Strange phenomena

The other day while returning from Fredrickton NB (great trip, no hassle from customs and got to see a lot of cool stuff), I was westbound out of Bangor ME at 8000 ft on the backside of a front. Weather was clear with some broken cumulus, very strong winds out of the north. While straight and level on Trutrak altitude hold, my airspeed slowly began to climb, from 160 IAS to 180 IAS. The altimeter and VSI never budged. After about 1 minute or so, airspeed went down to about 140 IAS. THat cycle began to smoothly and gradually repeat with a period of of about 3 minutes and repeated for about 15 minutes and slowly smoothed out. After close scrutiny, I did detect the nose rise about two degrees nose up above level during decelerations and drop 2 degrees nose low during accel up to 180. Speed changes were accompanied by appropriate groundspeed changes. Again, absolutely no changes in altitude or VSI. My theory was this gentle roller coaster ride was due to the Trutrak climbing and descending to maintain the barometric altitude as I tracked across closely packed isobars?? Does that seem plausible

Later on a second leg well west of the front, and in very calm air, the Trutrak held altitude and A/S prefectly.

Anyone else ever experience this?? When it first happened I thought I magically gained 20 Hp - or that my pitot system was some how failing - until I slowed backdown and then decelled. It was actually kinda cool. Wish the 180 knot cruise had stuck!!

Any possibility you were flying in and out of mountain waves? With the autopilot on altitude hold, the aircraft would have worked to stay level in altitude but would have had to lower its nose in lift and raise it in sink to do so. Your airspeed indicator would have displayed a "record" of these activities.

Lee...

gereed75 said:

...Anyone else ever experience this??...

Click to expand...

Yes, many times. Mountain wave (which can occur over flat terrain) is one factor that can make flight testing so difficult.

If you don't need to hold altitude, you can gain lots of speed by slowing down in the up part and speeding up in the down part (the exact opposite of what your autopilot is doing).

Funny you should mention it - Louise and I spent last Friday watching the exact same phenomenon as we crossed the Basin and Range region of the US west. Westerly winds set up the Mountain Waves as Larry said, and we enjoyed the ride when it got fast! Larry and other champion soaring pilots know how to use it to maximum advantage.

Paul

Yep, That must have been it exactly. Strong winds were out of the northwest and they had just passed over the Vermont mountains.

It was cool and now I know why. Is there any flying topic that escapes the expertise of this board?? Thanks guys.

Did you note what your GPS altitude was reporting?

Gary
Glad to hear you had a good visit to Fredericton. There is a group of RVer's here. If you have some time and you make another visit give me a shout. We enjoy meeting RVer's from elsewhere.

Joe Hine
Fredericton, N.B.
RV4 C-FYTQ

Mountain wave... you bet!

Flying transports, I have seen the autothrottle go from idle to near full power to maintain an airspeed while the autopilot is getting a workout to maintain altitude. The best waves in the US are along the eastern front of the Rockies. Great ride!

Surfing

What Smokey said.

It is even more fun going north-south in when the mountain wave sets up. Find the lifting air and stay in it by adjust course slightly east or west. I have stayed in it for 100 miles. At its best - or worst, I have seen so much lift that I had to throttle back to avoid busting the Class B ceiling. Nose was pointed down, going fast, and still climbing! Actually felt like surfing.

I was westbound into Albuquerque one day in a 172 with about 40 knots over the mountains in my face, and was in the wave going from full power Vy climb (dropping 1000+ per minute) to redline airspeed idle power and climbing 500-600 per minute, trying to maintain assigned altitude. I finally gave up, called center and advised "unable to maintain stable altitude due to wave activity", they gave me a block from 8000 to 14000 and I just put it in a Vy climb and left it there. I finally crossed the ridgeline just east of ABQ at about 11500 at a 45 degree angle for a good downwind escape option.

Mountain waves can be VERY interesting.

Sounds like wave. Since your altitude hold was on, you're not going to see changes in altitude - wen you're in the lift (or sink!) and clear of the rotor areas, it tends to be very smooth, not strong kicks like thermals. The autopilot will have plenty of time to react, and that's what's causing your speed changes.

Sometimes wave is weak, sometimes it's strong. I can't access youtube at work, but there are some great wave videos by Sebastian Kawa. The tow through the rotor is a bit rough, but once he's in the wave, it's smooth sail(plane)ing.

Now if you can earn a lennie pin in a RV (without the use of power!), I'll be seriously impressed.

TODR