CFI myths..Special VFR day/night et al

[northmountain]

Here's a new one. A fellow instructor told me that they changed the regulation and you need an IFR rating (and aircraft IFR equipped) to fly under Special VFR during the Day (as well as night). Oh really?

I thought I lost my marbles, but checking with other CFIIs and a lawyer, I can see where the confusion is:

91.157 Special VFR weather minimums
b) Special VFR operations may only be conducted--
(1) With an ATC clearance;
(2) Clear of clouds;
(3) Except for helicopters, when flight visibility is at least 1 statute mile; and
(4) Except for helicopters, between sunrise and sunset (or in Alaska, when the sun is 6 degrees or more below the horizon) unless--
(i) The person being granted the ATC clearance meets the applicable requirements for instrument flight under part 61 of this chapter;

SO in English...You need 1 and 2 and 3 and operate in daylight (ok, ok, between sunrise and sunset). Then, if you don't operate between sunrise and sunset, (in other words you now operate at night) then the instrument stuff comes into play.

What irks me is that this gets told to a student, who tells another student who tells...etc....etc....and PRESTO a myth is borne.

CFIs really need to watch what they are teaching! Students often look to us as the final authority.

Any other myths that you guys and gals were told as students that you later found out were total bunk? I know I have plenty.

We aren't talking about flying techniques; we're talking FARs, aerodynamics, systems etc.

How 'bout this one....."when you get into ground effect you are compressing the air below the aircraft" ARRRRGGHGGHH!!!

 

[Timebuilder]

North, I have never hear that particular reg misunderstood in that way. Clearly, the instrument rating is for the night op, not the day special VFR.

Once you straighten out your friend, you can encourage him to go back over this with his students and show them where this misunderstanding can develop.

 

[midlifeflyer]

Although there are some myths about flight itself that seem to persist (the dreaded "downwind turn" for example), Most of the ones that I've seen seem to be, like this Special VFR one (I have heard it from a ground instructor), seem to be in the regulation area.

The "logging" vs "acting as" PIC ones come to mind immediately. Of course some of the questions when the subject is bantered about get silly, but there are still a substantial number of CFIs out there who think that an ASEL rated pilot simply =cannot= log the training time that he receives for the complex, high performance or tailweel endorsements as PIC.

 

[northmountain]

Midlife...
What's this "downwind turn" misconception....I'm not familiar.
Jedi: Yeah, dihedral: It's there to drive you crazy and give you dutch roll, or lateral-directional oscillation. If I really said that a studen would look at me like this

 

[midlifeflyer]

>What's this "downwind turn" misconception....I'm not familiar.

Assuming a strong constant wind, you will lose airspeed when turning from upwind to downwind. So it's very dangerous turn from upwind to downwind at a slow airspeed or too much bank because you could easily stall.

 

[northmountain]

Oh, yeah, that's a good one. Reminds me of the old "you need more power in the left turn than the right turn" BS. Reasoning was that pitot is located on left wing so in left turn it is not going thru the air as fast; hence, registering a lower indicated airspeed. I saw a trig proof of this and it comes out to 1 knot or so. Hardly a reason to add more power. However, I guess one could argue that THEORetically, it is true

 

[throttlejockey]

Now if you can only get ATC to learn this reg.........many think that SVFR is no allowed at ALL at night.

 

[avbug]

I'm all for truth in advertising and correcting myths, but the downwind turn isn't all myth. In theory, you can initiate a turn upwind or downwind, watching only the aircraft instruments, and will see no change. The aircraft has no idea what's occuring with winds, as it's in a fluid environment. Ground track changes with wind, but airspeed does not. So goes the explaination of reason.

Those who do this explaining haven't ever had to make a hard turn downwind in a critically loaded airplane, and had the opportunity of watching the airspeed bleed to nothing.

I've heard the arguements made over the years, and I've even tried arguing both sides. The science of it says that in theory there's no such animal as the effected downwind turn, that wind has no effect on the downwind turn. Science is supported by inspection of many downwind turn accidents; we see that the pilot made the mistake of visually seeing groundspeed and turn radius increase, applied back pressure and steepened bank to compensate, and lost airspeed and eventually stalled.

Very common. However, there's more to it.

