Downdraft over level ground?

[NimpoCub]

A little 'shamed to ask this, cuz I feel I should know, but don't... so:
Is it possible to get hit with a downdraft when flying over relatively flat ground? I've always thought downdrafts are only from cold air sliding down hillsides, or waves rolling over the tops, or windshear when at altitude. I surely know there is turbulence nearer the ground, but an actual downdraft??

We had an incident near here recently where a downdraft "was reported" and I think it was over relatively level ground, @ 500', pilot applied full power but pancaked (all walked away). Just doesn't feel/sound right to me. I fly @ 300-1000' and wonder if I should be more concerned 'bout that!

BTW, when in the mtns, I'm always aware of which way I can turn toward lower terrain, etc, but I'm kinda complacent about downdraft when over flat areas. Now I'm not so sure.

[SJ]

Heck yah... We have HUGE updrafts here over level ground too.

sj

[aktango58]

short answer: Yes.

Longer answer: convective currents can cause some strange winds. Also, a slight uplift may give you a downdraft a ways away.

Your area gets hot, but some is tree cover. The dark trees will actually not heat as much, so the bare ground will give an updraft, and the trees downdraft... lakes down and so on.

The more heat the more convective current.

Also, a wind shear overhead can cause strange winds.

[AviatorPA11]

Sounds like a Microburst maybe?

[8GCBC]

Any wind that has the slightest amount of negative vertical (-) travel is a "downdraft". Which can occur at ground level. The Wind does not need to be at a 180 degree negative (-) vertical vector to produce "downdrafts"

Some contributing factors:

1) Orographic slopes (leeward)
2) Dying thunderstorms, microbursts
3) Gusty winds can create windshears with negative vertical (-) travel

Think about vertical (+/-) wind as a "crosswind". It can have a component (that is less than the velocity) up or down. But, it will hurt if not understood, just like a horizontal crosswind .

If you depart Lanai Island, Hawaii on a tradewind day you will know this! The slope is negative (wind direction) and the wind follows the negative slope, a downdraft at ground level!

[Patrol Guy]

I just finished reading this book. Jim Dulin in "Contact Flying" says to ride them out because you can't hit the ground. It might move you sideways, but just before the ground it goes some direction, and you should go that way too?

When in the mountains and you find yourself in one. he says to point your nose down and go full throttle to get out of a down draft fast and to save your altitude. When in a up draft slow down and point up, to stay in it longer.

[8GCBC]

I just finished reading this book. Jim Dulin in "Contact Flying" says to ride them out because you can't hit the ground. It might move you sideways, but just before the ground it goes some direction, and you should go that way too?

Consideration of terrain height is paramount. Riding a downdraft to low may be an issue, depending on what is in front, side and/or in back of your aircraft.

We lost a CFI on Lanai, because the aircraft simply could not climb, becoming to low and turning. He struck a pineapple field with full power at Vy. NTSB report = downdraft, and pilot error not seeing it (taking off).

[8GCBC]

Also, wing tip vortices ON the surface can rip a wing OFF! I.e. 777. They sound like a tornado!

[Bill Ingerson]

Got caught in a wind shear, down draft or microburst ? All I can say is that your eyes could not see the dash clearly, just a blur. My CFI gave the plane full power nose down and turned away from it and we got things back in control. I should pay him more money, hope he doesn't read this. We were at 1500 ft and lost about 500 feet.


Bill

[8GCBC]

During frontal season, I have seen 40 knot negative windshears from 0 to 180 degrees on the SURFACE. Amazing and scary...not navigable in a light aircraft! If flying at 50kts IAS you would hit 10kts IAS in about 10 seconds and drop like a rock. Watch GPS SOG/IAS when close to the surface to get an idea of wind component/shear in bad weather. I have seen some bad shears!

[AntiCub]

The airport I used to fly ultralights from has a 4 lane highway across one end. When landing on sunny days I quickly learned to expect an updraft crossing the highway, followed by a down draft on short final.

