sudden speed loss during climb...

[SmolaTheMedevacGuy]

The airport was in an area of a very strong high, with the QNH of 1040 hPa, and the temperature @ minus 12 degrees centigrade. There was a light headwind, and no turbulence. We were flying an ATR-72 about 1 tonne short of the MTOW. During the initial climb-out, I noticed that the ROC was 2000-ish and the pitch attitude in excess 15 degrees - rather unusual for the ATR-72, which, especially when heavy, isn't a good climber. Then, at ~1000 ft the speed abruptly dropped from ~130kts to ~105kts - about 10kt below V2. My first thought was power loss, but the engine gauges showed only slight (but clearly visible) variations of power. Then, with speed regained, the a/c settled at V2+10 and the ROC was about 1500ft/min (more ATR-like) and the engines showed no abnormalities.

I wonder what actually happend... My guess is a strong temperature inversion, checked the SAT some 1000ft higher, and it was some 5-10 (don't remember exactly) degrees higher than on the ground... But then again, the speed change was very abrupt (almost windshear-like, though there was no turbulence). Never experienced it before. Besides, the ATR's engines are flat-rated up to much higher temperatures, so it should make no difference if it's -5 or -15 degrees - the power output should be the same. My other suspects are a brief power loss, or some kind of windshear.

Thoughts, anyone??

[BEagle]

Lee wave activity?

Somewhere in Austria, Eastern Europe or Southern Germany, were you?

[SmolaTheMedevacGuy]

Central Europe - the area around the airport was totally flat - no single mountain within 100 miles :D

[BEagle]

Has all the hallmarks of lee activity; perhaps the temperature/lapse rate and wind direction were all just right to generate standing waves at a natural frequency. You might have taken off into gently rising air, then at 1000 ft entered gently decending air?

A similar, but much more unpleasant encounter happened to me many years ago in a heavy 4 jet leaving Canada. It was almost uncontrollable and there was little warning (nothing forecast). We lost a lot of radios and nav kit; fortunately most of it recovered after re-racking in the avionics bay.

[JABBARA]

No doubt, it was a windshear. Windshear is not necessarily to be in turbulent air. I believe it was the kind of windshear, which may be induced due to inversion. The inversion may prevent two different air masses from mixing each other. These two layers of air masses may have completely different wind speed and directions.

In your example, most likely you were getting stronger and stronger headwind as you were climbing. This wind speed change may be as much as 8 knots per each 100-foot climbing. So due to inertia of the airplane mass, the apparent (not actual) performance of the airplane increased for some seconds. However just after crossing inversion layer, the headwind ceased or -maybe even worse- turned out to be a tailwind. Again, due to inertia, the apparent performance of the airplane degraded.

I believe that's all.

Jabbara

[SmolaTheMedevacGuy]

Might be... Never thought you can get waves on a flatland. I suspect it must have something to do with that siberian high. Never seen a QNH of 1043 hPa before... Do you have any idea where I could find some additional info on the subject?


Greetings
Smola

[RatherBeFlying]

Don't know what you have over there, but in North America, the 850 Mb charts bracketing your flight time would show the wind observations at low altitude (1100 to 1500 metres or so depending on pressure height). All you have to do is get your friendly local met. office to dig them up for you:O

[swh]

checked the SAT some 1000ft higher, and it was some 5-10 (don't remember exactly) degrees higher than on the ground

Sounds like an inversion layer to me. Which are common in areas of high pressure.

The windshear would be a result of the inversion layer.

Can I ask why you didnt fly the normal pitch attitude for climb if the pitch attitude was unusual ?

:ok:

[Loose rivets]

Confirm that the aircraft was on autopilot, and also that your SOPs prohibit the uses of locking onto a climb rate.

I'm assuming that the PF reacted to the attitude fairly briskly. I would feel decidedly uncomfortable sitting in a 72 at that weight, while significantly below V2. I have to say, that my first thought would have had more to do with getting rid of that pitch attitude following a sudden drop in airspeed.

You may find that the slight fluctuations in the engine parameters were due to the vertical component of the airflow at the engine intakes. But I don't know.