If an airplane had no mass or inertia and simply floated as a speck in the wind, then turning upwind or downwind would make no difference. This isn't the case however. Take an airplane off loaded to the gills in a right crosswind, and make a hard turn to the right. Then do the same thing with a hard turn to the left. You're going to see a difference.

The airplane, if allowed to remain over a single geographical spot and simply rotate about the vertical axis, would see a change in airspeed simply by nature of a heading change. Science tells us that this is of no effect, because the airplane is in a fluid environment, and as it turns, the airplane is also moving with the wind.

An airplane may be turned rapidly such that it will experience an airspeed loss. I understand the theory, but I've seen it happen too many times to doubt it. Doubtless PilotYip will be here to berate that statement along with chides on the history of flight and nonprecision approaches, but he will be wrong. Those who doubt the efficacy of the effected downwind turn simply haven't had the experience themselves. Once exposed to it, it leaves an indellible impression.

I have the stick grip at my hangar residence, from an airplane years ago that stall spun during a downwind turn. It was the last thing that individual touched at the company for whom I flew, in the D model Pawnee before he hit the ground and had the pump brake handle driven through him.

While I realize that such statements will likely raise the ire of many self-opinionated purists, I can't deny what I've seen. I can't necessarily argue it too effectively, either. After all, one should reason that if what I say holds true, then one should be able to perform a steep turn and see a change in airspeed...when clearly this won't be seen.

My first commercial flight employment was ag work, right out of high school; I spent my time doing steep turns at 75' every 30 seconds, usually in wind, in airplanes loaded to the proverbial gills. I'll grant that sometimes climbing out of ground effect while making the turn into or away from an increasing wind gradient also played an effect, but I've seen it in effect toomany times to make a blanket statement that there's no such thing as the downwind turn.

Northmountain, some airplanes do need more power in the turn one way, than the other. Some require rudder input in the turn, others don't. Some have large right hand turning props, others to the left, and it does make a difference...particularly when slow, heavy, and at appropriately high power settings. The difference can be noticable.

 

[midlifeflyer]

avbug said:
I can't deny what I've seen....

Precisely. Most of the explanations I've heard of why the downwind turn myth is true come down to the pilot's perception — seeing a change and acting accordingly.

As you turn the airplane from upwind to downwind, especially in a strong wind, the airplane will begin to move at a significantly different groundspeed. The increase in groundspeed causes a visual illusion of a dive, so the pilot unconsciously pulls up. Bingo! Loss of IAS.

But that's the pilot, not the airplane. I watch my students do this during early turns around a point all the time. Eventually, though, they go round and round from upwind to downwind and to upwind again without a change in airspeed simply by holding a constant pitch. I've always seen turns around a point as the most difficult of the private pilot maneuvers. This is one of a number of reasons.

But for the pilot who reacts to the diving illusion of increased GS, indeed the downwind turn is more dangerous.

The other example you gave is also accurate — the "hard" turn. The sudden full/fast deflection of the controls is momentarily changing the airplane's relationship with the airmass. It's just like when sometimes the airmass suddenly changes it's relationship with the airplane — windshear — with the same result.

In either example, it's still not the change from going upwind to downwind that's causing the problem. Like so many other things, it's the pilot.

 

[avbug]

Mark,

That might be true for a student pilot. The student pilot or new pilot might be inexperienced enough to not realize that it's him doing such a thing. How about the pilot who makes fifteen thousand such turns a year, into and away from the wind, every thirty seconds or so, and has a chance to experiment?

It's not just the pilot. Perhaps it's just that the real experts on that particular topic aren't great orators...there are those that do, and those that talk, and those that have the most hands-on experience in this area often aren't big talkers.

As I stated before, I'll allow that good science dictates that the downwind turn is a myth, but I can tell you for a fact, that it isn't. And it's not just a matter of a slight airspeed loss.

Rolling into a turn slowly and steadily at altitude in a steady wind is one thing. Rolling hard into the wind in a heavy performance limited airplane is another.

It might be the pilot. However, given that a pilot who repeates the same turn in the same airplane thousands upon thousands of times in all types of winds conditions (calm, left, right, gusting, whatever), one might be given to believe that the pilot has some appreciation for what he or she is doing. If the same airplane is loaded the same way, and turned the same way by the same pilot in varying conditions, and different results are found, perhaps it's the conditions, and not the pilot...dontjathink?

