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Turbulence and Safe Speeds to Adopt


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There is another thread running in the European section, but I believe that this is such an important subject that it should be re-opened here in Training.

 

As an instructor who has conducted many, many Flight Reviews over many years, I can confidently state that the knowledge most pilots hold about 'safe speeds' in turbulence is frighteningly low. I've had pilots tell me that anything under Vne is fine. Others have stated that as long as your needle is below the yellow arc on the ASI - you're fine. Very, very few understand the relationship to gross weight and stalling speed of the aircraft. Very few recognise moderate turbulence when they feel it. It's no wonder that aircraft breakup in flight!

 

Aircraft which spend more time at low altitudes are known to suffer greater stresses in the airframe. Low level activities such as mustering, ag, and survey create lots more need for airframe inspections. I've heard some truly terrifying stories from maintainers about tailplanes virtually 'flapping' after thousands of hours mustering.

 

I've had some real frights with turbulence. After several, my legs barely supported me after landing! Australia might have an absence of high mountains, but it consistently has ambient temperatures 15-25 degrees above Standard - and that means lots of thermal activity. Diurnal heating should always be in mind when selecting altitudes - sometimes a little more headwind is better than an uncomfortable rough ride.

 

So, to start with - who knows the load factor limits of their aircraft, both clean & flapped? Who uses a G-meter to keep an eye on things? Who knows the formula to calculate Va. Is Va close enough to use as a proxy for turbulence penetration? Who adjust their cruise speed to suit the turbulence experienced?

 

I'm seriously interested - lets talk.

 

 

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Great questions, & the silence is deafening.

 

I for one confess abysmal ignorance on this. I just stay below the yellow arc when it's lumpy. But as a single seater with max 10kg baggage at reduced fuel uplift my options for exceeding the envelope are perhaps more limited than some. No excuse, though.

 

I look forward to learning more.

 

Bruce

 

 

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For My hornet my turbulence penetration speed is the same as my manuevre speed which is 70knots but I am too much of a sissy and prefer to sit on 50-60knots which also allows me to either throttle back and maintain height or leave full throttle on and climb up into hopefully smoother air. I don't like turbulence and no doubt like most of us rec flyers I avoid it where possible.

 

I did have one interesting time flying over the range with mechanical turbulence, I had a passenger in with me and had only climbed a couple of thousand feet higher than what the range was and we were still climbing as there was a nice 10-15kmot tailwind I knew to expect a little bump once over the range but was surprised by a solid hit and saw the ASI peak at 90 (vne) for a split second before returning to 60. Certainly an eye opener for me especially as it was only blowing 10-15knots I certainly wasn't expecting anything that tough.

 

I don't have a G metre and would find one interesting. I also don't have a brs and in turbulence is the only time I consciously wish I did.

 

My flap extension max speed is 60knots but on a rough approach I leave them up until down to 50ish just to add a bit of an extra buffer.

 

Avoiding turbulence like I do means I have a lot to learn so I also am watching this thread with interest.

 

 

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The yellow arc in my AUSTER begins at 95 knots but I cruise at 105 KN TAS which means I have to throttle back a fair bit in turbulence and leave sufficient margin for large fluctuations.

 

There is no AFM or POH for the aircraft so I'm really just guessing that this is sufficient in rough conditions.

 

Kaz

 

 

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But as a single seater with max 10kg baggage at reduced fuel uplift my options for exceeding the envelope are perhaps more limited than some. No excuse, though.

I look forward to learning more.

 

Bruce

I won't steal poteroo's thunder, but I imagine that this may be one of the common misunderstandings he was referring too.

 

Great thread! These are misunderstood at all levels.

 

 

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Great subject you have brought up Potteroo, I live on the East Coast and am on the Lee side of the Great Dividing range and have a lot of respect for Westerly winds, my strip is east west and have had a lot of westerly winds over the last few months mostly in morning and middle of day as the sea breeze kicks in noon to mid arvo. I have power lines 300m west of strip and am very aware of possible downdraft wave taking off to the west and have experienced it on several occasions. I am a former resident and aviator from the snowy mountains and am very use to mountain flying so I do not underestimate the downdraft threat but turbulence is a worry too ! I have done several trips in severe turbulence over mountain terrain ! The threat of structural failure or fatigue is very real as pointed out but the ability to outclimb a downdraft is very life threatening that is sometimes not considered by those who have not flown near real hills !

