Fluctuations in Indicated Airspeed

Guest Discussion
1

Okay, weather guys, help me out here. What’s up with the fluctuations in indicated airspeeds as I’m flying in level cruise over flat land?

Admittedly, it’s warm out over Arizona and California, and there was a bit of light chop, but nothing too dramatic. Fluctuations were +/- 5 knots, occasionally 10 knots, and occurred several times over 30 minutes while I was paying attention to Garmin density altitude and wind screen. Wind direction was varying a bit, but only 10 degrees or so. My confusion came about when I noticed that the Garmin TAS was different than the airspeed dial, even though I had carefully matched the indicated airspeed with the calibrated airspeed entry. So I started noticing that the IAS was varying. No apparent change in engine RPM, MP, or other indicator. And only a slight change in wind noise, but I might have started hearing things.

So, since I didn’t expect airspeed changes, I wondered what was happening… Is there a weather explanation?

Cheers
Rick

2

If you are using the autopilot in altitude hold an updraft will cause a speed increase as the autopilot pitches down to maintain altitude, a downdraft will cause a speed decrease as the autopilot pitches up.

3

Rick,
I am glad another ‘glider guider’ responded with the info I was going to offer. And I agree with Marty that it most probably was mountain wave that caused your airspeed variations. Also, mountain wave is completely laminar flow, so it will be extremely smooth. I have literally parked in wave in a glider and it was as smooth a ride as sitting in my living room. As a matter of fact, after I went to about 14,000 feet, I turned around to head downwind to the gliderport and cruised at redline of 160kts all the way back with a groundspeed of about 200kts. without the slightest bump. The area associated with wave that is turbulent is the rotor that is under the peaks of the wave at lower altitudes. Sometimes they are marked by scraggly looking cotton ball type roundish clouds. It can get REAL rough in there.

Anyway, your autopilot would have pitched the nose down a bit to maintain altitude in the ‘up’ part of the wave and then gradually transitioned to a nose up mode in the down side of the wave. Depending on the characteristics of the terrain causing the wave and the angle of the wind, the wavelengths could have been from a mile or so to 10-15 miles. Higher altitude also tends to widen the wavelengths. This wavelength would have given you an airspeed change every 15-20 seconds to as long as several minutes, depending on how long it took to traverse the waves.

4

Rick,

The changes you are seeing in airspeed do sound real. In other words, it doesn’t sound like a blocked pitot tube or static port.

Don’t know exactly where you were geographically. Were you on the lee side of a mountain range? Say within 100 nm? Very few clouds (no cumulus or vertically developed clouds)? What was your altitude?

I agree with Greg that what you were experiencing was a gravity wave (mountain wave). (We cover this on the CA CPPP.) By hearing a change outside the aircraft and coupled with the IAS change you were probably caught in a wave.

The typical scenario is very few clouds (if any) present, a temperature inversion (or very stable atmosphere) and a “strong” wind perpendicular to the mountain range. You can get rotor clouds to form if there is enough moisture (not usually the case in AZ).

It doesn’t take huge mountains to cause this and the wave may continue beyond 100 nm from the mountain range. If you end up flying parallel to the mountain range you can get yourself into huge trouble. And as Greg mentioned, it is usually coupled with very calm air. Almost an eerie kind of calm.

BTW, don’t rely on the Garmin to give you any accurate stuff. Unless it is coupled with an air data computer it is pretty useless.

5

Art’s right. The effect is especially noticeable in a mountain wave, but if the ride was smooth and there were no noticeable changes in attitude, I’d look for dirt (or bugs) in the pitot tube or static ports. If nothing is noticable, I’d make a dash to the A and P.

Marty

6

Its ironic that what the autopilot does is exactly wrong for fuel economy (although it is what ATC wants to see). The autopilot maintains altitude, which means it slows down in the downdrafts, and speeds up in the updrafts. When you fly a glider through those conditions you slow down in the updrafts to linger and gain the free lift, and dive to get through downdrafts in a hurry. In an updraft you want to be at Vy (best lift/drag ratio), and in a serious downdraft you want to be at Va. In cross-country gliding they call it porpoising, and unfortunately it would raise eyebrows at ATC.

