Maneuvering Speed

[kent4142]

Hi All,

My son and I flew through some moderate to severe turbulence last Saturday in my 81, Commander 840 with -10 engines at 27,000.  The Turbulence was forecast between 18,000 and 28,000.  The controller notified us about it's proximity to our location via a pilot report.

My question to the group is:

1). What would you do operationally?  Land, slow down, climb, descend, ignore everything and just fly etc.

2)  If you said slow down - what speed would you slow down to and why?

Thanks,

Kent

[donv]

I flew through some severe turbulence at FL280 a few years ago. I was warned, but it's so hard to tell based on PIREPs-- one guy can get really thrown around and someone else can go through with nothing worse than light turbulence.

In my case, since I had an idea it was coming, I slowed below 180KIAS (not much of a sacrifice at FL280). When the turbulence hit (it was smooth before), I disengaged the autopilot and let the altitude wander a bit. It was crazy! The thing I remember most is that I had a 100 knot tailwind, and when it settled down enough that I could look at the wind indicator on the PFD, it was a 15 knot headwind-- in a time span of less than a minute. I did not try to chase the altitude, but let it climb and notified ATC.

I went down after that, to FL240 (I think), and it was still bad, bordering on moderate the rest of the flight, about two hours. Then I had a nasty approach down to minimums at a little airport I had never been to before, at night...

So I would slow down below 180, and consider descending.

I also look at the winds as part of my preflight brief. As a friend of mine used to say, "stay away from the elbow of the jet stream!" Places where there are big changes in the jetstream are where you are going to find bad clear air turbulence. Often the mid-high 20s are actually the best place to be, as the wind velocities are a bit lower than the 30s. If you know you'll be flying near the elbow of the jet, I'd consider going lower to get below the big wind velocities, as that's what causes the turbulence.

This is a good topic!

[Bruce Byerly]

Kent - I can’t post a link since the NTSB site seems not to be working, but I’d suggest reading the full narrative of 690B N53LG accident we were discussing.

Keep it slow in the big bumps.  Just like any light plane I think.

Of course I agree with all the Don posted. 180 KIAS is easy when needed. These days it’s pretty uncommon to be surprised to find yourself in deep doodoo given all the technology, but if you’re really expecting to get whacked, I slow it way down.

Cheers - Bruce

[donv]

Accident report is here:

https://www.ntsb.gov/_layouts/ntsb.aviation/brief2.aspx?ev_id=20030401X00419&ntsbno=ATL03FA068&akey=1

The meteorological section is the interesting part of that report... and the fact that the 727 in the area only encountered light turbulence, even after descending to FL270. That's the problem with turbulence-- sometimes it's there, sometimes not. I do pay a lot more attention to the upper level winds than before.

So Bruce, how slow do you go? On the one hand, obviously you don't want to overstress the airplane. On the other hand, I don't particularly want to stall a Commander at FL280 either, especially given the full stall behavior of the airplane.

One other thing-- probably superstition, but at high altitudes (above FL250 or so), I turn the ignition on if I'm in moderate or close to it turbulence.

[Bruce Byerly]

Don,

The weather info was definitely what I thought important to take away.  But thinking about it a bit, that accident was around the time we realized some of the vertical caps were paper thin at the leading edge and Twin moved to retrofit all 690’s with the later model all aluminum caps.  So although the report doesn’t say it, that cap failure due to unchecked leading edge erosion may have contributed to the accident.  At least that’s what I recall being told. I don’t think they ever found the cap.

Anyway, for me it just depends on how rough it is. For comfort, the rougher it is (or I think it might become), the slower I go. It’s a rare day that I go to Va but I’ll do it if it’s really bad and may get worse. 170’s normally if it’s rough. But most of the time it never gets too bad and you pick the power up within a few minutes. 180 is also recommended for descent (maybe that was FSI’s recommendation?) but I think almost barber pole speeds seems to be more popular when it’s smooth. On that note, I have seen a few G600’s without any functional barber pole programming and also barber poles that don’t work. What percentage of your flight time do you expect moderate or worse turbulence?

Somewhat related to this discussion, I get asked occasionally about the Va. Some are confused and associate Va with aircraft strength when it really has nothing to do with it.  Commanders just don’t stall easily.  Even with approach power the stall occurs way below the documented stall speed. As I recall, the high altitude stall kit install requires testing which Includes some stalls up to FL350. It was surprisingly easy to fly that profile. So I don’t worry about stalling the plane at altitude and believe the stall characteristics to be conventional.   

