Turbulence and feet per second
 

The area that I have been flying is bumpier than what I'm used to, and I want to calculate or find a cruise speed that will not 'break' the plane if turbulence is encountered, but also maintain a reasonable airspeed.

I've been doing lots of research, and it seems like people are pointing to using Maneuvering speed. The problem is that Maneuvering speed in my plane is rather slow....about 110-120 kias. The aircraft that I'm using at the moment is the Cessna 210. For the most part, I find that people cruise around 145 kias, but can go up to 155 or even Vno. They tend to maintain that speed through light-moderate turbulence and not bother adjusting the power. My concern is the potential of increasing the load factor on the wings and thus over stressing the plane.

This has lead me to become curious lately about V speeds, specifically Va (maneuvering speed), Vb/Vra (rough air penetration speed), and Vno (maximum structural cruising speed). I have been looking, with no joy, for a formula to calculate Vra.

From what I've been reading, in GA airplanes, Vno or the top of the green arc is a speed at which the aircraft is able to withstand up or downdrafts of up to 30 feet / second without over loading the plane. The question then arrises, What category of turbulence is 30 FPS? Light? Moderate? In any case, if severe turbulence, or even sustained moderate turbulence is encountered, slowing to or below Va is probably a good idea.

To sum it all up, what I'm looking for is a way to find or calculate Vra, and understanding turbulence in terms of feet per second.

Thanks ;)

Admittedly, it's been quite a few years since I flew lighties, & furthermore I never had the pleasure of flying the C210. That said, most aircraft flight manuals publish a turbulence penetration speed, & failing that: a manoeuvring speed.

My point being: If the manufacturer publishes a recommended speed, then that's the speed they recommend, so that's the speed you fly. Simple. And with that being said, why bother trying to reinvent the wheel if the manufacturer has already done the math for you? Of course, it will mean having to slow down - which can be frustrating, I know - but them's the breaks. And 'break' you just may if continually ignored, especially with the age & fatigue issues facing so much of the GA fleet these days.

From what I have found.

Light - 5 Feet Per Second - 20fps
Mod - 20fps - 35fps
Severe - 35fps - 50fps
Extreem- >50fps

Maneuvering speed Va is the speed at which the wing will stall at the limit load of 3.8g
( remembering 1.0g of that is in level flight)

The wing will be over stressed at 3.8g above Va and potentially fail at 5.7g ( 150% above the limit load )

That's positive load factors, I think most aircraft are stressed for less in negative g ? At least the Vn diagram I was looking at has that.

Sagan is right.

From the Manual of Aviation Meterology, chapter on Turbulence

Light = 300-1199 fpm
Moderate = 1200-2099 fpm
Severe = 2100-2999 fpm
Extreme = >3000fpm

If you divide the above numbers by 60 you will get the numbers quoted by Sagan. Just highlighting where the numbers come from.

My rule of thumb when flying survey operations. Avoid Extreme turbulence at all costs. Avoid severe turbulence where possible and, if encountered or flying in forcast area, fly at Va.

Fly at Va if your in moderate turbulence.

Proceed with caution in light turbulence.

Its also worth noting that Va can change with aircraft weight. For the aircrat I flew Va was painfully low when you where light. The heavier the aircraft the better. We used to calculate it for take off and landing weights for each flight. This info should be in the AFM

Alpha

Yes Imagined I too have asked the same question and never got an exact answer.

On descent in the 210 you easily approach the top of the green arc and must be vigilant to keep from exceeding that.

My Manouvre speed is 125 knots which is a hell of an excercise to reduce to on descent if things get rough.

I will watch this thread with interest.

The C210 is deliberately limited by Cessna to avoid overstress in the un strutted wing/fuzelage attach area.

This has been the subject of debate for years but the end conclusion is that the way the wing attaches does warrant care in turbulent conditions.

Good question, and one I've wondered about too. I have no written references, so what I say is open to correction. However...

My understanding is that Va is a little too slow for best performance in turbulence. Yes, it protects the aircraft from structural damage due to excessive g load. But you could inadvertently stall in severe turbulence, which is just as bad.

Rough air penetration speed, or turbulence penetration speed, is around 5% to 10% faster than Va to reduce the risk of stalling. I don't know the precise calculation. The manufacturer expects the pilot to maintain an attitude in turbulence, specifically not to try to maintain an altitude, and this will reduce the chance of structural overload.