-Curt

7

My personal observation in the underpowerded 20 is that downdrafts can really cost up to ten knots for a few minutes if on ALT hold. However, when the downdraft ceases, the speed won’t recover completely but remain 5 knots low, so in order to get them back you have to apply more power for a minute, then go back to normal cruise.

Phil
N199CD

8

Could have been mountain wave effects, but not very dramatic ones. Was in eastern Arizona and flying across southern California along the border where the “mountains” are only 5-8,000 feet at their best. Weather was generally stable air with high cirrus, no cumulus anywhere. Wind was generally a quartering tailwind in Arizona and crosswind in southern California. However, there are lots of small ridges that could set up some interesting effects.

Thanks for the suggestions. Having flown through Colorado recently, I was tempted to learn much more about mountain flying. Clearly there is lots more to know…

Cheers
Rick

9

The typical scenario is very few clouds (if any) present, a temperature inversion (or very stable atmosphere) and a “strong” wind perpendicular to the mountain range. You can get rotor clouds to form if there is enough moisture (not usually the case in AZ).<

Scott,
I am sure you know that lenticular clouds are frequently associated with mountain wave. Who knows how frequently?! But, they’re presence is a sure bet there is wave action going on. Also, mountain wave does not need airflow perpendicular to the range. It is most pronounced at that angle, but it can be present when the wind is at less of an angle. As a matter of fact, since most ridgelines and mountain ranges are not straight lines, the oncoming wind is usually not exactly perpendicular.

Weather was generally stable air with high cirrus, no cumulus anywhere. Wind was generally a quartering tailwind in Arizona and crosswind in southern California. However, there are lots of small ridges that could set up some interesting effects.<

Stable air is the best for formation of mountain wave. Convective activity will disturb the mountain wave. However, I once climbed above cloudbase in a glider using convective wave. This was caused by the same airflow dynamics that cause mountain waves, deflection of air over an obstacle.

By the way, the biggest ‘mountain’ in North Carolina is about 6,600’ MSL and I have been in wave 100 miles downwind of that peak up to 15,000’ MSL. I climbed several times and then nosed over to penetrate farther up into the wave system and found the wavelengths to be about 20 miles wide 100 miles downwind and about 10 miles wide 50 miles downwind. As I moved up in the system, the lift and sink increased in strength indicating the wave got stronger in amplitude as well as a higher frequency.

10

Yep, sounds like mountain waves. The mountains don’t have to be large, however, the winds aloft (crossing the ridge) must be somewhat strong (> 25 kts). And as you noticed, they don’t have to be large waves either. Imagine what a “real” wave could do to you!

11

In reply to:

My personal observation in the underpowerded 20 is that downdrafts can really cost up to ten knots for a few minutes if on ALT hold. However, when the downdraft ceases, the speed won’t recover completely but remain 5 knots low, so in order to get them back you have to apply more power for a minute, then go back to normal cruise.

Phil,
Speaking of observations…I experienced a mountain wave here on the east coast of the US (under IFR) where I slowly lost 800 ft of altitude (happy I wasn’t flying at the MEA). I was in my Turbo Arrow at full power just to maintain altitude (I could not climb). Got a block altitude from ATC and this lasted approximately 10-15 mintues. The problem was that this airway was parallel to the ridges which kept me in the wave a lot longer than if I had made a turn to the right or left to get out of the wave.

12

In reply to:

I am sure you know that lenticular clouds are frequently associated with mountain wave…Also, mountain wave does not need airflow perpendicular to the range. It is most pronounced at that angle, but it can be present when the wind is at less of an angle.

Greg,

With sufficient moisture present, clouds can form (including standing altocumulus lenticularus) at the top of the rotors. Mountain waves occur anytime you get a strong flow over the ridge. And yes, you are correct, it does not have to be at 90°. I was just trying to paint the “typical” scenario, not necessarily every scenario.