 

[bigD]

Good to see you back, Avbug! Hope things are well with you.

As a science man, I know the "fatal downwind turn" is a myth, however I'm not going to just wave my hand and discount any pilot that has seen otherwise in actual practice. So there must be a logical explanation for what some pilots are seeing. For example Avbug, are you SURE that the wind you're dealing with in the instances where you see the airspeed fluctuations is constant? No shear at all?

 

[avbug]

Sometimes yes, sometimes no....but no, I'm not sure. In fact, in some cases, most definitely it was due in part to a shearing effect. I would attribute that in many cases to climbing from ground effect into an increasing gradient: certainly a quick pull up coupled with a rapid turn can cause this. Also, the types of turns I'm talking about are turns that most people won't ever see or do in their flying (specifically the ag turn).

Every ag turn is started with a pullup for obstacles and then a turn downwind, followed by a turn back into the wind to reduce the turn: the object being to get turned around as fast as possible and to complete the turn one wingspan farther upwind for the next pass.

Many would be quick to lable what I'm saying as junk science (and they'd be right), but I'd be interested to see what they'd have to say after a morning of back-and-forth with a little wind. There really is no such thing as a down-wind turn. Except.

I'm not back yet...just stealing some unauthorized access on a terminal that doesn't belong to me.

As a sidenote, I have recently observed that there are 24 hours in a day rather than the 18 I thought were available. Being internet-free does wonderful things to one's schedulebook.

 

[bugchaser]

Guys, I have been watching this discussion and can't help but put my two cents in. What Avbug is saying is absolutely true. I have seen this thousands of times and there is a difference between a downwind and upwind turns. I have observed this many times when flying a field upwind/downwind. When pulling out of the field going into the wind, the airplane will climb like a rocket. Often when loaded light it is possible to pull up almost verticle right out of the field. One the other end of the field, you will bust your *** if you even think about this kind of pullup. What is the difference in these two pullups/turns. The only difference is in the wind direction. I have thought about this many many times and believe that some of the explanation lies in the fact that often there may be a wind gradient at low levels. What I mean is that the wind speed increases as the airplane moves from just above the ground up through 200-400 ft. Also many times the wind is blocked by trees, etc. Anyway, the effect is that at low levels, the downwind turn is much more difficult to make and the airplane does not perform nearly as well. As Avbug stated, these kinds of things are much more evident when you spend lots of time flying around heavily loaded and right on the edge of a stall. You get very sensitive to small changes in the way the airplane behaves. I know many of you may still disagree and give examples from the "book", but until you spend a year or three down in the dirt, don't discount this as a myth.

 

[100LL... Again!]

I think that wind gradient and surface friction would explain this.
Even a 10 knot difference would yield a noticeable result - both upwind and downwind. Unles you can independently verify that there is no 'layering' of wind speeds, the science must hold fast.

As the aircraft climbs and turns, there is a very slight shearing effect from the increase in tailwind component from turning downwind.

Therefore, the theoretical physics of this question hold true, yet it does not explain the observed phenomenon due to the lack of inclusion of this additional factor.

bugchaser-
I notice you referred to pulling up into a turn. This correlates with the slight change in wind component. I'm guessing that you rapidly transit a few hundred feet of altitude.

This explains why no one sees this effect during turns around a point or steep turns.

See there? Everyone was right after all.

 

[avbug]

Several hundred feet? Thems some mighty big trees, thar.

75' to 150' is closer. The idea is to get turned around and headed the other way in the absolute minimum time possible. Time spent in the turn is time spent not spraying; that means time wasted and profit lost.

A properly executed ag turn resembles an aerobatic maneuver more than something you might picture a student pilot doing.

 

[100LL... Again!]

Still enough to account for a few knots of wind shift, which is enough to be noticeable.

 

[bugchaser]

I think the idea of the wind gradient makes a lot of sense. 100LL, I think you better explained what I was trying to get across.

Avbug, personally I like to pull up high during turns. That is just my style of flying and it fits the airplane that I fly. It is very common to be up 200-300 ft during a turn. You are right about not wanting to waste time during the turn. Don't worry I'm not wasting any time out there. Are you doing any ag flying this season? Just curious if you had a seat this year.