Here's what the good people at FAA.gov require (gusts are assumed to be 50fps according to rule 23.333, and the speed Vc is equal to 33 times the MTOW (lbs) divided by wing area (sq feet) according to rule 23.335.)
(d) Design speed for maximum gust intensity, VB. For VB, the following apply:

(1) VB may not be less than the speed determined by the intersection of the line representing the maximum positive lift, CNMAX, and the line representing the rough air gust velocity on the gust V-n diagram, or VS1√ ng, whichever is less, where:

(i) ng is the positive airplane gust load factor due to gust, at speed VC, and at the particular weight under consideration; and

(ii) VS1 is the stalling speed with the flaps retracted at the particular weight under consideration.

Couple of extra things, from others' posts above:

- Va protects against a single full and abrupt control input. It does not protect against multiple controls used simultaneously, or against one control input made multiple times.- Va does not necessarily protect against full forward stick. If you want to mess about with high negative 'g', Va is not the number to use.
- Va is not necessarily about the wing. The wing spar is usually over-engineered. Va might be limited by the tailplane or fin strength, or even by the strength of the aft fuselage (which takes a huge bending load at high g.) It is possible that the first item to fail might even be the engine mounts, or other high-load points inside the fuselage.

(Edit - I don't know much about the C210. My comments in the last sentence are for light aircraft generally, not the 210.)

Cheers,
O8

IBG I've just retired after completing 18 years and 4000+ hours flogging a C210 around the northern parts of Australia. I too used to worry about what was acceptable turbulence until I attended a Systems and Procedures course for C210's put on by the Cessna Pilots Association. The instructors were very knowledgeable and one had worked for Cessna as a test pilot during the testing of some of these aircraft. Someone asked him when was turbulence bad enough to require cruise speed reduction. He said when you're hitting your head on the roof every 30 seconds or so. He said they are very strong and we shouldn't worry about moderate turbulence.

I tend to adjust my speed to the level of comfort I need. I'm too old to be bounced around in a hot uncomfortable cockpit in order to save 5 minutes. If it's hot and you just know it's going to be rough below 8000 stay up in the cool smooth air until the time to destination in minutes equals the number of thousand feet you have to lose. Then drop the U/C. A C210 will come down nicely @ 1000 fpm and you will see a stable 140-145 K indicated without any of those scarey 160K excursions sometimes seen descending in turbulence with U/C up.

We were told that wing failure in strutless 210's invariably occurred beyond VNE. The wing fails at the point where the flaps end and the ailerons start. The outboard section bends and breaks downwards because at very high speeds this outboard section is at a negative angle of attack due to the large washout in C210 wings. All it takes is a big aileron input (such as our hero pilot would do as he came out the bottom of a cloud and found himself looking at the ground sideways) to provide enough additional downward force to fail it.

One other worry I used to have. In dry season if you take your headset off in flight you often hear dreadful noises which sound like the carry through spar is on it's last legs. It is actually the windshield which is fitted loose so it doesn't crack. When it moves about only a minute amount it makes these noises that you would never want to hear from your spar.

So to sum up: to fly smooth, safe and worry free fly high, descend slow, don't exceed VNE and don't take your head sets off.

Cheers, RA

I think I'd rather inadvertently stall in turbulence than have something break.

That said, whenever I've flown in fairly bad turbulence, on the "airspeed dropping rapidly" side of the 'bump' there seems to be a corresponding large decrease in loading. Down to zero G, maybe a bit negative. Stall speed at zero g is zero. (control will be a tad difficult at this speed, though.;))

I think it would be difficult to stall in turbulence, and the only severe encounters I've had, I've reduced to below Va, because the airspeed excursions easily shot the airspeed to well above Va, albeit briefly.At the other end of the green range, in the same turbulence, the stall warning operated, once again, briefly, and this also corresponded with much reduced loading.

Good post there 'rutan':-) 4000 hrs in a C210...you poor bugger!:E

Much has been written about turbulence & is a very subjective subject.
What one pilot would do to reduce it's effects on airframes & pax could be somewhat different to the next pilots reactions or lack thereof.
Often yr cruising in light turb at a typical cruise speed then hit almost severe trub in an instance & that being the case the recommended turb penetration speed for such a situation is instantly exceeded so like most things in aviation ones personal comfort factor is yr guide here.The book No's are really only a guide in some ways.



Wmk2

Light;

Occupants may feel a slight strain against seat belts or shoulder straps. Unsecured objects may be displaced slightly. Food service may
be conducted and little or no difficulty is encountered in walking.

Occasional – Less than 1/3 of the time.

Intermittent – 1/3 to 2/3.

Continuous – More than 2/3.