A drop of 25 knots, is in itself, not really very significant met-wise, things like this are going to happen quite often over the years.

[SmolaTheMedevacGuy]

Just to straighten this out. We were climbing @ V2+10 and the resultant pitch was somewhat higher than expected, but still within the limits. It just felt as if the plane was lighter. We were NOT locking to the VS, the PF's reaction was immediate and the speed was regained quickly.

Thanks for the answers

[Approaching Minimums]

Might be... Never thought you can get waves on a flatland. I suspect it must have something to do with that siberian high. Never seen a QNH of 1043 hPa before... Do you have any idea where I could find some additional info on the subject?

On my recent met classes I was told that standing waves could exist many hundred nautical miles away from the mountains. So it is not uncommon at all to get waves over the flat land. According to the UK AIC (144/1997 pink160 30th December) usually waves extend about 50-100nm to the downwind side of mountains but those has been witnessed extendind as far as 250nm from the Pennines or even 500nm from the Andes!

Best Regards,
AM

[Kit d'Rection KG]

Having flown a Discus (glider) in glorious wave over Cambridgeshire, I'll agree that wave persists a long way downwind of mountains, given the right conditions..

Two thoughts...

Having recognised that things looked unusual, did the PF give any thought to flying pitch and power, accepting the speed, and thus avoiding the low speed event?

Second, recollections are notoriously inaccurate. You could ask the company safety people to let you see the FDR trace, assuming it was captured by FOQA or similar... It may tell a much less interesting story...

[SmolaTheMedevacGuy]

Yep, that's what I want to do. Just have to wait 2 or 3 days more until they (the FOQA) download it. As for the wave... I did fly a glider in a mountain wave (last october acually), and know what it is. However, the departure airport was really far from ANY mountains, especially to the east (that's wehere the winds came from), so I think it must have been an inversion windshear after all

[Bre901]

You don't even need any mountain, wave can happen over flatland in some circumstances : "Cloud streets have also been observed to force wind from a right angle up and over, undulating downwind especially when there is another cloud street at some strategic distance between wave crests to enhance the wave shape and intensity downstream." (here (http://www.northwestsoaring.com/WaveSoaring.html)).
Coudn't find a picture online but there is one in "Cross Country Soaring" by Helmut Reichman (p56 in the French edition).This kind of wave do happens from time to time in western Europe and I've been lucky enough to experiment it once (1989).

However, this requires a cloud-street, i.e. a convective layer from ground to the inversion layer and a stable one on top. Given the temperature and QNH, I guess the bottom layer was stable too, so I'd go for the windshear at the inversion layer

[Miserlou]

15 degrees nose up is about 3-5 degrees higher than I'd expect for a heavy -72; that would give you a decrease in speed for a start. This may be because the flight directors were being followed too slavishly (not looking through them) and making a height/speed trade.

Bearing in mind that after acceleration altitude you want to be increasing to white bug (7-8 degrees pitch attitude)then the attitude was definitely too great. When you then reached the inversion layer and the wind changed you were in an unfortunate flight attitude.

[error_401]

I'd bet on an Inversion.

Happened to mee to the opposite side. Descending with an ATR 42 on autopilot maintaining 180 kt as instructed by ATC.

Temp above a cloud layer at about 2500 ft AGL 12 deg positive. diving into the layer the speed came up to 205 kt ! and the temp dropped to -6 deg celsius within not even 100 ft altitude difference. the cloude layer also looked smooth from above.

We were already on the LOC so I noticed a change in wind correction of about 3 - 5 deg.

In climb I'v seen that thing happen on a Piper Aztec ride for fun :cool:. expecting the speed and performance loss when climbing out (got smart and asked met office ...) once it started getting lighter i increased the speed to 140 mph. coming out of the cloud layer I hit the inversion. temp up some 10 deg C, speed down by about 10 mph.

If you know there is an inversion basic pitch mode could be best practice.

I only climb at VS in cruise for a level change and keep my hand on the glareshield to be sure not to forget setting it back to IAS once it reaches climb speed.

Miserlou,

on cold days with -10 or more you can get quite impressive climb figures out of an ATR :E and his 72 wass 1 ton short of MTOM :}