Something else to consider is a heavy 1000’s Va is close to 160 KIAS. A lightly loaded 840 is probably closer to 120. 

[donv]

We should do a separate thread about stalls. You've probably done more full stalls in Commanders than I have, but I'm not convinced that the stall characteristics are all that benign. And I'm definitely not convinced that if you let a spin develop, it's recoverable.

I had to do full stalls after my horizontal stab leading edges were replaced, and it was fine, but I was also very careful.

[Steve binnette]

There are some pretty handy tools for Flt planning that give you turbulence forecasts.

One is on the Jepp app.  They have a turbulence function with a slider for different altitudes, WSI is another product that has a turbulence function, finally foreflight also provides one.

I try real hard to avoid or lessen turbulence with the family on board.

Flt planning is the first step, then if I hear a report of bad turbulence I will avoid it primarily by changing altitudes.  It is also important to understand the pirep, who gave it and what are they flying.  I will not fly into a report of severe turbulence without somebody else going first.  Severe turbulence is no joke I will descend as low as needed to avoid it! 

Like Don says look at the wind charts and see what the wind is doing. Ask the controller for help isolating the altitude and area where the turbulence is located.

Flying slow helps 130 to 180kts will soften the ride

[Bruce Byerly]

The short wing planes seem less likely to drop a wing but I’ve never had any model do anything radical. I’m no aerobatic pilot so try not to get into the spin zone and I quit doing takeoff power stalls A long time ago as the deck angle is usually pretty wild. I think the biggest thing to watch is coordination and fair use of the rudder. Uncoordinated, asymmetric power, poor rigging, plus all the mods like heavier props, major airframe repairs, aftermarket winglets, etc and who knows what any particular plane will do compared to when they were certified so I can understand not wanting to find the edge of the envelope. Hoover used to spin the 500 at low altitude and I think he quit doing that for a reason but I don’t know for sure.

I wish we had some old factory flight test engineer to contribute.  Also, there was a fantastic presentation on gust loads and design strength years ago at the Aspen Twin Commander University.  I wish I could remember who gave the presentation and if we could revisit that.  I remember it being very educational. I’ll ask Geoffrey Pence next time I talk to him. Maybe Mark Matheson or Mark Twombly would have a record of it.

[donv]

Steve, that Jepp suggestion is a good one. I had never looked at the turbulence section in the app, but I will do so going forward. Have you found it to be reasonably accurate?

Also, there is an old picture of your airplane and some interesting discussion about it on Facebook. Apparently it was used by the FBI in the search for the Unabomber-- maybe you already knew that.

[kent4142]

Hi All,

I am very grateful for all the input.  Bruce and I talked about this on the phone a few days ago, and I thought the group would want to talk about it as well.  (I am not rejecting your input Bruce!    ). Oh, and I did read about two Commander accidents, (as Bruce recommended), both in the South where I fly - always.  My standard route is Central Florida to southern MN typically 6 or 7 trips per year.  So I get it all nearly, Thurnderstorms, tornados, 200 knot winds, ice, and clear air turbulence.  What am I forgetting.  Not that you guys don't fly through the same stuff with long distance airplanes.

I have some great take aways so far - but I hope there are more.  Compared to my old 414 I am flying "over" quite a bit more weather, so I was getting some what cavalier.  I actually use the radar a lot less because I am topping a lot of the winter precipitation.  But this turbulence is a new thing, that I have not focused much on.  I have really been lazy with the preflight weather briefings.  I look at the map see where the bad weather for take off and landing might be and see "oh yeah" another front or two, along the way.

I appreciate the tips on forecast turbulence for Foreflight.  I use foreflight, but I have not been looking at the forecast turbulence. Also, a new red flag for me is the intensity of the winds.  Last week the winds were over 200 kts.  I remember reading about 40 years ago, a Buck Taylor book about turbulence at the edges of the jet stream.  I am going to pay way more attention to that.  I never heard of the danger at the elbow Don.  Thanks for that also.

Another take away is the slowing down part.  Last week we were looking at about a 5 hour and 30 minute flight.  Most of the winds were 60 to 100 knot head wind component.  I honestly did not want to slow down.  I thought if we slowed down we would be up there all day long.  "Better late than never", some one said.

