Would the benign spiral maneuver be one alternative to dealing with being "stuck above" or inside a cloud?

Question

Recently I provided an alternate answer to a question involving "hands off" recovery techniques. In this I briefly illustrated the benign spiral maneuver. For completeness I will reiterate what I said in that answer:

This (benign spiral) maneuver is considered to be the safest way of recovering in a glider when one has found themselves above a hole-free cloud layer or inside a cloud. My club teaches this maneuver as part of initial glider pilot training, though it is not (yet) part of the US FAA Glider Practical Test Standards.

The steps my club recommends are:

• Full spoilers
• Trim slightly aft (Other references suggest 1.5 times the stall speed)
• Hands off stick
• Feet off rudders

The effect of this maneuver is the glider will settle into a gentle spiral with a roughly constant (and not load-exceeding) airspeed and altitude loss. The maneuver is also taking advantage of the aerodynamic stability mentioned in other answers which is inherent in most (all?) modern gliders.

Here are some additional sources that discuss this maneuver:

• Scottish Gliding Centre's Airfield Procedures, Page 17 - 18
• This post, which quotes "Transition to Gliders, A Flight Training Handbook for Power Pilots" by Thomas L. Knauff.

That got me to thinking about whether this technique would be applicable to recovery from the same situation in a powered airplane. Being "stuck above" or even inside a cloud has certainly been the cause of accidents. One harrowing story that really hit home for me is told in this video from the Air safety institute.

So, my question: Would a benign spiral be one alternative to dealing with being "stuck above" or inside a cloud? Lets assume for the sake of argument that the airplane involved is a typical single-engine, forward prop GA aircraft (e.g. Cessna 172, Piper Cherokee, etc.)

Answer 1

It works in most gliders because they tend to be neutrally stable in roll and - the big one - have speed limiting spoilers/dive brakes. I'm not sure it's safe to do that in an older glider without speed limiting spoilers.

In any case, most power planes, once disturbed beyond the ability of dihedral effect to restore level flight, will naturally descend in an ever steepening spiral without pilot input. There aren't any ways to limit the speed on a GA power plane (gear and flaps won't cut it) the way spoilers/dive brakes on a glider can. It would need a speed brake that can hold the plane to below Vne in a vertical dive.

Answer 2

I would not recommend this as a technique to penetrate a cloud layer. My answer assumes that the pilot involved in a non-instrument rated pilot. Otherwise they would just pick up an IFR clearance for the penetration.

Before beginning any penetration of a cloud deck without clearance the pilot would have be certain of their position and ascertain the lowest altitude to which they could descend without fear of CFIT. An Emergency should also be declared. Being in a sustained bank without visual references is a recipe for spatial disorientation - even for instrument rated pilots. If a penetration must be performed I would recommend doing so in a straight ahead manner by slowing the aircraft while VMC and configuring in a manner to increase your drag to minimize the time spent in IMC. Fly the aircraft in a straight, unaccelerated descent.

It is imperative that you obtain information about where the bottom of the cloud deck is in your specific area so that you can be certain that you will break out in VMC before reaching your minimum safe altitude. If you need to descend through a hole in an undercast and it is thick enough that you do not think you can do so in a straight ahead manner with the descent rate available, then I would recommend flying a descending box pattern on cardinal headings in lieu of a spiral for the reasons above. Flying cardinal headings means that your aircraft should drift with the hole you are descending through if there is any appreciable wind.

Here is a reference for you that describes the types of spatial disorientation that would be highly likely to occur using the technique you initially posited and why I recommended against it.

Answer 3

When I was a teenager, I was taught to do a benign spiral in a 2-33. While it works in some older trainers, it really doesn't work in many (most?) high performance sailplanes. Many sailplanes don't have enough stability to establish a steady benign spiral. Some, such as my ASW-15 are unstable in phugoid motion, so releasing the controls will result in ever-increasing pitch departures. When I was a CFIG, I did teach benign spirals, but I always made sure that my students understood that it damn well might not work, otherwise, they might develop a false sense of security.