The biggest danger with a downdraft at low level, is that as you come out of it you'll have a tailwind as the sinking air spreads out away from the downdraft along the ground. At low speed, like on short final, it can cause a stall.

[M1]

Logan: For sure, you can find updrafts and down drafts over all types of terrain. In your area on a hot summer day, there are often lots of updrafts or thermals and corresponding downdrafts. I often use the updrafts to aid in a climb, or in level flight you can speed up. Avoiding the downdrafts is really the key.

[NimpoCub]

OK, thanks y'all, but is there any way to precict/expect shear, or "thermal downdrafts" while I'm flying along fat/dumb/happy? I hate the thought of having to stay above 1000' all the time!
I understand that cool (light colored) surface next to a dark surface will be suspect, but just flying along...??
Thanx for the thoughts.

[8GCBC]

A few items:

1) Windspeed over obstructions
2) Laps rate
3) Fronts (cold and warm moist air mixing)
4) Thunderstorms (ending)
5) Orographic terrain
6) Vortices (wingtip, helicopter)
7) Jet blast (My friends's C150 flipped by a DC-10)

[M1]

Logan: you need to become a glider pilot!

[bigrock]

I had to look up orographic. Check out how this tiny island effects the wind.
http://en.wikipedia.org/wiki/Fileave_cloud.jpg

[bigrock]

http://upload.wikimedia.org/wikipedi...Wave_cloud.jpg

[King Brown]

At two locations I've had significant sink at roughly 100 feet altitude nearing approaches runways, one in Hamilton, Ontario, and in Stellarton, Nova Scotia. They were well known to local pilots who said they were caused by spring-fed, colder ground.

[benflyn]

I just finished reading this book. Jim Dulin in "Contact Flying" says to ride them out because you can't hit the ground. It might move you sideways, but just before the ground it goes some direction, and you should go that way too?
.

Care a full;
That air might go sideways at the ground, but you and your bird might be like the bug that meats the windshield!!

[cubdriver2]

I've lived in this 3500sf 1860's house for 31 years, the third year we lived here on a calm moist May day I was taking the studded tires off of the wife's car and I heard a strange noise and a few seconds later half the roof of my house was standing up on end and flying through the air coming in my direction and I dived under the back of the car as the debri pelted me in the garage and some of it was found 2 miles away, it only lasted for about 10 seconds and then was dead calm again. I later learned on the local news that places 60 miles away had had the same thing happen to them, when I looked on a map it was a straight line of destruction over 90 miles long. I hope that I am never flying on the day that that happens again.

Glenn

[8GCBC]

http://upload.wikimedia.org/wikipedi...Wave_cloud.jpg

Classic picture of a mountain wave. The moisture is condensing on the crest and evaporating on the trough of each wave. This shows the washboard effect of the clouds. The washboard effect is not always seen! The waves could be invisible!

Waves that pulse on the horizontal axis (i.e. ocean swells) are easy to understand and see ( i.e. the photograph of the island [above]). Waves can/will also pulse on the verticle axis. I.e. wind coming around a cliff, mountain or even a tall hangar. These waves can NOT be defined with a trough/crest scenario. Instead of trough/crest, you would say + pulse and then - pulse for the energy movement.

[M1]

Yeah sure; ride it out, maybe you would want to put in a downwind turn at the same time. Don't believe everything you read.

You can do a lot for yourself by really observing the environment and weather each time you go flying. Early in the morning and evening there is often not enough heating to make the thermals work, so you will have nice calm flight, once there is enough heat and the correct air mass, you can expect thermals. Remember that where air goes up, its going to come back down somewhere else. Glider pilots will often refer to a "house thermal", or thermals that seem to form in the same area on a regular basis. There is a place I fly that has what I think is a thermal that forms regularly right where you would turn final. I have seen this many times, and it was a site of a fatal accident several years ago, maybe this was a factor, who knows for sure except that a bit of extra speed and coordinated flying is your friend when down low.

