Turbulence penetration question

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Turbulence penetration question

Unread post by Slam and Go » Mon Feb 10, 2020 11:59 am

Hi all

So far I have encountered 2 schools of thought on what speed to fly in heavy turbulence.

1.Fly at the halfway point between Vs and Vne
2. Fly Va

What do you guys think?

Edit: for aircraft without a published speed for this
Last edited by Slam and Go on Fri Feb 14, 2020 4:24 pm, edited 1 time in total.
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Re: Turbulence penetration question

Unread post by Volo » Mon Feb 10, 2020 12:49 pm

Fairly straight forward according to my flight Manual and herewith the following operating speed (IAS) limitations :-

Max permitted speed - VNE - 135 knots
Max speed landing flap - VFE - 86 knots
Max speed rough air - VRA - 102 knots
Max Maneuvering speed - VA - 102 knots
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Re: Turbulence penetration question

Unread post by Slam and Go » Mon Feb 10, 2020 12:58 pm

Thanks for the info.

Now we all think of Va applying one control input abruptly. In heavy turbulence there is often rolling and pitching going on.

I see the logic to fly below Va in this case.
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Re: Turbulence penetration question

Unread post by StressMerchant » Mon Feb 10, 2020 1:30 pm

My first response would be something along the lines of "what does your POH / AFM suggest?"

Part 25 aircraft and some Part 23 have a defined rough air penetration speed that should be published in the data available to the pilot. The wording on the previous version of Part 23 was:
"FAR 23.1585(a)(3) A recommended speed for flight in rough air. This speed must be chosen to protect against the occurrence, as a result of gusts, of structural damage to the airplane and loss of control (for example, stalling);"

Older Part 23 aircraft may not have a defined rough air speed in their manuals. In the absence of a defined speed, an airspeed less than VA, but greater than Vs by a margin seems reasonable.
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Re: Turbulence penetration question

Unread post by jimdavis » Mon Feb 10, 2020 2:19 pm

StressMerchant wrote:
Mon Feb 10, 2020 1:30 pm
My first response would be something along the lines of "what does your POH / AFM suggest?"

Part 25 aircraft and some Part 23 have a defined rough air penetration speed that should be published in the data available to the pilot. The wording on the previous version of Part 23 was:
"FAR 23.1585(a)(3) A recommended speed for flight in rough air. This speed must be chosen to protect against the occurrence, as a result of gusts, of structural damage to the airplane and loss of control (for example, stalling);"

Older Part 23 aircraft may not have a defined rough air speed in their manuals. In the absence of a defined speed, an airspeed less than VA, but greater than Vs by a margin seems reasonable.
Stressors, am I correct in thinking it gets less when the aircraft is light?

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Re: Turbulence penetration question

Unread post by Flooi » Mon Feb 10, 2020 3:52 pm

LSA with light wing loading.... as slow as safely possible.... keeping in mind that in turbulence, the aircraft may unexpectantly find itself below stall speed. Also maintain safe altitude.
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Re: Turbulence penetration question

Unread post by Romeo E.T. » Mon Feb 10, 2020 5:58 pm

I was taught:

1) Below Va - full abrupt control surface application, the control surface will encounter aerodynamic stall before becoming overstressed
2) Above Va - full abrupt control surface application, the control surface, the attachment point, or further internal structures can experience structural damage
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Re: Turbulence penetration question

Unread post by Jean Crous » Mon Feb 10, 2020 6:18 pm

Myself flying mostly LSA aircraft albeit with a PPL, my self made rule has always been in very rough turbulence to fly at Vy. And I am still here to tell the tale :lol:
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Re: Turbulence penetration question

Unread post by Flooi » Tue Feb 11, 2020 1:52 am

Jean Crous wrote:
Mon Feb 10, 2020 6:18 pm
Myself flying mostly LSA aircraft albeit with a PPL, my self made rule has always been in very rough turbulence to fly at Vy. And I am still here to tell the tale :lol:
Agreed.... my POH gives Vy at 68knts IAS. If turbulence is high, 55 knts IAS mushes with more comfort. Maybe the high aspect wing ratio makes the difference. Not sure.....
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Re: Turbulence penetration question

Unread post by yesbhobho » Wed Feb 12, 2020 7:13 am

I once did a license conversion in Namibia with a gentlemen called Leon and was asked about Va and what it means. To cut the long story short, I was told that it's not my fault that I didn't know, but he blamed the person who taught me. After reading parts of this thread, I'm now more inclined to agree with him about this aspect not being taught well.