 

By the way, I consider 15 knots from the west a limit to fly but would not put a limit within reason on NE to SE as there would be no turbulence or downdraft ! N and S are crosswind and 14 knot for aircraft and 20knots for me, haha ! Been there ! A lot more than that experienced in a C172 in Broken Hill 15years ago ! 54knots wind 45deg across strip ! What a ride ! Can laugh now ! Remember every moment ! Perfect landing ! Hard to park and open door ! Turbulence was wild around airport and town due to the local hills, was reasonably smooth before got near hills !

 

 

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From another discussion:

 

"Va is the close approx for Vb if it is not published in the aircraft flight manual. Most light GA aircraft dont have a published Vb.

 

Vb = design speed for maximum gust intensity(best speed to give protection of both stall and structural failure) i.e. might be at a higher certification assumption e.g. 45fps

 

while

 

Vno = maximum structural cruising speed (where maximum turbs by "definition" is 30 fps vertical)

 

Vno (green segment upper limit) the aircraft must be able to with stand a vertical gust of 30 feet per second without STALLING or STRUCTURAL FAILURE at Max weight. You would have to make sure that it did not exceed the Vno speed or the 30fps gust while at Vno to avoid stall or damage, hence the Va recomendation by some engineers for light aircraft.

 

30 fps gusts can be found in the vicinity of thunderstorms, while 45 fps can be found near thunderstorms, and in them can be well over those values

 

The calculation for Va is often quoted as

 

Va = Vs x sq rt Gmax "

 

For the AUSTER (no manual or POH) Vs without flaps is 32 knots and the max loading is 4g

 

Va = 32 x 2 = 64 knots which would feel terribly slow to me compared to Vno of 95 knots.

 

Kaz

 

 

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My flap extension max speed is 60knots but on a rough approach I leave them up until down to 50ish just to add a bit of an extra buffer

Very sensible: and it's because the calculation of Va is dependant on the square root of the load factor x the clean stall speed.

 

The LF of most RAAus and GA aircraft would be 3.8 nil flap......BUT.......only half that with flaps extended! With a lower Vs1 than Vs, and a LF only half that for unflapped flight - the sensible thing is to use nil flap if it is exceedingly rough in the approach.

 

Va = 32 x 2 = 64 knots which would feel terribly slow to me compared to Vno of 95 knots.

The Auster is very tough and these calculations may be less applicable to it because we really don't know just what standards they were originally built to handle.

 

Notwithstanding, I would be getting well under Vno of 95 whenever it became seriously rough. If not for the structural failure reasons, rather for the reason of maintaining controllability without losing it. A 'target' IAS of half way between your Vno and Vs would place you in a safer spot I'd think.

 

happy days,

 

 

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?..The Auster is very tough and these calculations may be less applicable to it because we really don't know just what standards they were originally built to handle.

 

Notwithstanding, I would be getting well under Vno of 95 whenever it became seriously rough. If not for the structural failure reasons, rather for the reason of maintaining controllability without losing it. A 'target' IAS of half way between your Vno and Vs would place you in a safer spot I'd think.

 

happy days,

I read elsewhere that the wing on the AUSTER is stronger than that on a 150 HP 8KCAB. I did some initial aero training on one of the latter but I'm always conscious of the Auster's age...we were both built the same year!

 

I had a very rough trip up to Bourke a few weeks ago in forecast moderate turbulence and low cloud base to around Cobar. Most of it I did at around 80 knots while laying off a fair bit because of the very strong cross-wind.

 

It feels a bit odd when you see more of what's directly ahead through the side screen than the windscreen. I also don't like babying the O-320 engine at 2200 -2250 rpm. Feel much more comfortable when up at 2450-2500.