Moderate;

Turbulence that is similar to Light Turbulence but of greater intensity. Changes in altitude and/or attitude occur but the aircraft remains in positive control at all times. It usually causes variations in indicated airspeed.
or
Turbulence that is similar to Light Chop but of greater intensity. It causes rapid bumps or jolts without appreciable changes in aircraft altitude or attitude.

Occupants feel definite strains against seat belts or shoulder straps. Unsecured objects are dislodged. Food service and walking are difficult.


Severe;

Turbulence that causes large, abrupt changes in altitude and/or attitude. It usually causes large variations in indicated airspeed. Aircraft may be momentarily out of control.

Occupants are forced violently against seat belts or shoulder straps.
Unsecured objects are tossed about. Food Service and walking are impossible.


Extreme;

Turbulence in which the aircraft is violently tossed about and is practically impossible to control. It may cause structural damage.


The Vb speed (turb pen.) is not usually any different from the Va speed for light a/c and is defined at 66'/sec of vertical gust.

In general, any time you are in moderate turbulence you should be not above the Va speed adjusted for weight.

Note, even though it is expressed that you will not break the aircraft if you make full deflection at Va, DO NOT go from one full deflection (eg. Full left rudder) to the other extreme.:=

There's some great information on this. Thanks everyone!

IBG
Wally is quite right-turbulence is a very subjective subject. Based on Makeithappencaptain's excellent definitions I slow down in moderate chop and the less moderate it is the more I slow. "Loose objects moving about " would be an easy way to judge that it's time to slow down. Perhaps you could place your mob phone on the glareshield and rename it your "Oh F*#k-o-meter" When the meter goes up you slow down.
Safe Flying RA
As the young pilots apple cheeked old grandmother used to say " I want you to be real safe so you fly real low and real slow"

MIHC, Ive always found it difficult to walk around in a 210, regardless of how strong the turbulence is...

Totally agree Lasiorhinus. LMAO

The above guide, whilst it is well written for an airliner, does it really apply to a lighter a/c ?

Ive been flying and encountered what I feel as top end of moderate turbulence in the circuit area in a C182. Later visually seen other light a/c getting bumped around in the circuit, but watching a 737 on downwind it appeared to be smooth as silk ?

Is it the same as comparing a 4.5m aluminium boat to an 80m floating gin palace ?
I can tell you the ride difference between a 4m tinny to an 8m fibreglass boat in 2 foot chop is spine breaking to champagne sipping.
Are these comparisons fair ? Both are basically vehicles travelling in a fluid environment although water is somewhat harder.

There's the old coffee test for turbulence.

If the coffee is just slopping about within the cup its light.

If its slopping out of the cup its moderate.

If you cant find the cup its severe.

Onya Mr RA for an excellent explanation.

I did not know about the windshield being fitted 'loose' in the 210.
However it does tell me why, in VH-KWW for example, the windscreen used to creak like crazy - especially on the descent, so I used to slow it down using the u/c method to keep it 'within my tolerances'.

But, alas, it was still full of 'internal' cracks which were a 'bugger' in the afternoon sun when landing to the west...

One 'famous' 210 driver in WA wrote this one up -
'Windscreen creaks badly on descent - frightens passengers - F#*in' TERRIFIES pilot!'

Cheers:ok:

Las and Lanc,

Yeah, but these are the quoted "definitions" so I'm not complaining when the skimpy passes me a coffee from the middle row.:E

"Eh pahlut, you wanna drink? It's inna VB can but":}

Actually, maybe this is a complaint in a way.......

Now all of this food for thought has brought me to another question.....

How many G's can Light, Moderate, and Severe turbulence produce based upon their prescribed FPMs? I'll do some more digging and see if I can find a rate for 'Gravities'.

Your question's answer is relative to the airspeed of the aircraft. The faster you travel for a given vertical gust, the smaller the resultant change in relative airflow.
WRT the lift formula, a 10% increase in speed will give an 11% increase in lift and (as LF=L/W) the resulting LF. Then you need to factor in the changing Cl as AoA varies, so you really need to look at it on a type by type basis if you want an accurate answer.

All you need to know is that going faster than Va in moderate or higher turbulence is bad.:cool:

(Another way to think of it is that it won't matter how deep it is, you'll be in the siht regardless.)

That one goes on my "quotes to memorise" list. Thanks TB!

Just an extra thought. If you do go too fast in turbulence, don't expect anything to "break". What will happen is reduced airframe life and possibly some very slight bent parts, by no means necessarily the wing itself. Airframe abuse is often insidious and hard to detect.

But I continue to use turbulence penetration speed to penetrate moderate turbulence, assuming of course I haven't been able to avoid it. If it's not quoted, I guess Va is a close approximation, even if you're one of the nine out of ten pilots* who don't factor it for aircraft weight at the time.

