Unexplained speed variations

[Tom Davis]

Yesterday, I was flying from Asheville, NC (KAVL) to Washington, NC (KOCW). I was using my KAP-140 autopilot. I was flying at 7000 feet, the throttle was set at 21.1 inches, the prop was set at 2400 RPM, and the fuel flow was set at about 8.4 gph. The air was reasonably smooth. My indicated air speed slowly dropped from about 115 knots to about 93 knots. The true airspeed decreased significantly too as did the ground speed. The vertical speed indicator stayed at or around 0 ft/minute I expect minor indicated (and true) air speed variations, but this seems excessive. The air speed picked up to what I normally see, but I have never noticed this much variation.

If it makes a difference, the most significant decrease came while I was flying over mountains.

If my ride was bumpy, I would understand it, but that was not the case today.

Any comments will be appreciated.

Thanks.

[Colin]

I would put my dollars on mountain wave action. You were in a sink. Your autopilot held the altitude but had to exchange some speed to do it.

[Chris B]

Hi Tom -

Sounds like mountain waves in otherwise smooth air.

Chris

[blsewardjr]

I experienced the same thing Sunday flying from Roanoke VA (KROA) NE up the Valley at 9000. KIAS varied from 100 to 140 KIAS with altitude the same and smooth ride. Like the other commentators I think it was mountain waves.

[rwtucker]

You are fortunate that this was your experience with a mountain wave. I fly in an area where they can be quite "upsetting" if you don't give them 2,000 feet vertical clearance. What were the winds showing and in what relation to the mountains?

[dafzero]

I was flying in the opposite direction yesterday from Asheville KAVL to Gwinnett KLZU outside of Atlanta, and we saw some pretty significant variations in the wind speed and direction that all faded once we were clear of the mountains. We didn't have your experience Tom but did bounce around for a while. We were at 6500. Nice coming back today though!

[CFIDave]

Mountain waves for sure! Any time you have winds aloft of more than 30 knots that are perpendicular to mountain ridges, they can be expected. Usually they happen the day after a frontal system comes through and strong winds are coming from the NW.

I've experienced them over the Alleghenies in PA so bad that in a DA40 I was forced to reduce throttle to avoid exceeding Vne during updrafts, and had the plane "hanging on the prop during downdrafts" almost into a stall. The GFC700 autopilot was doing a beautiful job of maintaining cruise altitude (since I was flying on an IFR clearance at the time).

The next time this happens while flying IFR I'll request a "block altitude" clearance -- then I'll be able to ride the updrafts and downdrafts without varying airspeed so much.

One other thing: You can sometimes see these mountain waves by the appearance of wave clouds, perfectly aligned and spaced to match the the mountain ridges that generate each of them, potentially also extending downwind. Just stay clear of rotors (more of an issue when flying in the western US).

[rwtucker]

My experiences flying in the east are limited but mountain waves have the potential to be life threatening out west where we have real mountains . Oddly, even though I fly mostly over the mountain northwest, I have had the worst experiences with mountain waves a few miles north of Phoenix and in another area near St. George, Utah. (BTW: If you didn't get a chance to land at the the old KSGU airport before they dug it up, you missed a classic.)

Mountain waves are not always the relatively smooth "up & down" kind that you can ride or in which you would want to leave your autopilot engaged. They can be deadly and sometimes the best course of action is an immediate 180. I have been caught in two that were serious, one of which almost resulted in an inversion. Also, mountain waves are entirely different than the rapid wind-induced up and down drafts and local turbulence you will experience when flying in the mountains, below the level of mountain wave effect. You have to respect both. A 10,000 hour Navy fighter pilot lost his life in a C182 near McCall because he was flying 500 feet AGL through the mountains and found that he could not out-climb a precipitous 3,000 FPM downdraft (even though most last only a second or two).

If I am in a mountain wave zone and the winds are > 20 kts. my personal minimums are 2,000 feet AGL above the highest point of the mountain I am flying over. Also, remember your basic training and approach mountains at a 45 degree angle, especially if you are unfamiliar. In the mountains, visualize the winds at all times and have a constantly evolving plan B at hand.

[AlexanderBingham]

...out west where we have real mountains ...

I live in CO and yes... this sounds like mountain waves to me. I try to follow some simple personal minimums: 1500ft AGL and no more than 25kts winds at the peaks, plus get in early and out early. Personally I avoid Monarch Pass as the DA40 can make that one a bronco ride!

[Steve]

Westbound crossing at El Paso a few years ago we had a 55 knot headwind! It definitely was a "bronkin' buc". Had a 2500 foot AGL cushion, used almost a third of it. 10 miles further west, smooth (still had a 45 knot headwind though...).

Steve