 

Kaz

 

 

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And the rougher it gets .......... the faster you want to get there and down again

 

"moderate" turbulence in a forecasts seems to be a loose term meaning "we arent sure"

 

I agree running low rpm for much time isnt good

 

 

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And the rougher it gets .......... the faster you want to get there and down again"moderate" turbulence in a forecasts seems to be a loose term meaning "we arent sure"

If you are flying an aircraft with a 'G' meter, (and most EFIS units have these as standard), then the following applies to Turbulence:

 

Very Low: - < 0.05G - light pitch, yaw & roll

 

Low: - 0.05 - 0.2G - choppy, cobblestone effect

 

Moderate: 0.2 - 0.5G - uncomfortable bumpiness

 

Severe: 0.5 - 1.5G -abrupt bumps, difficult handling

 

Very severe: > 1.5G - aircraft thrown about, near loss of control.

 

But, when you consider just the vertical gusts, (as do the engineers with their Vn diagrams), then the following applies:

 

Weak: 5-10 m/sec

 

Moderate: 10-15 m/sec

 

Strong: 15-25 m/sec

 

Severe: >25 m/sec

 

I am lucky enough to have Dynon D-180 EFIS/EMS in both my RV and Brumby so can measure the amount of turbulence on any one flight. It is instructive for both myself and student because it allows for some level of 'quantification' of the conditions that we've just experienced. I have always planned to advise my LAME of any severe experiences that we have in the aircraft and that would be backed up by knowing the 'G' numbers. Standalone G - meters are not that expensive anyway, and anyone who flys close to ranges and in the hotter parts of Australia should consider one.

 

happy days,

 

 

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I read elsewhere that the wing on the AUSTER is stronger than that on a 150 HP 8KCAB...

It's fairly easy for designers to measure the G-force capabilities of major components like a wing, but what about all the other unimportant bits like elevator, tailplane and ailerons? My Jodel's spar is rated at +9G, but that was before mods. I have no idea what the other bits will take, so I'll be backing right off (and risking glazing the bores) in rough stuff.

 

 

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It's fairly easy for designers to measure the G-force capabilities of major components like a wing, but what about all the other unimportant bits like elevator, tailplane and ailerons? .....

Someone has already done the hard work (for small conventional airplanes):14 CFR Appendix A to Part 23, Simplified Design Load Criteria

 

 

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I still don't know what I'm doing, but I guess that's why fin and tail plane are cross-braced?Kaz

Yes, I'm missing something here too:

I found the G force vs perceived effect (light pitch/yaw to near loss of control) enlightening.

 

But I'm not able to make any (similar) subjective connection with the table of vertical gust speeds.

 

Would that not be useful if that is what is used to calculate Vn speeds, or am I missing the point entirely?

 

 

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Innt

 

Yes, I'm missing something here too:I found the G force vs perceived effect (light pitch/yaw to near loss of control) enlightening.

But I'm not able to make any (similar) subjective connection with the table of vertical gust speeds.

 

Would that not be useful if that is what is used to calculate Vn speeds, or am I missing the point entirely?

To really connect all the terms, I have read Turbulence-The Aeronautical Forecasters Handbook, from BoM. I have a digital copy for anyone who is into this stuff - but beware - it's heavy going in places.

 

From this book, on p5 - Table 1 connects IAS variation to vertical gust fps to G load to aircraft reaction:

 

Intensity IAS variation vertical speed G load aircraft reaction cockpit reaction

 

light 5-14 kts 5-19 fps 0.15 - 0.49G slight little

 

moderate 15-24 20-35 0.50 - 0.99 rapid bumps,jolts strain on belts

 

severe >25 36-49 1.0 - 1.99 large changes, objects off floor,

 

loss of ctrl forced into belts.

 

extreme >25 >50 >2 out of control - damage likely-injury likely

 

I recommend this book to interested pilots.

 

I found the following article from Vans Aircraft quite informativehttps://www.vansaircraft.com/pdf/hp_limts.pdf

Yes, the high altitude stuff in regards to VNE and TAS isn't likely to be of great relevance in Australia because we are usually well under 10,000 ft - whereas in the western states of the US it's not unusual for RV owners to cross the Rockies at 13,000 - 14,000 ft. Many RV's have O2 and many RVs have 180HP and above engines. They also fly at 75% power everywhere.