 

[avbug]

No crop work (yet), but probably taking a leave to do SEAT work in the drom again.

 

[VNugget]

I don't see what the big mystery is, without even having to bother with wind gradient. If the downwind turn (assuming you were tracking straight in the crosswind) was such a myth, wouldn't we all be taking off downwind half the time depending on our mood?

 

[JimNtexas]

How about turning in the jetstream? If you turn a 360 over a solid overcast at night in smooth air, but with high winds present, will you notice the airspeed changing wrt to the wind?

 

[SPBRIAN]

I enjoy science to make sense of things, and seeing a performance increase because of the wind doesn't make sense. But I will admit that there are things that science will not explain, and I have limited ammounts of flight time to have seen thise phenomenon.

Bugchaser I think that the reason that you notice the difference is quiet possibly the fact that you are trying to set up runs across the ground, and paying attention to the ground when turning with the wind of course the plane will not "perform" as well because of the increased groundspeed, so you will appear to be moving much faster while climbing at the same rate?

And on the phenomenon side, at our local airport is a fairly well known aerobatic pilot with many many many hours of low level airshow flying. When they are practicing he will swear up and down that when doing hard pulls to vertical he will get a few hundred more feet when he does the pull while going downwind. I should go bug him for an explanation... Time to put on the investigator hat...

-Brian

Glad to see ya AVBUG, hope the season is going well and everybody is staying safe.

 

[CLCAP]

Mountain wave:

Rising air - airplane tries to maintain altitude - speed increeses - Falling air - airplane tries to maintain altitude - speed decreases.

 

[VNugget]

I have heard the statement many times "Wind direction and/or speed has no effect on IAS". True. But, the rate of direction change or speed change does affect IAS. 'splain this one to me: If changing wind direction or velocity (at an extreme rate) has no effect on airspeed, then why have airplanes crashed when encountering microburst type windshear close to the ground. In a wind shear, you encounter a change of plus 30 or so down to minus 30 or so (just numbers) in a few seconds. Been there, done that....you WILL see an immediate IAS change, and it usually ain't good!

That's because airplanes have inertia, just like any other object in the physical world. If it's moving a certain groundspeed, it wants to stay with that groundspeed (Newton's 1st law.) So of course if we're flying a headwind and a gust blows, the airspeed will increase.

"Wind direction and/or speed has no effect on IAS" refers to steady states. That's why you can fly backwards in perfectly controlled flight on a really windy day.

 

[Timebuilder]

"Wind direction and/or speed has no effect on IAS" refers to steady states. That's why you can fly backwards in perfectly controlled flight on a really windy day.

I remember the first time I achieved a hover in a 172 on a windy day. I thought "Wow! Just like a wind tunnel!" Then the wind increased, and I started flying backwards. This was the best illustration ever that IAS was independent of either wind speed or goundspeed. A sharp turn of 180 degrees would have brought my TAS to almost zero, and I would have had an immediate loss of lift.

While the downwind turn is not a myth, it is hardly ever a factor that most pilots will need to worry about.

 

[A Squared]

A sharp turn of 180 degrees would have brought my TAS to almost zero, and I would have had an immediate loss of lift.

No, this is absolutely incorrect. If you weren't looking at the ground, you would have felt no difference nor seen any in your airspeed indicator. Granted, if you banked steeply enough, you *could* cause the plane to stall, but this would happen at the exact angle of bank at which the airplane would stall in still air. Let's say that you were flying at a TAS of 70 knots whan you were doing your hover. If you turned down wind you would accelerate to 140 knots. It seem appealing to beleive that the air just can't accelerate the airplane 140 knots so it will just drop out of the sky...nothing could be further from the truth. THe airplane is perfectly capable of accelerating to a ground speed of 140 iknots. What happens when you are doing the same 180 degree turn in still air? You are accelaerating from a groundspeed of 70 knots in one direction to 70 knots in the opposite direction, a velocity change of 140 knots, which is exactly the same velocity change that you get in a 70 knot wind.

140 knots is 140 knots, neither the air not the airplane care what the ground is doing underneath them.

