From the Editor
The Impossible Turn RevisitedDo what you’ve been trained to do
By Patrick Panzera, EAA 555743, firstname.lastname@example.org
Throughout the years, I’ve read seemingly countless articles in various magazines concerning “the impossible turn.” For those not familiar with the phrase, it refers to turning back to the runway behind you in the event of an engine failure. The concept is simple, unless you have sufficient altitude, your rate of descent is too great for your rate of turn, and simple math will tell you that you’ll hit the ground before completing the turn. Quick example: A 500 feet/minute rate of descent coupled with a “standard rate” turn (2 minutes to complete a 360-degree turn) means that you need at least 500 feet of distance between you and the ground to complete a 180-degree turn. So unless you have a minimum 500 feet above ground level (AGL), the turn is “impossible.” There are other issues here such as wind speed and direction, needing additional altitude to realign with the runway, etc. But like I said, it’s a simple example.
One particular article I read prompted me to write a letter to the editor, who printed it. The article recommended to the readers to steepen the bank to complete the 180 faster. The author recommended a bank angle as great as 60 degrees. The thing that I took issue with and wrote in my letter was the lack of any recommendation to go out and practice this maneuver to perfection before adding it to one’s repertoire. I was certain (and still am) that trying it for the first time in an emergency situation close to the ground would surely end in a stall-spin.
One issue of concern is that stall-speed goes up with the bank angle. So if you were taught that, in the event of an engine failure, you are to pitch for best glide speed (let’s say 70 knots for this exercise), but then you rack it over to 60 degrees where the stall-speed is now 90 knots, you might have a problem. In my letter I referenced the charts in my Cherokee 180 pilot’s operating handbook to make the point.
Two people wrote to that same magazine stating emphatically that I was wrong for referencing this table, as it assumes maintaining altitude while banking. Both persons seemed to have missed one little detail in my note which said, “If we are trained that in the event of an engine failure we should immediately go to best glide speed…” Although we’re most certainly not taught to maintain altitude during an engine failure (duh!), we’re taught to maintain a specific speed, which while banking, requires a certain amount of back-pressure on the stick just as it does to maintain altitude while banking. This back-pressure will add to the wing loading, which will increase stall-speed – perhaps not as bad as trying to maintain altitude, but it will nonetheless. As both persons pointed out, adding to the wing loading will increase stall-speed.
Putting It to Practice
Taking my own advice and wanting to find out firsthand if my theory holds any water, I went to the local flight center and rented a Piper Tomahawk (the same model of plane in which I learned to fly powered aircraft – a plane I’m very comfortable in) and climbed to 5,000 feet (about 4,750 feet AGL). It was a normal Central California spring day, pretty close to standard, so I felt comfortable that weather wasn’t going to skew my attempts to find my own answer to this “controversy.”
All maneuvers were executed a total of four times, twice to the left and twice to the right. All turns were 180 degrees, using roads as my reference. The plane was trimmed to 70 knots and flown straight and level for several seconds after returning to 5,000 feet before entering each turn, to allow all the instruments to quiet down and to make sure I wasn’t in an inadvertent climb or dive. Carb heat was used as a precaution when the throttle was reduced to idle.
First, of course, I tried what I was taught: power to idle, 70 knots, standard rate turn (3 degrees/second). I didn’t pay any attention to the bank angle, but suffice it to say, it was rather shallow. The vertical speed indicator (VSI) was reading 800 feet/minute, and of course after turning 180 degrees, I lost 800 feet.
The second test was exactly the same except I didn’t add any back-pressure to maintain speed. As I turned, the nose dropped and the airspeed indicator (ASI) indicated 75 knots. The VSI was reading 1,000 feet/minute, and since I was maintaining a standard rate turn (bank angle again ignored), I lost 1,000 feet making the 180-degree turn.
Next I tried banking to 45 degrees, using the attitude indicator and ignoring the rate of turn. Holding back-pressure to maintain 70 knots, the VSI was indicating 1,000 feet/minute, but as expected, the stall warning horn came alive with its burbling, not a full horn. I got around the turn in well under 60 seconds, netting an altitude loss of only 600 feet.
