In the March issue of AOPA Pilot magazine, I wrote an article titled: In My Humble Opinion.... In it I said that restricting yourself from applying flaps in a turn is an unfounded idea. I still think that's true, in spite of a few of my fellow pilots who were worried that they are exposing themselves to dangers if they ever did this. While there are many things to worry about, in my humble opinion, applying flaps in a turn isn't one of them. At least not in a modern day general aviation airplane.
The reasoning behind the belief is that a flap might fail, and the resulting split-flap condition would cause the airplane to roll uncontrollably. The plane might also get hit by lightning and I suspect that the probability is greater for a jolt from above. There are several reasons for that.
First, split-flap failures are exceedingly rare.
Second, there is a 50-50 chance that the flap failure would be on the outside wing and thus roll the plane level, a rather benign result.
Third, and perhaps most important, modern GA aircraft are deliberately designed to cope with such a situation.
Most of our general aviation airplanes have flap interconnects, which makes a split-flap condition unlikely -- in my opinion, no more likely than an aileron or an elevator failure. Those airplanes not having flap interconnects must be shown flyable under split flap conditions. I believe the Super Cub, Aztec and Navajo fall into this category. Here is an excerpt from Part 23 that supports this point:
Part 23 -- Airworthiness Standards: Normal, Utility, Acrobatic, and Commuter Category Airplanes
Mechanical interconnection requirement of Sec. 23.701(a)(1). This requirement is to ensure against hazardous asymmetrical operation of the flaps after any probable single or probable combination of failures of the flap actuating system. A probable combination of failures should be considered when the first failure would not be detected during normal operation of the system, including periodic checks, or when the first failure would inevitably lead to other failures. (Systems where a probable combination of failures may occur could include the electrical and hydraulic systems.) The airplane must also be shown to be capable of continued safe flight and landing without requiring exceptional pilot skill or strength following these failures. To demonstrate that the airplane is safe under these conditions, tests should be conducted with the flaps being retracted on one side and extended on the other during takeoffs, approaches, and landing. If there is a probable hazardous condition, a separate positive connection that is not part of the flap actuation system is required.
Here's an excerpt from Aviation Safety magazine (January 1st issue, 1986) that speaks to split flap conditions:
A Cessna spokesman said most Cessna single-engine aircraft have undergone split-flap testing, even though the typical Cessna cable interconnect is considered to fall under the failure-proof part of the rule [FAR Part 23]. According to Cessna, a split-flap condition in flight "will not produce a great deal of change. Normal pilot responses in roll and yaw can control the aircraft from Vfe [maximum flap extended speed] down to the stall speed.
Considering the efficiency of Cessna's Fowler flaps, airplanes having flaps that produce less lift and drag seem to me even less likely to produce controllability problems in the event of a split flap condition.
Intuitively, the failure of a single flap seems no more likely than an aileron or an elevator failure. Yet, we don't restrict our usage of these other controls in any phase of flight for fear that they may stop working. It's true that flaps are probably subject to more aerodynamic pressure when they're extended. Structurally, however, they and their hinging mechanisms are designed to compensate for any increased loads. In fact, you could make the case that flaps applied anywhere below Vfe [maximum flap extended speed] come nowhere near the maximum stress they're capable of handing.
I searched the past 16 years of NTSB data and only found one instance where a plane -- a homebuilt -- (an ANDREW D. MOORE HP-18 [I believe this is a homebuilt]) had a split-flap condition (www.ntsb.gov/aviation/atl/86a179.htm). This accident resulted from the flap having been modified with pop rivets at the attach point.
Here's another thought worth considering. The Piper Arrow had a wonderful device that automatically lowered the gear when the airplane got too slow (unless, of course, the pilot chose to override the automatic device). Apparently Piper thought about, designed and applied a device to keep pilots from landing gear up because this was something that happened frequently ? at least frequently enough for such a device to be put on an airplane. Isn't it interesting that there is no device on any general aviation airplane (at least that I'm aware of) that restricts a pilot from applying flaps in a turn? I've never heard of anyone even suggesting such a device. I think we might infer from this that there is simply no danger associated with lowering flaps in a turn.
Even if none of the above were true, even if there was danger associated with lowering flaps in a turn, a pilot still has no reason to avoid applying flaps in a turn if he or she applies flaps correctly. Why? If the pilot applies flaps properly, the failure of a single flap shouldn't present an insurmountable difficulty. It's simply good operating practice to apply flaps in increments, rather than all at once. They should be retracted the same way. If a flap fails after 10 degrees of flap application, you immediately retract the remaining flap and minimize your problem. This certainly provides a more controllable condition than if you lower the flaps all at once and experience the failure of a single flap. Also, you should never take your hand off the electric flap handle until the flaps have extended within the range selected. Why? If a flap fails, you can immediately retract the flaps and, hopefully, minimize any difficulty.
Here's one more thought. If applying flaps in a turn is supposedly such a bad thing, then it's bad because it subjects us to a greater potential for an uncontrolled roll if the inboard wing flap fails. This is the same thing as saying, "Don't apply flaps in a situation where the failure of a single flap might expose you to greater risk." Given this assumption, shouldn't we avoid applying flaps below 500 feet AGL? After all, if we applied flaps below 500 feet AGL and had a split-flap condition, this would certainly expose us to greater risk. The risk is certainly greater than having the same split-flap condition above 500 feet AGL, right? This thought leads to the logical conclusion that we shouldn't touch the flap handle once we're below 500 feet AGL. Yet, pilots are not at all hesitant to manipulate flaps at any altitude or at any (appropriate) airspeed, even during a low speed, full flap go around. Hmmm? It seems that we're willing to isolate our concern for a split-flap condition in one area (turns) but we're not willing to apply the same concern to other supposedly critical areas of flight (operations on final below 500 feet AGL, for instance).
You would certainly think the airlines, who are in the business of transporting people safely, would prohibit the application of flaps in a turn if it were indeed dangerous. As far as I know, there's no airline that does such a thing.
If you want to restrict flap application in a turn, then do so. But don't let the prohibition keep you from using flaps in a turn when necessary. The example I gave in my March AOPA Pilot article is a case in point. I wrote:
Suppose this fellow had an engine failure and needed to descend quickly while turning toward the only available landing spot. A delay in applying flaps could result in overshooting the field.
If you need to apply flaps in a turn to make a field during an emergency landing, then do it.
If we don't have a good reason for doing or not doing something, then we should try and get one. If we can't find a reason supporting our behavior, then maybe that behavior isn't as useful for us as we thought it was.
For more information on this subject, see "Turns: Straight Talk About Curved Flight."
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