Microburst?

[PeteK]

What do you make of this? I took the Archer up tonight - I had to drop something off at the clubhouse, and I couldn't resist taking it for a quick spin. Anyway, winds were from the South at 3 knots when I took off. Clouds were at 12,000, but you could see that the air was unstable tonight just by the look of the nasty overcast way up there and a bunch of lenticular clouds. This is in Greeley Colorado, by the way.

Made one nice landing, wind had shifted - now it was from the West at 3 knots. On the upwind leg, at about 600' AGL (and at Vy) I hit some heavy moderate turbulence, the stall warning began blaring continuously, and the VSI showed -1200 FPM even at best climb pitch. Naturally I needed to nose down a bit to maintain speed but I was descending fast. By the time I was able to climb again I was about 150' AGL and, uh, a bit concerned.

The turbulence again struck as I tried a gentle turn to crosswind and I saw 30 knot airspeed fluctuations. Halfway down downwind I came out of it and I was only able to get to about 500AGL by that time. I let the dude on takeoff roll know what had happened and he aborted.

Needless to say that was my last landing of the evening - when I did land winds were now 15G22 from the East. Before I left the airport winds had shifted to being from the Southwest (!) at about 10 knots.

So, that plane in that config can climb out at Vy at 600 FPM - since I was descending at 1200, was this a microburst or just wind shear? Have you guys experienced this before (especially so close to the ground), and if so would you classify my experience as a "close call"? Any tips on anything I could better next time. Scared the hell out of me but at least it wasn't just 2 boring landings.

 

[Brother Francis]

Do you recall what the temperature was and whether it changed?

Personally, I prefer boring landings.

 

[PeteK]

Temp was about 74F and I believe it dropped just a few degrees. Although there were no T Storms forecasted or seen, it did rain for about 3 minutes when we were putting the plane away. I guess the most significant change I saw was the changing wind. The ceiling remained very high, but had the look of boiling water, or kind of a popcorn appearance. Very strange looking - I do know we have a low pressure area right over us right now, but no low clouds and no frontal activity. The mountains had many, many small lenticulars just beyond them.

 

[Brother Francis]

Without knowing all the facts, SWAG it was likely a dry microburst.

Rain from high clouds evaporates in low relative humidity as it falls, cooling the surrounding air, forming a descending column of air. Little or no rain due to evaporation. Temerature drop afterward is a marker.

Avoidance clues are cumulus (steep lapse rate), virga (evaporating rain), and high temp and dewpoint at the surface with a large spread (high moisture content, but low relative humidity).

150 AGL and 1200 down? That's too much fun for me.

 

[PeteK]

Yes there was some virga in the area, above the airport looked kind of nice though. I spoke to a friend of mine about it and he said that when you see thos weird, curdled and bubbly looking high cumulus that they are evil. I figured that since no virga was nearby and the cloud bases were so high I'd be fine, especially since at first there was no wind. Funny thing is, once I landed I was laughing about the whole thing, thinking how I had gotten waayyyyyy more experience than 0.3 hrs out of this little adventure.

 

[avbug]

If your max descent rate was 1,200 fpm, it was a very mild microburst. Most likely nothing more than a downdraft. (Granted, a microburst is a downdraft, but generally a very nasty one, exceeding 6,000 fpm in many cases).

I've had two encounters with microbursts in which ground contact was an issue, both in light airplanes, and in each case, the VSI was pegged. In the first case, I was unable to maintain better than a 3,000 fpm rate of descent, and recovered at 150' AGL. The descent began under similiar meteorological conditions to what you describe, with a high flat base, no virga, and only precip visible around the hills. I was in a Cessna 206, and the descent commenced at about 2,500 AGL. Everybody was pinned against the ceiling for the initial part of the descent, and it occured rapidly enough to pull my hands from the controls, and pin my feet off the rudder pedals, against the underside of the panel. There was no turbulence or any warning prior to the descent.

Control input at the time was full power, and a rotation to best rate (Vy), followed by a rotation to best angle (Vx), followed by a rotation to minimum airspeed, without any noticable decrease in vertical speed. Vertical speed exceeded the instrument's capapability, and the VSI remained pegged.

The second event occured in a Cessna 172 just after departure in a rough area, and resulted in a descent almost to ground contact. The fact that ground contact did not occur indicated that the severity was nowhere near as bad as the first encounter, but it was enough to cause a great deal of concern. Again, there was very little warning, and conditions did not suggest that this was a liklihood, prior to encountering the downdraft

A third event occured which did not affect me directly, but was very graphic. I departed PHX in a Seneca II, northeast bound. I was VFR. No clouds were in sight. Just after starting the takeoff roll, I heard a windshear alert on tower frequency. I was given an early turn, and very quickly after that heard two 737's go around. I looked back to see just west of the field, a ring of dust rising up that was perhaps about 500' high. It was spreading rapidly, and clearly showed a microburst. Aircraft on final had little or no warning, and reported going missed in IMC.

The view was shocking. It looked as though a very large explosion had taken place, with a shock wave spreading slowly, outward and upward. A solid wall of dust in a perfect ring surrounded the center of the burst, and was shortly several thousand feet high. I don't recall what the wind values peaked to on the surface, but I seem to remember them being close to 70 knots or better, with shears in the 40+ range. I was away from the area affected, and only saw it, but it was very impressive, and at the time, I'd never seen such a thing with no visible clouds or precip.