In reply to:

Stable air is the best for formation of mountain wave. Convective activity will disturb the mountain wave.

By definition a mountain wave must have a non-convective, stable environment. When you get orographic lifting as air flows up and over a mountain during an unstable or conditionally unstable atmosphere it can cause rising air to continue to rise. But I would say that this is a different process and not classified as a “mountain wave.” In other words, I wouldn’t expect to feel any wave effects “downstream” of this rising air.

13

Scott,
At the risk of boring other forum readers, I’d like to make a few further comments so that anyone interested doesn’t have a mistaken impression of wave systems. Besides, I rarely get to have a discussion with someone about obviously knows something about it :-).

With sufficient moisture present, clouds can form (including standing altocumulus lenticularus) at the top of the rotors. Mountain waves occur anytime you get a strong flow over the ridge. <

Agree with this, but only because the lenticular clouds happen to be at the peak of the waves and the lower level rotor turbulence underlying the wave system happens to be underneath the wave peaks as well. To have the correct understanding of a mountain wave and the clouds associated with it, I didn’t think it appropriate to say the lenticular clouds were located at the top of the rotor, they are actually interspersed at various altitudes in each of the waves. A fine point, but worth mentioning.

By definition a mountain wave must have a non-convective, stable environment. <

Well, I would only like to add that the stable non-convective environment must be present only at the altitudes where the wave system is occurring. I have ridden strong thermals many times up into wave systems. As a matter of fact, that is about the only way I could get to them because I was too cheap to pay for a high tow behind the Pawnee. Usually, you could not get to the wave until you get to about 5,000’MSL, and that is anything but a stable air mass around here. That is a great day for soaring! I know that sometimes, the mountain wave system was not organized and laminar until you were at or near the inversion, where the air became isothermic or stable. But, there was clearly ovelap between the convective altitudes and wave altitudes. In many cases, I am confident the wave system could have been organized at lower altitudes if the inversion had been at a lower altitude. This demonstrates your point about wave systems occurring only in stable air, but it left the impression that if a guy took off and climbed through a lot of convective activity, he would not have to worry about wave at all.

When you get orographic lifting as air flows up and over a mountain during an unstable or conditionally unstable atmosphere it can cause rising air to continue to rise. But I would say that this is a different process and not classified as a “mountain wave.” In other words, I wouldn’t expect to feel any wave effects “downstream” of this rising air. <

As you probably already know, orographic lifting is a function of surface winds and obstacles to their movement. For mountain wave to develop, these surface winds must be supported by mid and upper level winds from more or less the same direction. I would agree that if they are not supported by the mid & upper level winds, you would not expect a wave system to form with any significant tertiary waves.

14

I’ve experienced a mountain wave in an SR22 on the east coast (sort of). The conditions were strong winds, 30 - 45 kts, from the west. The winds were associated with the back side of a front in mid spring. I was in western North Carolina (or was it South Carolina) and I hit one full cycle.

Each part (up or down) lasted about a minute or two It was very noticeable. First the nose slowly pitched up (the auto-pilot pointed the nose up to hold altitude) and we lost around 10 - 15 knots. We felt like we were dragging the tail similar to mushing even though the IAS was 135 +/-.

Then, very smoothly the situation reversed. The nose pitched down (the A/P pointed the nose down to hold altitude) and the airspeed gained the 10 - 15 that we previously lost plus another 10 or so. Felt like we were in sustained descent or like we were surfing a big wave. Other than the ‘funny’ feelings and attitudes, it was very comfortable and smooth. The key was recognizing the events.

The mountain tops were between 5,000’ and 7,000’ in that general area, but I don’t know the elevation of the ridgeline I was lee of. I was probably at 9,000’ MSL.

The A/P handled it perfectly and didn’t care that we were in a wave. Better than I would have done. The A/P just reacts, In VMC, which it was, I would have been confused until I figured out the cause. My wife was a little spooked, even after I 'splained the situation. But she handled it better than the front we crossed 15 minutes later.

Marty

15

Thanks Greg for the clarifications!