Then I tried the same as above, but I didn’t hold any back-pressure. The nose dropped to 95 knots which is a fairly steep nose-down attitude, one which would alarm anyone who is close to the ground. The VSI jumped to 1,500 feet/minute again, something that would freak out anyone trying it this close to the ground who has never tried it before. Getting around the turn took very little time, netting only a 300-foot loss in altitude. I was impressed.
So now it was onto the 60-degree bank that was recommended in the letter I responded to. Let me tell you – that was one wild ride! Certainly not one that anyone would want to try for the first time during an engine-out emergency. It was also difficult for me to execute the maneuver exactly the same way during each of the four attempts (I’m not an aerobatic pilot), so my reporting on the numbers contains a few ambiguities.
At (or about) 60 degrees, there was no maintaining 70 knots. Had I tried it, I’m sure I would have entered a full stall. And this basically proved my point, that when someone melds the two practices – maintaining a specific speed as trained, but adding in the suggested 60-degree bank – bad things will happen if he isn’t experienced with this maneuver. But I digress. About the best speed I could maintain while at the 60-degree bank angle was 80 knots, with the stall warning horn blaring. I totally gave up on the idea of controlling airspeed at 60 degrees; it was just too dangerous (for me), so I went on to simply letting the nose drop.
At a 60-degree bank with no back-pressure, the ASI jumped to just over 100 knots (nearing the yellow arc). The nose-down attitude was uncomfortably steep as I saw through the eyes of someone 700 feet from the ground trying this maneuver for the first time. My windscreen was filled with farms, roads, buildings, and not much in the way of a horizon. The 180-degree turn was accomplished very fast, so fast that the VSI never stopped moving. It peaked at well over 1,500 feet/minute, close to 2,000. As I pulled out of the dive/turn, I picked up 100 to 150 feet while converting the excess airspeed to altitude, netting a total loss of 500 feet. Although a 500-foot loss seems okay, I had to lose close to 700 feet to get it. If the ground was 600 feet below me when I started the turn, I would have been a smoking hole in the ground.
For me, I certainly won’t ever be executing a 60-degree bank if I need to make it back to the runway in a Tomahawk with an engine failure on departure. But from this experience, I would certainly be tempted to bank as steep as 45 degrees and let the nose drop while doing so.
Why Am I So Interested in This Subject?
Several years ago while training for my instrument ticket, I was left seat, under the hood on the missed approach. I had a highly competent CFII (certificated flight instructor – instrument flight rules) in the right seat who is perhaps the most safety-minded individual I’ve ever met. Certainly a credit to his rating. He had recently attended an Aircraft Owners and Pilots Association (AOPA) convention and sat through a seminar on the subject of steep bank angles during the 180-degree turn back to the runway. Sometime earlier, he even presented this information to our EAA chapter.
At about 750 feet AGL, on the left crosswind leg of our departure from the pattern, the Cherokee we were flying suffered a broken crankshaft: 100 percent loss of power. I tossed the hood in the back seat while my training kicked in. I knew we had enough altitude to make it back safely to the runway, so while pitching for airspeed, I entered a standard rate, 180-degree turn back toward the airport. No sooner had I established best glide and a 2-minute turn did my instructor grab the yoke and announce aggressively that we needed to speed up the turn if we were going to make it back.
The 1963 Cherokee we were flying had only a stall warning light on my side (which was illuminated at the moment), out of sight of the instructor who was looking outside the windscreen anyhow. As I told him we were getting close to stall (the ASI was indicating well over 90 knots – best glide is 69), he rolled out of the turn. We were now on what would be considered a close right base, with the runway rising in the windscreen. We weren’t going to make it. Looking at the instruments, I saw that we were still doing over 90. I told my instructor that we could trade this speed for altitude, and he relaxed his grip on the yoke. The plane was still trimmed for 70. It then became apparent that we would most definitely make it back with no problems, so I asked the instructor if I could take the controls back. We had enough energy left to taxi halfway back to the tie-down area.
Long story short, although my instructor had the best intentions and the appropriate head-knowledge, his choice to execute a maneuver which he never practiced in this plane (as opposed to just doing as he was trained, has practiced, and has even trained others to do) could have led to a disaster.