To fully understand this concept of Va, one has to look at the reason why it reduces with weight, like Oom Jim said, or vice versa. You also have to know what the G load limit of your aerie is. This has to do with the load factor as the angle of attack is increased. (YouTube has very nice explanations of you search for why Va decreases with a decrease in weight)

In basic terms, Va speed will ensure that your wing stalls before exceeding the g limit which acts as a relief on the structural strength. So, if your aircraft does not have a stipulated turbulent penetration speed, fly at VA.
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Re: Turbulence penetration question

Unread post by rare bird » Wed Feb 12, 2020 9:23 am

better to avoid turbulent air - might mean you fly a few extra miles.
(e.g. flying down the West coast at midday will be bumpy - we all know that! so fly early in the morning, or fly over the sea, within glide of the land.)
i.e. better planning
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Re: Turbulence penetration question

Unread post by heisan » Wed Feb 12, 2020 10:32 am

jimdavis wrote:
Mon Feb 10, 2020 2:19 pm
Stressors, am I correct in thinking it gets less when the aircraft is light?
jim
There seems to be a lot of confusion about V speeds...

First, some definitions:
Va = maneuvering speed (safely apply full control deflection without structural failure).
Vno = maximum structural cruising speed (only go faster is still air).
Vb = design speed for max gust intensity.
Vra = rough air penetration speed (max safe speed in severe turbulence).

Vno and Vb are fairly similar (IIRC Vno assumes max 30fps gust and Vb max 45fps gust). No idea how Vra differs from Vb. I have not seen any light aircraft manuals that specify Vb or Vra though...

Secondly, these are book numbers, and DO NOT CHANGE. (Some books do give different numbers for different weights - but the V speed is the number given in the book for the weight.)

Theoretically, you could safely maneuver a little faster than Va in a heavy aircraft, but do you really want to be a test pilot.

As for the other speeds, all bets are off... Are you in part of the envelope where stall determines the limit (higher weight = faster), or part of the envelope where structural loads determine the limit (higher weight = slower)? Do you really want to try find out?

Rather fly the numbers as they are in the book.
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Re: Turbulence penetration question

Unread post by rare bird » Wed Feb 12, 2020 11:28 am

certainly keep within the numbers, however do remember it is a 3D envelope
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Re: Turbulence penetration question

Unread post by stefan » Wed Feb 12, 2020 8:25 pm

Just a correction, Va is the speed at which you can apply full deflection of any ONE flight control without overs stressing the airframe. So you cannot go do a roll at Va like some think you can. You do get away with it for a while but like Justin said "do you really want to be a test pilot"
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Re: Turbulence penetration question

Unread post by StressMerchant » Thu Feb 13, 2020 12:23 pm

Rough air penetration speeds and manoeuvre speeds have similar issues regarding definitions and applications. You’ll find differences in approach between older aircraft and newer aircraft, between single engine and multi-engine, and between Part 23 and Part 25. So a degree of confusion is understandable.

Design vs Operating: The engineers design the aircraft to withstand manoeuvre and gust loads defined in Subpart C (Structures). The pilots operate the aircraft in accordance with information specified in Subpart G (Operating Information and Limitations / Flightcrew Interface and Other Information). In some cases the distinction between the two is not clear. And it doesn't help that in big companies they are implemented by different teams.

Looking at Subpart C:

Old Part 23: Aircraft is designed for static loads as follows:
VC: Gusts up to 50 ft/s
VD: Gusts up to 25 ft/s

In addition, for multi-engine aircraft certified after 1987:
VB: Gusts up to 66 ft/s

For new Part 23 aircraft, the regulation requires that:
The applicant must determine the structural design loads resulting from the following flight conditions: (a) Atmospheric gusts where the magnitude and gradient of these gusts are based on measured gust statistics.

For Part 25 aircraft, the requirements are:
VB to VC: 56 ft/s
VD: 50% of the value at Vc

(all sea level values, reductions are specified for increasing altitudes)

Subpart G requires that the designer include speeds in the Airplane Flight Manual, but these speeds do not need to be the same as the Subpart C speeds. Logically they would not be higher than the Subpart C speeds, but they may be lower for reasons for crew comfort, handling characteristics, or airframe fatigue considerations.

Part 23: Up to 1993, Part 23 aircraft were not required to specify a recommended rough air speed. For Part 23 aircraft certified after 1993:

(a) For all airplanes, information ….. must be furnished, including-- (3) A recommended speed for flight in rough air. This speed must be chosen to protect against the occurrence, as a result of gusts, of structural damage to the airplane and loss of control (for example, stalling);

The new Part 23 requirement is:

“(a) The Airplane Flight Manual must contain the following information— (1) Airplane operating limitations; (2) Airplane operating procedures; (5) Other information that is necessary for safe operation because of design, operating, or handling characteristics.”

Part 25 requires furnishing of a Rough Air Speed, VRA. This is a recommended turbulence penetration speed, and is established relative to Vmo. The relationship between VRA and VB is indirect.
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