Since I have been flying in the flight levels, I have kind of dis-regarded the controllers and airline pilots giving ride reports.  I though that was just airliner stuff.  Most of my life in general aviation we just tightened our seat belts and pressed on - "turbulence smurdulence".  That is NOT the case now.

I am still a little confused about maneuvering speed.  I have been reading in AOPA about slowing down to a speed comfortably above stall speed, a speed at which a test pilot could safely use full control movements without over stressing the aircraft.  But the caveat is that a pilot does not want to move the controls rapidly in succession and multiple controls at the same time.  I am not sure why any pilot would want to, and I did read about the airline pilot who cycled the rudder like 5 times in 6 seconds and failed the vertical stabilizer.  I can't imagine doing that, but I will keep it in mind.

So my final question.  With moderate to severe turbulence eminent within 30 miles or so of my position slow to 150 kts indicated assuming 27,000 or 28,000 or if possible just descend to 18,000 or 19,000?

Thanks All!!!!!

[donv]

The first thing is, I'm not that sure about predictions of severe turbulence. You just never really know. In the Commander accident Bruce cited, a 727 in the same area close to the same altitude got nothing worse than light turbulence.

That said, the edges of the jet stream, or where it bends back on itself, are likely spots for clear air turbulence and the bigger the winds, the bigger the turbulence. So if you know you are going to fly through an area of 200 knot winds, especially if you are crossing the jet stream or flying on the edges, I'd plan to go lower. Generally the really big winds are in the 30s more than in the 20s, but not always. You can look at the wind speeds on fltplan.com or your weather briefing and look at the altitudes where the winds drop off.

Listening to the other airplanes on the frequency is a good tool. The airlines don't like moderate turbulence, and they really try to avoid severe turbulence, so if they're all at FL250 or below (which I've seen), that's a pretty big clue. Garmin Pilot app will graphically show you PIREPs if you turn that feature on, which can also be helpful.

Sometimes, going lower isn't better. That can be true for big winds flowing across the Rocky Mountains. In some cases, higher is better. So you need to know where you are going... but in the southeast, I would generally think lower is better.

If you are in moderate turbulence and getting bumped around, and you're concerned it could get worse, I would probably slow to 170-175.

The thing to remember is that maneuvering speed simply is the speed at which, with a full deflection of the controls, the airplane will stall before the load limits are exceeded. So I wouldn't slow all the way down to that. Most jets have a "turbulent air penetration speed" (250 knots or Mach 0.73 on the Learjet 35, for instance), which is intended to provide you with the maximum buffer between low speed stall and high speed overload. I kind of treat 180 knots in the Commander similarly, although I might slow down a few more knots if I thought it were really going to be bad.

So my final question.  With moderate to severe turbulence eminent within 30 miles or so of my position slow to 150 kts indicated assuming 27,000 or 28,000 or if possible just descend to 18,000 or 19,000?

[donv]

Also, some airports pretty much always have moderate turbulence at low altitudes. Palm Springs is one such place-- virtually any day after 10am or so, you'll get moderate turbulence in the descent. All you can do is slow to 180 before you get down into it.

[donv]

Bruce, what speeds do the fire guys operate at? I assume they are in moderate-severe turbulence regularly?

[Steve binnette]

Steve, that Jepp suggestion is a good one. I had never looked at the turbulence section in the app, but I will do so going forward. Have you found it to be reasonably accurate?

Also, there is an old picture of your airplane and some interesting discussion about it on Facebook. Apparently it was used by the FBI in the search for the Unabomber-- maybe you already knew that.

I have found the turbulence predictor to be accurate.  This time of year going to Hawaii is bumpy and it comes in real handy.  Sometimes I will Flt plan for FL280 the entire way to avoid the bumps.  Once you are in Oceanic airspace changing altitudes can be a bit of a crap,shoot.  If you can make that call on the ground using the turbulence tool it helps.   

The jepp matches what I get on WSI which is a weather app provided by the airline.

I don’t have Facebook and have never heard the story involving my airplane and the FBI.  I knew the feds operated it but that’s it.

[Bruce Byerly]

Bruce, what speeds do the fire guys operate at? I assume they are in moderate-severe turbulence regularly?

My understanding is normally pretty slow if they are just loitering. They definitely don’t want to get in the smoke column.  In addition to the wing, that has permanently bent some ailerons before.  But if they are leading in, good question.  I’ll ask.