending on the airplane and how it’s configured, you may run out of rudder authority before the stall break. And when flying a twin and demonstrating VMC, you definitely want to recover well before that point. When demonstrating VMC in a twin, limit your rudder input to approximately half of what’s available. When you no longer can maintain heading, the demonstration is concluded. Recover back to straight-and-level, all-engines flight.
Many CFIs offer this tip about slow flight: Practice making something akin to flat turns using rudder alone. That’s because the roll/yaw behavior of many airplanes couples so well that you can add gentle rudder pressure to precipitate a near-flat bank angle while yawing through the compass points. Practicing 90-degree turns using mostly rudder helps a pilot develop the feel of the airplane before the onset of a stall or spin.
Should the slip/skid ball want to move to one side or another during a slow-flight demonstration, you can back away from the potential for a spin by either easing off the rudder pressure or adding bank angle to bring the turn into coordination—with the little black ball back in the center of the tube. Remember to “step on the ball.”
What did we just learn from our slow-flight exercise? Well, we should come away with a refresher on how the airplane handles at low speed, plus the shallow nature of the bank required for a heading change. The whole thing is kind of like vertically balancing a broom handle in your hand—there can be a lot going on.
One takeaway is that rejected landings are flown quite similarly and begin with the aircraft trimmed for its landing configuration. Next time you’re at VMCA, quickly increase power while stowing the flaps in steps blessed by the aircraft’s POH. Once the flaps finish retracting, you’ve established a positive rate of climb and a comfortable climb speed, the aircraft should be in its normal climb configuration and ready for more slow-flight drills.
Back in the practice area and at your practice altitude, repeat the exercise without flaps. You should experience all the same feelings and sensations, though at higher airspeeds. Repeat regularly and you should never find yourself unknowingly flying too slowly or without correct roll-yaw coordination.
Many aircraft flying in the slow-flight realm treat us to a long solo (or the blinking light) of the stall-warning system. That’s a good thing. But there’s something else that should happen around that time: your aircraft’s particular manifestation of pre-stall buffet. Many airplanes start an aerodynamically induced pre-stall buffet well before the stall system activates. With others, there’s not that much buffeting to begin with, and its onset is well below the stall-warning horn’s onset.
It begins with a slight shake or vibration, increasing to outright wing buffeting as the plane nears stall. Practicing slow flight helps us recognize both types of stall warnings—the artificial one and the inherent one created by aerodynamics. As with all of our flying, practice is the key.
What Else You Need to Know
If you’ve been paying attention the last few months, you probably have heard that the FAA significantly revised what we used to call the PTS—practical test standards—into the airman certification standards, or ACS. Implications of the changes have been seen ongoing discussion, including an article in this issue, which discusses in part the new ACS format for the instrument-airplane rating. There’s a new ACS for the private pilot-airplane certificate, also.
The ACS document breaks down tasks to be demonstrated or performed by student pilots flying their private pilot checkride. Much of it applies as well to the sport pilot certificate, and it’s all built around how closely to the test standard the candidate performs. The sidebar on the opposite page details that portion of the ACS dealing with slow flight.
The new ACS for the private ticket includes a checklist of slow-flight skills the examiner can ask a candidate to demonstrate, so it’s a good place to start when practicing and perfecting your slow-flight skills. Ace this section of your checkride and you should never worry about inadvertent stalls during slow portions of your flight, such as when flying the airport pattern and on final approach.
Dave Higdon is a professional aviation writer/photographer with several thousand hours of flight time in hang gliders, ultralights and airplanes.