Anybody have any thoughts on lightning strikes on our planes? Turbulence effects? Icing? Hard rain? Hail? Maneuvering flight (maneuvers, stalls)? Stability? Loading, or aft loading?
Editor:
I will try to share what we have learned through both reading and from corroborating personal experience. I do not consider myself an expert on weather, turbulence, or lightning. I will be stating some things as factual, but they will be factual within our personal experience, as opposed to being the findings of extensive studies. I hope my input doesn’t sound too dogmatic.
Depending on what you read, most commercial USA airliners get multiple lightning strikes every year. We only hear about the random (and relatively rare) instances in which some damage is done, such as a cracked windshield plate. All FAA-certified aircraft that are Type-Certificated for IFR operation (not just the repetitive pitot-static and transponder check), have to meet a certification standard for lightning “tolerance”. An IFR TC’d airframe will (for example) probably have bonding straps that cross junctures such as flight control pivot bushings. They may or may not have static wicks, which are more a matter of radio performance. If the lightning protection is intact, the airframe will probably tolerate a strike without significant damage. The occupants will likely be unhurt, and there may or may not be radio damage.
I know of a V35A that took a strike in the left wingtip. It left a sooty spot with some burned-contact appearance of the aluminum, but no other apparent damage. To our knowledge, we have never taken a strike in our Sierra, though we have been too close for comfort a few times. While we certainly try to avoid it, I have come to believe that a lightning strike would not be a fatal experience; just potentially expensive. I have seen a fair amount of it while airborne, just because this is Florida. You would never fly all summer long, neither IFR nor VFR, if you flew only when no lightning could be seen anywhere. I made it a key aspect of my IFR training to fly all my IFR cross-country time in actual weather conditions, to see what more experienced people (my CFII and Examiner) seemed to expect.
The side effects of a strike can be unpredictable. An example is magnetization of large ferrous components (chromoly airframe tubing, engine mount tubing, engine crank, etc.) that screws up the magnetic compass. My understanding is that the engine manufacturers have a mandatory inspection protocol if there has been a known prop or engine strike. Glass-composite construction is a whole new ballgame, whether certified or home-built. A certified glass-composite plane that is being manufactured to meet an IFR Type Certificate has to have integrated lightning protection. While it varies, a common method is to include a bonded wire-mesh layer beneath the surface skin. This adds a lot to the cost and weight, and is rarely included in homebuilts. Cirrus’ most recent announcement of a lower-cost trainer is VFR-only, and does not have this lightning protection. You cannot legally just add IFR avionics to a certified plane having a VFR TC, and go file IFR. Testing has shown major damage from lightning strikes on conventional lay-up and molded composite airframes. There is usually major debonding, or the appearance of a low-scale explosion, due to localized heating. I know of one confirmed report of a strike on a Lancair kitplane. It was damaged but landed successfully; probably pretty lucky. With the composite fuel tanks, etc. it is impossible to predict (or protect) the path that lightning might take through the airframe. Unless, of course, it is designed-in to meet IFR certification requirements.
Each of us has to set our own limits, based on personal research and personal experience. Our hope is to learn the most from the research side first, so that we can benefit from what others have experienced. Paula and I have experienced a lot of weather through Level 3, and including some snow and light icing. Note that the Sierra does not tolerate much icing; if you see it, get out right now. Before you can even really see the ice, the first clue will be a need to add 100 RPM to maintain altitude; followed shortly by a need to add another 100 RPM as airspeed decays. At that point you may be barely seeing it on the windshield base or leading edges.
Weather is also very regional in nature. I have flown directly over localized and isolated rainstorms in the Everglades that were a quarter-mile in diameter and had tops of only 4,000 feet. They had black rain shafts with lightning to the ground, and were classified by ATC as L03. I have been flying in Jackson Hole, WY beneath a solid overcast, in very light rain; no more than L01. The cloud layer capped the highest of the Teton Mountains, was only 2,000 feet thick, and had blue sky and sunshine above. There were multiple lightning strikes occurring all along the ridgelines, but none in the open valley area. It takes air movement to generate the static charges that lead to lightning. The turbulence can come from mountain updrafts and downdrafts, or from thermals and convection in the flatlands. The cloud thickness doesn’t matter much. If a stratus layer is 10,000 feet thick but has no convection, it is unlikely to create lightning if it has only L01 mist and rain. It may have some bumps due to the inconsistent nature of water vapor, but is unlikely to be turbulent. If the drops are large enough to suggest L02, it implies growing convection that is forming the larger raindrops. With L03 and above, lightning is likely. L01 rain and heavy mist sounds about like frying bacon. L02 sounds like light gravel hitting the plane. L03 sounds like machine gun fire or hail hitting the plane (very loud clicking and hammering). L03 will have turbulence that makes it difficult to retune a radio frequency. I can’t tell you what confirmed L04, L05, or hail sounds like, and I hope I never can. Hail will usually be found on the downwind side of a moving cell, often in the clear air and preceding the rain itself, below the base of the anvil. I always deviate to the back (upwind) side of a moving storm.
Paula and I tolerate L01 and L02 with no concerns. I try to use an altitude that will give me at least occasional views of tops and density. If I am solo I will accept brief shafts of L03, if I am seeing no indications of lightning anywhere around. Paula won’t, and expects me to avoid them no matter what, if she is aboard. Some initial clues of nearby L04/L05 are the appearance of a very dark gray-green wall extending left, right, up, and down, as far as you can see. There will probably be a roll cloud that you may penetrate first, if you don’t see it coming in time (usually you will see it first). This may give you vertical rates of as much as 3,000 FPM up (wrapped ROC needle), with the gear out, the throttle near idle, and the aircraft in a level attitude. In most cases this will be the surrounding updraft feeding a serious cell, and it is time to turn and/or land. This encounter does not hurt the airframe, if you don’t fight it. You won’t be able to change radio frequencies, but you can talk. Tell ATC that you can’t maintain altitude, that you are holding attitude, and will be diverting immediately. If you are able, you can tell them “left” or “right”, but your main concern at that point is to get turned without unusual attitudes, and before penetrating the main area of severe weather.