* Personal experience from examining. Present company excluded, naturally!

You seriously test people?

http://www.brisbanetimes.com.au/ffxi..._470x307,2.jpg

Gotta agree with Uber on this one.

The difference between the limit (normal ~3.8) and the ultimate LF (+50%) might apply when the aircraft is new, but metal has memory and every time it gets flexed back and forth (ie by exceeding Va in bumpy stuff aka overstressing the airframe), it weakens just a bit further.

How long until the ultimate LF is equal to the limit LF? You can't tell and I ain't gonna try and find out!:cool:

Most of the light aircraft we fly today were certified under the old CAR part 3 regs (pre '65). The certification criteria dictated that when operating in the green arc, the aircraft structure had to withstand an instantanious '50 per/sec vertical gust without bending anything. That's around 30 knots. I reckon if you happened hit a vertical gust of this magnitude whilst punching along at 160 kias in the 210, it'd certainly capture your attention quick smart. I've got absolutely no doubt one would be looking out the window at the wings rather suspiciously (mistakenly as it turns out).
When in the yellow arc by the way, the airframe has to be able to handle a '30 per/sec vertical gust, again without bending/breaking anything.

Another misconception that you quite often hear, is that it's OK to descend at cruise power in smooth air with the IAS in the yellow. Inncorrect.
The regs (CAR 3) state that the yellow arc is only for inadvertant excursions of airspeed . In other words, if you find yourself on the high side of Vno, get yourself and the aircraft back in the green arc on the ASI.

The C210 was limited by Cessna (as was any other model manufactured by Cessna) to meet the requirements of CAR 3 under which the aircraft was certified. To say that it was hobbled by Cessna because the wing was strutless or because of the way that the wings attached to the fuselage is incorrect. The strutless wing easily met the requirements needed to gain certification under CAR 3.
The wing on the 210 when compared to say, an A36 or Saratoga/Lance, doesn't warrent any extra consideration other than the normal precautions that a pilot might take when operating in turbulent conditions.
The fact of the matter is, the spar carry through structure on the 210 is stronger the the strutted wing of the 206. So if you do happen the hit that 50' per/sec gust in your 210, relax.

As an aside, one of the strongest set of wings that Cessna ever put on aeroplane were strutless. These belonged to the 195 series. Because of a number of design considerations that were made at the prototype stage, the wing was overbuilt (not on purpose) i.e. it was heavier than it needed to be.
When Cessna tested the wings, they didn't break when they were supposed to. They went way, way past the point at which they were supposed to let go. So rest assured, when out driving in your 195, you'll break before the wings do!

das uber soldat, and makeithappencaptain,

That aeroplane did not crash because the pilot went ten or twenty knots too fast in turbulence. If you have read the report behind it, perhaps you might tell us what led to the crash. (Exceeded Va by 10 or 20kts in moderate turbulence on an otherwise fine day? Ha ha.)

There is a difference between going slightly too fast and going 50% too fast, or going into a mature thunderstorm. It's because I know that, that I spent years being allowed to test people. And seeing them, for example, do max rate turns above Va. Naughty naughty. But yet we didn't crash. No, of course I didn't condone it though.

If a pilot reading this forum goes away thinking that flying through moderate turbulence above Vb or Vra will break something, he will be surprised when he accidentally does it one day. Because there probably won't be anything to see to the casual eye - unless of course ten or a hundred other pilots have done so before him, or if he went up to Vne in moderate or severe turbulence! But it's still bad, because of fatigue and hidden parts that are damaged if only slightly. So even if there seems to be no damage, report it at least verbally to a friendly LAME who knows what look for.

Have you seen the slight wrinkling of an overstressed fuselage? It isn't obvious. Have you felt the excess play in a worn-out stabilator bearing? Again, not necessarily obvious.

That's the message I'm trying to say. Nothing to do with flight through thunderstorms.

All the more reason not to do it. How do you know previous pilots haven't gradually worn down that nice limit-ultimate margin? I'm not saying a 10 knot exceedance will bring you undone and I'm sure no-one will be "disappointed" the airframe didn't fail, but Va is set for a reason and to introduce a healthy fear/respect of exceeding this parameter is definitely not a bad thing imo.

I agree with using Va in lieu of Vb, as previously stated, the littlies don't have the same certification reqirememts.

My agreeing with Uber had nothing to do with the commander photo specifically, just not happy with the generalised understatement of what can happen. Worst case scenario, I know and no apologies for that, but it's the old/bold scenario that will keep you alive the longest. Remember the age of Australia's GA fleet.:ok:

If you are flying geophysical survey in a 210 then it alters the dynamics somewhat. Going high isnt an option, and if you slow down too much you can risk driving into the side of a hill.

