Question for all Ag pilots, Past or Present

[windsock]

Hey guys I don’t usually post on this forum, but I have a question for the Ag pilots here, and I’m taking it to you because you’re constantly exposed to the environment pertaining to my question.

What it is, is that I’ve observed a discussion amongst some pilots who are not Ag pilots, who are debating the merits about the dangers of the downwind turn being a myth.

Their assertion is from their knowledge of what’s written in books, or on online aeronautical engineer/aerodynamics websites, which is that the downwind turn is not dangerous, especially when you get to the downwind part, it’s all a myth.

Their assertion is there’s no performance loss on the final downwind leg, especially when crop dusting, low and slow, that the danger lies in the actual radius turn close to the ground. They say approaching the completion of the downwind radius turn, the pilot subconsciously pulls back on the stick, because their closeness to ground would give the false “illusion” of too fast speed, and that’s the culprit. They say it has nothing to do with flying into a downwind airmass at low levels.

It’s also their assertion that upon takeoff from a runway in a crosswind, and turning downwind too soon is a problem, and not the downwind turn itself. If there’s a problem, it’s because of the ground speed illusion-the tendency to pull back on the stick, or because the aircraft hasn’t gained enough of a positive rate of climb and wasn’t established enough yet to be able to turn, and not the low altitude 90 degree turn into the downwind airmass.

I’m inclined to listen to the ones who actually do this kind of flying rather than the armchair quarterbacks reading popular books and mags. You guys do this kind of flying day in and day out thousands of times a year, what say you about the downwind turn being a killer? Thanks for your responses.

[Rarely Dble Amber]

A level turn, and let me emphasise LEVEL turn, over flat terrain in any wind conditions produces zero performance loss during the downwind portion of the turn. In a Level turn you dont lose airspeed as you come into the tailwind portion of the turn and you dont gain airspeed as you come into the headwind portion of the turn. Ag ops are usually either climbing or descending turns though and that changes things. If you pull out of the field with a 5 kt wind from the left, and you turn right.. you will most likely be climbing and turning into a wind shear situation,(5kts on the ground may mean 10 to 15 kts windspeed 200 feet up) and lose airspeed. This is one of the many times that turning downwind causes an actuall loss in airspeed. Terrain also plays a big part. If there is terrain of any sort that needs to be outclimbed, you turn the thing into wind and take advantage of the higher ANGLE of climb, if not a higher RATE of climb. That has a bit to do with the illusion of performance in strong winds as you mention. No point climbing into wind if there is a mountain in that particular direction, but if there are mountains in every direction, keep it into wind. The theory of downwind turns may all be very well, but you will find that most ag guys will instinctivley turn into wind when slow and heavy.
A level turn downwind = no ACTUALL performance lost. A climbing turn downwind at low level = most likely a loss of airspeed. A descending turn downwind = probable increase in airspeed. But of course you are picking up speed anyway in that case.

There you go, thats my opinion as an ag pilot. :ok:

[windsock]

Thanks for the insight, I just want to make sure I understand what you're saying. So the danger is when ascending or descending downwind, the wind shear's downward force is a factor that will cause loss of performance. And I bet the strength of the wind shear factor is unpredictable until your actually in it, am I right?
Maybe that's why some consider it a myth, they might have gotten away with it several times with not much happening, until that one time when there's incredible unforseen windshear, and then bad things happen.
But what about making a level sharp 90 degree turn into the wind near the ground, when flying crosswind or downwind, won't you see a performance increase? And what about making quick level radius turns close to the ground as in dusting rows in a field, don't you see an airspeed loss on the downwind section?