Others have given good advice about just keeping the plane near level, while accomplishing an exit. You must not fight altitude excursions with strong pitch changes; that’s what breaks up an airplane. If you pull the throttle and toss out the gear on a Sierra, and just keep the nose close to level, it won’t get hurt. You’d get badly beaten up in your seatbelt long before the airplane would incur damage. The updraft (and any downdrafts) will change rapidly within a short distance; usually within as little as a few hundred yards. It can be pretty exciting at the time, but it doesn’t have to be dangerous. If your IAS is maybe 100 MPH or even 140 MPH, it is almost irrelevant what your vertical speed is showing; that’s just air mass movement. Exciting and abnormal, but not dangerous. Most of the cases of airframe break-up have been determined by the NTSB to have been pilot-induced, through a misguided effort by the pilot to maintain an assigned altitude. There may have been more, but I have only read of one instance of a Sierra airframe breakup, in what was later determined by the NTSB to be a L05 cell or greater, with recorded Doppler evidence that implied tornadic activity (hooked image). The entire Musketeer line is generally regarded as having a higher margin of structural integrity than most of the GA fleet.
When it comes to Dutch rolls, the Sierra is a non-issue. You should see what it is like in a 35-series V-tail Bonanza. Even in calm and clear air, the wingtip will be showing a constant S-pattern on the horizon. Talk about marginal stability! Some short-coupled homebuilts are the same way. The single biggest improvement in the modern homebuilt field (in my opinion) came when the manufacturers quit making the fuselages too short and the tail surfaces too small. Our Sierra is rock-solid in this mode, compared to anything else I have flown. While I can’t pretend to understand how it can be, I’ve read of planes that could be placed five or ten degrees off-angle in yaw, and which simply continued to fly sideways until corrected. The 35 Bonanza makes significant wing-wags in any light turbulence, unless they are constantly damped out with timed rudder. Automatic yaw dampers are big sellers for the 35 series, for that reason. Most people just never go and both train and experiment (within certification limits) to see what an airframe can do. If you spend the time and money to get an hour in an Extra 300 (and do some spins), and get your CFI to take you and your plane to its limits in Chandelles, Lazy 8’s, semi-wingovers, etc., you can come to appreciate that there is much more to safe and confident flying than just TO’s, cruise flight, and safe landings. Look at how much your ailerons move during normal cruise. Sometime when you are under 144 MPH indicated (or your placarded maneuvering speed, and in smooth air), make a smooth but sudden full aileron deflection, and see how the plane reacts. Get it up to the edge of the stall and keep it there by whatever rudder deflections it takes to keep the ball centered, until it eventually gets away from you; then just let it fly out. Put it in a climb with higher power, with a map on the glareshield, then smoothly push the nose over until the map floats gently into the air, and keep it there. Keep an eye on your oil pressure, and reduce throttle during dive recovery. The plane can do far more than most of us ever expect from it, and can tolerate much more than most of us can. It can also be very subtle. You can be holding it at the very edge of the stall, with a sink rate of 500 FPM going straight ahead, and just move the yoke a quarter-inch forward to slowly fly out of the stall. It does not require the dramatic forward pitch that most people get taught for stall recovery (most planes don’t).
The phugoid oscillation can be pilot-induced or turbulence-induced. It is one of the stability tests demonstrated during certification. When testing is performed in calm air, the plane has to return to near-level flight after a fixed number of oscillations, with each successive oscillation being less severe (positive pitch stability). If the oscillations stay the same, it has neutral pitch stability; if they get successively worse, it has negative pitch stability. Only positive pitch stability will pass certification; one of the many differences between certified aircraft and homebuilts. Neutral and negative stability can be caused by turbulence and by loading. Anywhere near uneven ground there can be waves in the air that will cause pitch “hunting”. It doesn’t happen only in the mountains (where it can be pretty dramatic and can last for hundreds of miles). It can also result from clear air wind shear. I was once VFR heading to Atlanta’s DeKalb-Peachtree (from St. Augustine, FL) in mid-Winter, when Atlanta Approach put me down to 2,000 feet while I was still 80 miles out. I just about got beat to death; the turbulence was just like driving down a washboard road, for nearly the entire rest of the way in. One side effect was having to watch altitude like a hawk. It would have been the same in any other airframe. Inconsistent air has to result in inconsistent airfoil effects and trim. Another side effect was always filing IFR into PDK from then on.
If you are struggling to maintain level flight, and IAS isn’t varying much more than 5-10 knots, but groundspeed seems to be varying a lot more (even as much as 30-60 knots!), you are almost certainly involved in some form of turbulence or wave effect. If you aren’t really fighting it much but are having a hard time getting pitch trim to “settle down”; and you are seeing IAS and GS changes that coincide (and which are 5-10 knots or less), you are probably dealing with an aft loading issue. It won’t matter much in smooth air (unless you are doing stalls!), but aft loading will really show up when even light turbulence disturbs trim settings. You can find yourself constantly changing trim and struggling with altitude excursions during cross-country flight. Do some experiments with this and you’ll see what I mean. There is a fine line between safe handling, aft loading for improved cruise performance (reduced nose-up trim), and pitch trim stability in cruise.