 

The article on HP choices for RV9 series RVs is interesting. I 'disobeyed' VANS by fitting a Superior IO-360 to mine, and the dyno tests showed 190HP. (it has both injection plus EI on one mag). Now, on reading his article you'll note that he is talking about 180 HP engines being flown at 75% power, (and because most US aircraft are CSU fitted as well - his story includes this performance enhancer as well).

 

But, I believe VANS misses the point here. He is not recognising that Va is calculated on the published positive load factor,(3.8), and the actual stalling speed clean, (Vs). With a Vs = 49KIAS in my 9A, and then x that by the sq.rt. of 3.8 (=1.95), I calculate a Va of 96KIAS. This quite a lot lower than the 106KIAS that is evident in the V-n diagram shown in the VANS article.This has nothing to do with HP per se. But it does have a deal of relevance for flying an RV9A in Australia. For a start, we don't usually fly at 75% power because avgas is much more expensive in Australia, and it's often a long way between refuelling stops. So, the average Aussie 9a is flown at lower IAS than in the US. When they start quoting 156KIAS, and we are more often looking at 130 KIAS at 8500' - there is quite a difference.

 

And, from talking with other 9A owners with higher HP engines, I find that most use them mostly to enhance rate-of-climb performance - not trying for higher en route cruise speeds. Most seem to target a particular KTAS for cruise, eg 140,145,150, and then set up power and lean to achieve. So, the probability of sustaining damage due en route turbulence is lower here than in the US scene.

 

If you look at the numbers published by VANS - you'll see a Va of 112 given. It's shown as a blue line on their analog ASI gages. How did they derive this?? It's a long way above the Va that I derive for my 9A, and mine is based on the real life stalling numbers as I measured them using real life weights. (Sure, I'm using IAS instead of CAS, but the differences won't be much and my ASI was found to be quite accurate on test). Why would this be? Whose numbers do I believe? Mine of course - because they are the more conservative. Using my numbers allows me to traverse some rougher air than I might otherwise tackle, allowing for the skirting of stormy areas and frontal and trough lines at my <100KIAS self imposed limits.

 

I realise that these examples using RV's are some way from the average RAAus aircraft doing 90-100KTAS - though the principles remain the same. Just because you cruise at 85KIAS doesn't mean that you can ignore your operation in turbulent air. Have a look at your Vs at usual weights, then x by 1.95, and you have your Va. My guess is that many of you are in for an awakening - you'll end up with numbers in the 60-70KIAS range and that's not surprising because you can't have low Vs and high Va unless the aircraft has been built stronger than + 3.8G.

 

happy days,

 

 

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There is another thread running in the European section, but I believe that this is such an important subject that it should be re-opened here in Training.As an instructor who has conducted many, many Flight Reviews over many years, I can confidently state that the knowledge most pilots hold about 'safe speeds' in turbulence is frighteningly low. I've had pilots tell me that anything under Vne is fine. Others have stated that as long as your needle is below the yellow arc on the ASI - you're fine. Very, very few understand the relationship to gross weight and stalling speed of the aircraft. Very few recognise moderate turbulence when they feel it. It's no wonder that aircraft breakup in flight!

 

Aircraft which spend more time at low altitudes are known to suffer greater stresses in the airframe. Low level activities such as mustering, ag, and survey create lots more need for airframe inspections. I've heard some truly terrifying stories from maintainers about tailplanes virtually 'flapping' after thousands of hours mustering.

 

I've had some real frights with turbulence. After several, my legs barely supported me after landing! Australia might have an absence of high mountains, but it consistently has ambient temperatures 15-25 degrees above Standard - and that means lots of thermal activity. Diurnal heating should always be in mind when selecting altitudes - sometimes a little more headwind is better than an uncomfortable rough ride.

 

So, to start with - who knows the load factor limits of their aircraft, both clean & flapped? Who uses a G-meter to keep an eye on things? Who knows the formula to calculate Va. Is Va close enough to use as a proxy for turbulence penetration? Who adjust their cruise speed to suit the turbulence experienced?