Put your drink bottle on the seat next to you. If said drink bottle is regularly floating around the cabin, and you are also feeling those sickening big thumps every so often that are a bit painful, its time to go home....so go home!! The survey lines will still be there tomorrow to fly when hopefully it isnt so rough. Especially if it is my survey plane you are flying, they cost too much to fix!!!!

On another note, everybody worrying about the wings falling off may want to take a closer look at the tailplane of their 210. It will give up first you will find. My 210 has had its tail rebuilt 3 times now but the wing has only broken once.

Aileron 69,
I'm curious. When you say your C210 tail has broken 3 times was it the horizontal stabilizer to fuselage (forward attach fittings) that failed and if so after the first failure did you have the doubler mod fitted ?
Cheers RA

The only catastrophic in-flight failure of a C210 that I can recall, occurred near Cloncurry back in the 70's. Wing failed at the flap/aileron junction, if I remember correctly.

Dr :8

There isn't enough information in the Accident Report to really determine what happened. However if the limited information available is accurate it appears this may have been a flutter event. I once heard an aircraft flutter and a sound a bit like a misfiring engine is a reasonable description. The witness statements that the aircraft had commenced a right turn do not fit the overloaded outboard section of wing failing in the downward mode scenario. If that scenario had occurred the right wing would be the broken one. Turning right would speed up the left wing and perhaps that was the straw that broke the camels back. C210's don't have a great flutter margin but it is of no concern providing it is flown inside the design envelope. There is an aileron beef up mod available. This was developed when people started playing with much more powerful engine installations such as turbines.
Cheers RA

I don't see that in CAR 3. (I am aware of a reg which states that a pilot must do what the AFM states.)

The standard equation for gust load factor:
http://adg.stanford.edu/aa241/struct...ges/image5.gif
with: a = (dCL/da) - lift curve slope - try 5 for a simple calculation
Ue = equivalent gust velocity (in ft/sec)
Ve = equivalent airspeed (in knots)
Kg = gust alleviation factor - try 0.85 for a simple calculation
W/S is wing loading in lb/sq ft
(I hope my memory has worked for the "try" numbers, I haven't had any coffee this morning yet)

Rutan:

Its broken in different places each time. The forward attach fittings have broken, so have the rear. We do now have the doubler mod fitted. It also needed the rear 2 or 3 (cant remember how many) bulkheads replaced as they cracked. Was a rather expensive process. I would say part of the cause of the bulkhead damage would be directly related to the stinger on the tail.

Cheers
Aileron

Thanks Capt! I wasn't aware that there were witnesses.

Bit of a mystery really.

Happened around the time I first started flying 210's - gave considerable food for thought.

Dr :8

Wasn't there an SB that came out recently as a result of a stinger equipped a/c with this damage?

Sure was, it was the rear attach point on the Horizontal Stab breaking, and that caused the stab to move, jammed the elevator, and nearly brought the aircraft down. When other operators checked their aircraft about half found the rear attach points cracked and needing replacement.

Just had this arrive from Cessna today regarding Spar cap inspections.
This could ground a few aircraft.

https://support.cessna.com/custsupt/...df?as_id=37240

Thanks for that link 69.

For most readers of this thread I'm probably teaching my grandmother how to suck eggs with this C210 pre flight check to determine if the horizontal stabilizer to fuselage attachments are ok.
Grip the outboard end of horizontal stabilizer and push it up and down a few inches (fairly slowly.don't use excessive force.)

You should hear an oil canning noise as the skin flexes. This noise is ok
If you hear a crunchy or grating noise DON'T FLY.

If this test only prevents 1 accident it was worth boring those who already include it in their pre flight checks.

To recognize the difference between an oil canning noise and the dreaded crunchy noise I
recommend the following procedure.

1) Empty one aluminum beer can

2) Lightly squeeze and release the can several times between thumb and forefinger. That noise is oil canning or in this case beer canning.

3)Now using a screwdriver or knife rip and tear a break approximately half way round the circumference of the can. Grasp each end of the can and slightly rotate the ends back and forth in opposite directions. That is the crunchy "DON"T FLY" noise.

When Mrs RA politely inquired why I had just drunk 8 VBs I just as politely informed her that the recommended procedure would have to stand up to peer group review and one couldn't be too careful. The mower shed isn't all that cold and uncomfortable.

I recommend the above procedure to anyone unsure of the noises they should be listening out for whilst performing the essential horizontal stabilizer pre flight check.
Cheers RA