[8GCBC]

Yeah sure; ride it out, maybe you would want to put in a downwind turn at the same time. Don't believe everything you read.

You can do a lot for yourself by really observing the environment ......

I agree!

An ounce of prevention is worth more than a ton of bent metal! If you are riding a downdraft as mentioned, big mistakes have been made in flight planning! It is like going over the falls at the Pipeline (total screw up, Brah)!

--8GCBC

[NimpoCub]

Damn.
I used to really enjoy flying low, now I'm not so sure.
Damn.

[Little_Cub]

Years ago my father got caught in downdraft(s).. (no indication of anything abnormal) he was taken below_tree_line not once but continually for 10 minutes over and over again.. each time it would let him out (full throttle) at the surface and he would attempt to climb and get away (different directions). He wasn't convinced it would have let him out in time if he was over the trees.

[texmex]

My two cents worth NimpoCub. "flying over relatively flat ground" you will not encounter a downdraft that will put you into the ground. Yes thermals are concentrated rising air and therefore the air has to be descending else where. But it's not as concentrated and it's been cooled. Convective heat thermals can fairly belt any aircraft but the air is rising.

Enjoy your low flying.

Caveat: No mountains within 30 miles or so depending on wind velocity, no big black clouds overhead.
Any flying below 200ft without a thorough survey is suicidal.
Below 1500ft and poor visibility is unsafe with all the bloody mobile phone towers around these days. (Oh for the GOD's)

[skywagon8a]

Below 1500ft and poor visibility is unsafe with all the bloody mobile phone towers around these days.

...AND Windmills. Those are scary, particularly when you pop over a hill and there is one right in front of you spinning away.

[arcticairalaska]

What you are calling down drafts over flat ground are wind shears due different air masses mixing. I do a lot of low level bird surveys at 125" and below and have seen the wind switch direction 180 degrees and velocities of 20kts in 200 meters. If you are paying attention to winds on the water or leaf angle of the trees you will have warning and can be prepared. It will still be a rough ride but not unsafe.

[Speedo]

What you are calling down drafts over flat ground are wind shears due different air masses mixing. I do a lot of low level bird surveys at 125" and below and have seen the wind switch direction 180 degrees and velocities of 20kts in 200 meters. If you are paying attention to winds on the water or leaf angle of the trees you will have warning and can be prepared. It will still be a rough ride but not unsafe.

Experiencing low level wind shear has been a new, but frequent, experience for me. It's especially fun when encountered on short final with a heavy plane, going into a runway without a working windsock. I've learned to look for stove or boiler exhaust, but those darn Toyo's are so efficient that they don't produce much of a plume, so you have to look really closely.

[Patrol Guy]

I am no expert, but I do have 13,000 hours flying between 200 and 500 feet and I am still here. Now I am beginning to wonder why??

[8GCBC]

I am no expert, but I do have 13,000 hours flying between 200 and 500 feet and I am still here. Now I am beginning to wonder why??

You have common sense!

I flew SARs at 200' to 500' in all kinds of Wx. I.e. Gales, night, IFR. I check weather a lot during my flying, it is free and easy!

[Dave Calkins]

Care a full;
That air might go sideways at the ground, but you and your bird might be like the bug that meats the windshield!!

We can all understand bugs and windshields, having seen the effect many times.

Knowing that uneven heating/cooling of the atmosphere, and also "mechanic" turbulence from terrain features provide the updraft/downdraft and turbulence............KNOWING it's there...consider the truth of how YOUR A/C is the bug.

[Frodo]

As far as micro bursts, they teach airline pilots to throttle up and pull back to the stick shaker (stall warning). This is because the relative wind is coming from above and you need to get back to a positive AOA. Consider a cub pilot flying at 60kts through a 60kt down draft in a micro burst (not uncommon). The relative wind is now coming from 45 degrees above the horizon, and until he gets his nose up that high, he has a negative AOA which does not help in terrain avoidance!!