[Rarely Dble Amber]

So the danger is when ascending or descending downwind, the wind shear's downward force is a factor that will cause loss of performance.
Im talking wind shear, thats a variation in wind strength as you climb or descend. Not airmass that is moving down or up as in a thermal or a microburst. So at ground level the wind speed is say 10 kts, at 100 feet it might be as much as 20 kts, give or take 5, depends on the terrain and tree cover really.
until that one time when there's incredible unforseen windshear, and then bad things happen.
Doesn't have to all that incredible really. It isnt uncommon to be just a few kts above stall speed at times. If you pull up and turn downwind into a wind gradient situation that takes away 10 knots, things can get very soggy feeling. You dont even have to turn at all to experience the wind gradient/ shear effect, but its pretty rare to pull up and just fly straight ahead :rolleyes:
But what about making a level sharp 90 degree turn into the wind near the ground, when flying crosswind or downwind, won't you see a performance increase? And what about making quick level radius turns close to the ground as in dusting rows in a field, don't you see an airspeed loss on the downwind section?
Many will argue this, but no you dont. The airplane doesnt care where you are or what you are doing in relation to the ground. It simply responds to the "parcel" of air it is in. Sharp turn downwind or upwind will have no effect at all, just the lost performance due to the turn itself. For Many reasons it may feel as if there is lost performance when turning downwind, but all else being equal, there simply isn't an actuall performance loss.
Also, think of this.. At low level, you make turns usually to place yourself in a position reletive to something on the ground. So Imagine flying a perfect orbit around a ground based object, for example, you have spotted the farmers daughter sunbaking topless. In order to maintain that perfect orbit you need to tighten up the turn going into and coming out of the downwind portion of the orbit, you also have to open it up in the upwind part of the orbit. So what is happening is that you are demanding more performance out of the airplane in the downwind portion of the orbit and less in the upwind. This is another example of the Illusion of downwind turns. And also the Illusion of farmers daughters.. they all look pretty good from 200 feet and 120 kts :D

[SNS3Guppy]

There are several aspects to the downwind turn, some myth, some real, some illusion, some not. The idea that merely turning downwind will cause an airspeed loss is myth. The idea that a turn downwind under the right circumstances can cause a performance loss is not.

Already mentioned is windshear due to surface friction and increasing gradient in the climb. This may occur due to leaving ground effect and surface friction into a stronger free airstream while climbing and turning, or may be due to obstacle-induced windshears near the surface.

An aircraft making a hard turn, especially coupled with a changing wind gradient, may also experience a performance loss in excess of what it might under other circumstances. An aircraft that can be rotated to face downwind faster than it can accelerate with the wind, will experience an airspeed loss and performance loss.

The downwind turn also leads to increased groundspeed and increased turn radius. Those operating close to the ground sometimes tend to pull harder and thus bring themselves closer to a stall by increasing their angle of attack. This is part illousion, but also very real if you happen to the be one stalling.

From an ag point of view, the airplane is typically in a reduced margin situation in the turn to begin with, meaning it's closer to a stall, operating at a higher angle of attack, and has little altitutde for recovery if something goes wrong.

The typical arguement that an aircraft in a moving airmass isn't subject to a performance loss because it's moving with the airmass is far too simplistic, and in many cases, in error...though few will admit it.

[biggles190]

I thought the reply was simply and it was part of my ag rating, a 15 knot gust on the nose causes extra lift, having a 15 knot gust up the rear in a climbing turn with high angle of attach when air speed is not a lot above the stall is not fun. Experiencing it once was enough to make me keep my speed up in those conditions.

It would also depend on the pattern you are flying (racetrack- back to back) as to weather the problem was a nature or pilot created stall. Either way, ease back and do a go-around and live to fight another day and keep the insurance companies happy (if possible) and your skin in one piece.

[BallsToTheWall]

There are those who read when they fly,there are those who feel when they fly, those who read will never feel to fly!

[windsock]

Thanks everyone for the replies, I've been away from the board for some time and was surprised to see that there were so many responses.

There are many out there that don't do this kind of flying (and never will), who will never go beyond what they read in books.

There are those who will always claim that there is no substanciation for your views about the downwind turn, and there is so much out there published by the "experts" that say the opposite, it is very hard to convince them otherwise.

But thanks for the response, it's appreciated.