 

[Timebuilder]

If I were wrong, there would be no such thing as wind shear, as the new wind direction would have no trouble accelerating a aircraft to the new airspeed.

Just as a change in wind can cause a plane to literally drop from the sky, a ligh maneuverable plane can have a radical change in the relative wind under the circumstances I descrtibed.

And it's the relative wind that keeps the plane flying.

 

[Timebuilder]

It would undergo the exact same change in relative wind as a plane turning in still air.

Not so.

In still air, a turning airplane has a nearly constant relative wind.

In my example, the the airplane is a "rest" for the purposes of Newton. The only reason it is flying is the relative wind. A sudden turn to the opposite heading will begin to to accelerate this resting body by means of a tail wind. Until the airplane is sufficiently acclimated to this new relative wind, it will have insufficient relative wind to support flight.

Airplanes are not capable of going from zero to 140 without the use of a steam catapult.

 

[VNugget]

Originally posted by Timebuilder

If I were wrong, there would be no such thing as wind shear, as the new wind direction would have no trouble accelerating a aircraft to the new airspeed.

Wind shear is a change in relative wind, so it has nothing to do with this topic, as we're talking about a steady wind. "Accelerate" is a very bad word to use, because it usually refers to a change in velocity due to a net force in a direction. It's best left to refer to groundspeed. Just to keep things clear, I will point out that I understand what you're saying, because a plane has inertia, and therefore wants to stay at a certain groundspeed, and therefore when a gust of headwind blows, the airspeed will increase, just as you said. (Of course, if it's not just a gust but the wind stays with its new speed/direction, the greater drag will cause the plane to gradually decrease its airspeed to its orginal value... and decelerate to a lower groundspeed.)

Anyway, back to the 70kt hover... we are dealing with a steady wind. There is no reason (if there is, give us one) that the plane would behave any different in the 180 degree turn with a groundspeed of 0 or 70. Let me emphasize that we are ignoring the ground, and NOT trying to keep any particular ground track... we are just turning in the air.

As A^2 said, the net work done on the airplane is still the same, regardless of whether we went from 70 to -70 or 0 to -140; and centripetal acceleration will be the same with any given bank angle. The turn will take the same amount of time to complete. Everything will be the same.

And it's the relative wind that keeps the plane flying.

Completely true.

a change in wind can cause a plane to literally drop from the sky,

Completely true.

a ligh maneuverable plane can have a radical change in the relative wind under the circumstances I descrtibed

It would undergo the exact same change in relative wind as a plane turning in still air.



When thinking about all of this, keep in mind that Newtonian physics has nothing to do with motion, but acceleration. Any pre-existing motion is just the baseline from which further changes in motion (by defintion, acceleration) takes place. Imagine that you want to fly a R/C plane inside a cargo plane at FL350. Would it behave any differently? Of course not; any forces (and the subsequent accelerations) on it would change the baseline velocity, but to the R/C plane, it feels no different. Come to think of it, you don't have to do this thought experiment at all, because it's already happening: we are on the surface of a planet that is spinning at god-knows-what-angular velocity around itself, and at god-knows-what velocity around the sun, which is spinning at god-knows-what velocity around the core of our galaxy, ad infinitum. Does it make any difference to us? No. In fact, we didn't have the slightest clue for the first several thousand years of our civilization.

You are flying a real plane in a mass of air, which just happens not to have walls, unlike the cargo plane.

 

[VNugget]

Whoops, trivial correction... angular velocity does matter, but it's so low in this case that it's insignificant... most of the time. Note that the Earth is wider at the equator.

 

[SkyWestCRJPilot]

Here's one to think about. You're flying on a 180 degree heading. The winds aloft according to your GPS or FMS say from 270 degrees at 10 knots. You notice at your 1 O'clock position a hot air balloon about 5 miles away. Are you on a collision course? (Here's a hint, it doesn't matter how fast your airplane is flying.)

Here's another: Say your flying a freight plane full of 500 ducks. Total weight of the ducks (no cages just birds walking around) is 2500 lbs. Your airplane's takeoff weight is 100,000 lbs. Once you're in the air all of the ducks start flapping their wings and are flying around the cabin. Does your airplane now weigh 97,500 lbs. or does it still weigh 100,000 lbs?