 

[Clownpilot]

Goody

>>I've had two encounters with microbursts in which ground contact was an issue, both in light airplanes, and in each case, the VSI was pegged. In the first case, I was unable to maintain better than a 3,000 fpm rate of descent, and recovered at 150' AGL. >>

How many trees and 2X4's were you ingesting into your engines during these events. Also, I'm sure you had several fires occuring onboard simultaneously. I guess it's good there weren't any buildings over 15 stories in the area during your heroic brushes with disaster. Please take your delusions elsewhere. As Jack Nicholson said in the movie 'As Good as it Gets', "Peddle crazy somewhere else, we're all full up here".

 

[PeteK]

Why do you question what Avbug says? While I was unsure if what I ran into classified as a minor microburst, I sure as hell know that 6000FPM descending air within one is entirely possible if not common.

What Avbug says about being 150AGL makes perfect sense as well, since the air mass from the burst will disperse in all directions as the air mass reaches the ground.

Since I'm pretty sure my encounter didn't involve some strange, evil, one-of-a-kind event, one can obviously deduce that more severe instances of this phenomenon occur, so what exact part of Avbug's story seems to be BS? I have in fact heard several interesting stories from pilots around here about this phenomenon which were quite similar to Avbug's.

 

[brucek]

Hi Pete,

I'm in Boulder, just down the road from you (although I wasn't up today). The conditions were very gusty around here at 6-7PM local, there are a number of cold fronts stacked up to the north that either have come through today or may tomorrow, you may have been caught in one of those ( resultant microburst). Yes, flat skies and rather ominous stratus, with imbedded lenticulars.

Here's 2 successive METARs, for 0000Z and 0010Z, which may describe your issue today:

2002/09/21 23:55
KGXY 212355Z AUTO 14004KT 10SM SCT070 17/02 A3023 RMK AO1 10180 20120 402700030 52034

2002/09/22 00:55
KGXY 220055Z AUTO 03019G26KT 10SM BKN060 BKN075 12/01 A3031 RMK AO1

Note the difference in wind over the course of an hour here.

In any event, I'm not looking forward to an encounter such as your's. I have had the tower at Jeffco (my home airport) call a microburst in the vicinity, I've gone off for 30 minutes or so, and listened closely to ATIS before returning to the area. You have an AWOS up there, so it's much slower than a controller maybe at defining an event such as this.

Bruce.
BJC, Jeffco, CO

 

[TDTURBO]

Pete,

ClownPilot believes Avbug embellishes a bit and somehow feels he may not be a pilot.

I think he just likes ripping on him.

I for one do not hold his sentiments, Avbug's great advice and experience is second to none. It's amazing he is alive with all he has been through. It must mean he is either one hell of a pilot or the luckiest guy on earth.

Regardless of what Clown says about Avbug, you will find no other individual more apt to handle complex questions ranging from FAR interpretation, proceedures, to airframe and power plant concerns. He has graciously and selflessly graced this forum with his vast knowledge and wisdom, and for that we should all feel grateful.


I think Avbug is cool, if he doesn't know the answer to a question, it usually means there isn't one

 

[Clownpilot]

You're right

>>I think Avbug is cool, if he doesn't no the answer to a question, it usually means there isn't one [/B][/QUOTE]>>

Frankly, I think Avbug is extremely interested in aviation, probably works in a field very close to airplanes, may have a private license and a very minimal amount of time and....................................................................

laughs his ass off that any of you believe his truly fantastic stories of heroism, courage, selflessness, and betrayal of gravity and basic physics.

NONE of his stories ring true about actual piloting, he has a good knowledge of some aspects of aviation, and frankly I think the fact that he's duping you really shouldn't make you idolize him.

Ya no (sic)???? ............ most of us spell 'know' K,N,O,W.

 

[metrodriver]

Clown, if I were you I wouldn't say anything about Avbug's spelling. I have seen some of your collegues at AA, as a matter of fact same brand of military service write things like 'web sight' and 'there' when it should have said 'their'. Quite confusing when you read something like this: company ABC keeps there airplanes their and you should check there websight.
And regarding Avbugs story: better read it again and learn something from it. The list of microburst accidents is quite long.....

 

[Clownpilot]

Add yourself to the list...... colleague

>>

Clown, if I were you I wouldn't say anything about Avbug's spelling. I have seen some of your collegues at AA, >>

my collegues? what are those?



>>And regarding Avbugs story: better read it again and learn something from it. The list of microburst accidents is quite long.....

>>

Sorry buddy, I've got predictive windshear radar and I only fly into airports that have windshear warning systems. But thanks anyway. Email me again when you learn to spell. LOLOLOLOLOL

 

[skyslug]

Clown, you might want to turn the volume down on that pilot-god attitude of yours just a bit. It's currently rather loud and annoying!

 

[Clownpilot]

Lighten up Francis

Clown, you might want to turn the volume down on that pilot-god attitude of yours just a bit. It's currently rather loud and annoying!

Don't EVER talk to God like that again.

 

[Brother Francis]

moi???

You're not trying to drag me into this, are you?.

Ain't gonna work.