Turbulence Penetration Speed

[donv]

Twin Commander posted this article in Flight Levels recently:

https://flightlevelsonline.com/2021/winter-2021/control-column-turbulence-penetration-speed/?fbclid=IwAR12hJrfCG6XOH7-HrbG9p9oTq64hLbiU1c1X0YZRaHAbEU_T8gxF-BpWcM

The main part is here:

The problem with using maneuvering speed as a limit is that by its nature, turbulence isn’t constant. While you or the autopilot may be able to largely maintain maneuvering speed, the gusts associated with the turbulence could quickly and easily exceed the limit, erasing any protective benefit.

Turbulence penetration speed takes these gusts into account and offers a lower target airspeed. The speed has long been known to operators of turbojet aircraft, as it’s required to be established for Part 25 aircraft. Being a Part 23 aircraft, the Twin Commander doesn’t have a published turbulence penetration speed. But Boyd and others offer some guidance on how to develop your own. Writing for Air Facts, O.C. Hope references the FAA’s design regulations to say that turbulence penetration speed must be low enough that the associated gusts don’t make the aircraft exceed maneuvering speed. It’s clearly impossible to know exactly what the gusts will be in a given scenario, but research has shown that even moderate turbulence can result in momentary gusts of 25 knots.

Hope suggests splitting the difference and flying a speed approximately halfway between stall speed and maneuvering speed. That would give the same margin above stall and below structural failure, which seems reasonable. By using the autopilot to maintain attitude, airspeed will be allowed to vary up and down.

I'm not sure I agree with the author, though, but I'd be interested in other opinions. Turbo Commanders carry a placard limiting airspeed to 180 knots in moderate or greater turbulence, so that's a good place to start. Clearly stay below 180 knots.

Maneuvering speed in my airplane, if I remember correctly, is 137 at max gross weight, or 135 at 10,000 pounds (a more realistic enroute weight). Stall speed at MGTOW is 77 knots, so using the author's theory, you should fly about 107 knots in severe turbulence.

That just seems way, way, way too slow.

The AFM has a specific procedure for flight in moderate to severe turbulence, and it basically is slow to maneuvering speed (137). In severe turbulence, that makes sense to me (I only recall being in true severe turbulence in a Commander once, and I was going faster than 137 but less than 180), but in moderate turbulence, which is not that uncommon, I typically just slow to 180.

What do others do?

[Badger]

Very interesting.  I have been using "maneuvering speed" in my 500B for turbulence penetration (155 mph), but I do see the author's point.  Certainly worth thinking about.  I, too, would be interested in hearing others' opinions..

Ed

[donv]

It sounds like the maneuvering speed in the 500 and the maneuvering speed in the 980 are about the same. The 980, though, is typically flying 50-60 knots over that speed at a normal cruise, and sometimes 70-80 knots over in a descent.

[kent4142]

This may be a slight "bunny trail" from the exact speed for turbulence - but I am going to blab away my thoughts anyway.  My flying experience up until the Commander was Cessna 414's, flying in the low 20's at the highest.  I never understood why everyone was always talking about ride reports and turbulence on the frequencies.

Enter Commander 840 flying.  No one in my training really talked about turbulence except for the 180 and the 137 knot numbers.  Well - one day coming home from MN to FL my son and I experienced a few seconds of severe turbulence which we were warned about by ATC.  Big surprise for Mr. Bold pilot (me)!!!  There is no excuse for stupid on my part.

From now on we check the forecast turbulence for our route and deviate around it, just as if it were imbedded thunderstorms.  If there is no getting around it, we descend below 20,000 feet until forecast turbulence passage.  (This information is available with the GTN-750 in our airplane.)

Everybody overloads their Commanders (in my opinion).  This puts us in an even more susceptible position for turbulence.  Luckily I lived to fly another day.

[Bruce Byerly]

Years ago at a Commander University in Aspen there was an engineering talk about gust loads and certification standards etc. It was very informative and I wish we had a record of it and who gave the talk. Among the take aways was it’s a strong design but you can’t depend on a published maneuvering speed to take an unlimited load.

Commanders have a huge wing that doesn’t like to stall. By that I mean, the published stall speed may be 77 or what have you, but the reality is that, with even a little power, you can see stall speeds in the 60 knot range. Go try departure stalls and you’ll see what I mean.  It’s a great feature in general but pounding through severe turbulence is not something you want to do on the barber pole.  The heavier you are, the higher the stall speed hence the reason a heavy 1000 has a higher speed relative to a light 840. The beautiful thing about today’s tech is that we have unbelievable info relative to the old days to avoid severe turbulence. There’s simply little reason to be surprised by it. Back in the day, I swear we just turned on the radar, hoped for the best, and pounded through stuff that we can easily avoid today. I can’t recall the last time I saw all the dirt in the carpet head for the ceiling.

For me, if it’s obviously going to get gnarly, I’ll be at 130. Just can’t remember the last time I had to do that. If it’s dark and I’m not sure on the descent, I’ll do 180 or so.

Just watch the shear zones with upper winds aloft and turbulence info, look at the pireps on your XM and don’t pound through big puffy clouds and you’ll greatly improve your ride quality.

[donv]

I don't agree that severe turbulence is that easy to avoid, and I particularly don't agree with that in regard to moderate turbulence. In many cases, moderate turbulence is just a fact of life.

In my experience, there are three types of severe turbulence:

1) Convective activity (thunderstorms). Yes, this one is pretty easy to avoid, at least most of the time, given modern technology. It's also the most dangerous, so it's good that it's reasonably avoidable.

2) Clear air turbulence at high altitude related to the jet stream, and shear. As a friend of mine used to say, "stay away from the elbow in the jet." However, it's not always there, and it's not always that bad. But sometimes it is. And the jet stream covers a big area, so sometimes you can't get to where you are going without flying through some of those areas.

3) High winds and mountainous terrain. Sometimes if you are going to mountainous areas, you are just going to get rocked around.

In desert areas, moderate turbulence is common. Often going into the Palm Springs area, you will get moderate turbulence. If you don't fly through it, you won't get there. Same thing with cold fronts on the west coast. It is just a fact of life. I flew an ILS the other day that seemed like I was on the inside of a washing machine.

[Bruce Byerly]

Well I don’t know Don, I’m pretty sure I can count on my fingers and toes how many times I lost control of the airplane, even momentarily, while violently hanging against the seat belts.  Anything less than that isn’t severe turbulence to me. While I’ve been caught off guard by CAT or a possible wake a couple times, I didn’t expect it so wasn’t anything to prepare for.

Of course 95% of my flying is east of the Rockies and most of the rest is looking down on it from 28,000 or higher glad I am not down bouncing around and picking up ice, etc.

I don’t want to be hanging on the belts nor do I want to see the winglet going up and down relative to the rest of the plane so I definitely slow down in moderate turbulence. However, if 180 is the desired speed as placarded, that’s same as cruise speed in a -5 @ FL270 and commonly 5-15 knots lower than dash 10 IAS so not a huge deal.

[donv]

The one time I can recall being in what I would consider to be severe turbulence in a Commander was at FL280 in completely clear air. I was slowed down because the airliners were reporting moderate, and in some cases, severe, turbulence.

It was smooth and serene with about a 120 knot tailwind. When I could read the instruments again, we had a 30 knot headwind... in the space of under a minute.

I admit that the winds aloft chart was a clue, but on the other hand, going across eastern Oregon / Idaho that evening, there was no way to avoid it other than going low (below FL180, probably) and I'm not sure even that would have done it.