These days, AHRS is dirt cheap, and it is available on many common flight computers and varios. The only solid solution is to install AHRS in our trainers, and to provide simulated instrument training. There's a good reason why private airplane pilots are required to log 3 hours of instrument training, and it's very unsettling that glider pilots are still being taught to let go and hope for the best.

Answer 4

Yes, in some airplanes. Gliders tend to be more unstable in roll than power planes, due to the over banking tendency due the long wingspan. A high sink rate provides a stabilizing effect in roll, due to roll damping, so be sure and bring the power back to idle and use full flaps. (This stabilizing effect due to sink rate due to roll damping may also be accentuated with a long wingspan, so it's hard to say definitively whether the technique would work in any given powered airplane with modest wingspan.) I have heard an instructor advocate this technique for one light airplane; I believe it was a Cessna 172. Carefully test it first before trying in cloud!

A better technique for descent with no working gyro instruments in some airplanes might be to use the magnetic compass, using rudder inputs only (hands off the stick or yoke), using a target heading of due magnetic south (if you are in the northern magnetic hemisphere.) At mid latitudes this can work suprisingly well in some aircraft; don't knock it till you try it. Why it works could be the topic of another question for ASE. It undoubtedly would not work at high magnetic latitudes, and it also might not very well at very low magnetic latitudes, i.e. near the magnetic equator.

With a working attitude indicator or even just a working turn rate indicator, there are better options than either of the above techniques for anyone with even a bare minimum of instrument training, but they still retain some value as interesting demonstations of the physics of flight.

Answer 5

The benefit of using the benign spiral as a means of descent through cloud is to overcome the tendency of pilots with either extremely low instrument flying experience, or no instruments, to make extraneous control inputs based on erroneous sensations. This can result in the pilot believing they are in a normal flight attitude when in fact they are in an extreme attitude. The hands-off stable approach of the benign spiral minimizes this problem.

The situation is not very relevant to power plane pilots. Even the most basic power licenses teach some instrument flying, enough to be able to make a controlled descent through cloud without leaving the normal flight envelope. Glider pilots often have literally zero hours of instrument flying (and sometimes no attitude instruments), and are much more susceptible to the above problem. A powered pilot is probably better off remembering their instrument flying training and making a controlled descent.

While powered pilots have many accidents during descent in clouds, these are more likely to be descent into terrain or other collisions. The benign spiral does nothing to prevent these accidents, and may make them worse, since you have little control over your direction of flight.

As pointed out by John K, power planes are not usually as stable as gliders in these situations. If you are not sure that your plane will stay in a benign spiral then the technique is at least as likely to result in an extreme flight attitude as an attempt to make a controlled instrument descent.

I suspect also that the benign spiral is actually taught as an exercise in controlling the aircraft rather than as a technique to be used in any but the most exceptional circumstances.

Answer 6

Let's start with the air safety video. Being familiar with the "canyon" the pilot tried to fly through (I-80 northeast Utah crossing Parleys Summit) his odds of survival were significantly low. Sadly, he may have been better off attempting to land in Coalville on the interstate.

Flying into those conditions, against repeated advice was fatal, as would be attempting a spiral descent in that type of terrain (the Wasatch mountains).

For a powered plane, a 180 degree turn, or any turn, away from clouds, along with pre-trip weather awareness, keeps it safe. What happened in the video was slightly different than a thermal chasing glider getting pulled into a fair weather cumulus cloud. We are also taught to stay below clouds flying VFR.

In a powered plane, if one is crazy enough to get into that situation, but sane enough to save themselves, number 1 (after carb heat ON) would be a slow turn away from the mountains, then fly straight and level as high as I could away from the weather making sure, with my rudimentary instrument training, to fly straight and hold altitude after completing the turn. Then, get on the radio, ask for help, and get a vector to safety.

The deceased pilot did not follow instructions.