 

I'm seriously interested - lets talk.

There is another thread running in the European section, but I believe that this is such an important subject that it should be re-opened here in Training.As an instructor who has conducted many, many Flight Reviews over many years, I can confidently state that the knowledge most pilots hold about 'safe speeds' in turbulence is frighteningly low. I've had pilots tell me that anything under Vne is fine. Others have stated that as long as your needle is below the yellow arc on the ASI - you're fine. Very, very few understand the relationship to gross weight and stalling speed of the aircraft. Very few recognise moderate turbulence when they feel it. It's no wonder that aircraft breakup in flight!

 

Aircraft which spend more time at low altitudes are known to suffer greater stresses in the airframe. Low level activities such as mustering, ag, and survey create lots more need for airframe inspections. I've heard some truly terrifying stories from maintainers about tailplanes virtually 'flapping' after thousands of hours mustering.

 

I've had some real frights with turbulence. After several, my legs barely supported me after landing! Australia might have an absence of high mountains, but it consistently has ambient temperatures 15-25 degrees above Standard - and that means lots of thermal activity. Diurnal heating should always be in mind when selecting altitudes - sometimes a little more headwind is better than an uncomfortable rough ride.

 

So, to start with - who knows the load factor limits of their aircraft, both clean & flapped? Who uses a G-meter to keep an eye on things? Who knows the formula to calculate Va. Is Va close enough to use as a proxy for turbulence penetration? Who adjust their cruise speed to suit the turbulence experienced?

 

I'm seriously interested - lets talk.

There is another thread running in the European section, but I believe that this is such an important subject that it should be re-opened here in Training.As an instructor who has conducted many, many Flight Reviews over many years, I can confidently state that the knowledge most pilots hold about 'safe speeds' in turbulence is frighteningly low. I've had pilots tell me that anything under Vne is fine. Others have stated that as long as your needle is below the yellow arc on the ASI - you're fine. Very, very few understand the relationship to gross weight and stalling speed of the aircraft. Very few recognise moderate turbulence when they feel it. It's no wonder that aircraft breakup in flight!

 

Aircraft which spend more time at low altitudes are known to suffer greater stresses in the airframe. Low level activities such as mustering, ag, and survey create lots more need for airframe inspections. I've heard some truly terrifying stories from maintainers about tailplanes virtually 'flapping' after thousands of hours mustering.

 

I've had some real frights with turbulence. After several, my legs barely supported me after landing! Australia might have an absence of high mountains, but it consistently has ambient temperatures 15-25 degrees above Standard - and that means lots of thermal activity. Diurnal heating should always be in mind when selecting altitudes - sometimes a little more headwind is better than an uncomfortable rough ride.

 

So, to start with - who knows the load factor limits of their aircraft, both clean & flapped? Who uses a G-meter to keep an eye on things? Who knows the formula to calculate Va. Is Va close enough to use as a proxy for turbulence penetration? Who adjust their cruise speed to suit the turbulence experienced?

 

I'm seriously interested - lets talk.

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Thank you for posting this topic. This issue came to my mind only yesterday while (for work) on a medical retrieval coming in to land from aprox 3,000 feet between a mount range was 'choppy'. While flying through this and landing with a significant xwind I was contemplating how the little sports star that I fly (for fun) would handle this. I had visions of the whold plane breaking up! I questioned to myself when flying these conditions how does a pilot know exactly what stress the plane is experiencing at that point in time? This is a question I will be asking my pilot collegues next time.

 

While aware of VNO and VNE aircraft limitations, for someone like myself with as little as 13 hours PIC I will be very interested in this conversation. From reading so far, the G meter sounds like a good recommendation to record and report aircraft stressors. Thanks again. Safe and happy flying ... avoiding turbulance!

 

 

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I always understood Va (or probably Vb is more accurate) to be the speed at which the wing will stall as a result of an excessive vertical gust (turbulence) therefore unloading the wing before it reaches its maximum G-load. This speed gives you an automatic safety valve against excessive G forces. Interestingly the lower your weight is the lower the safe Va speed is if I remember rightly.

 

 

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