My Experience with Mountain Waves

By Austin Kemink

Early in my instructing career I was teaching at the University of Alaska in Anchorage when I had my first experience with a true mountain wave.  Why do I call it a “true” mountain wave?  Simply put, our aircraft didn't have sufficient power to maintain altitude or airspeed.  How could I get into that situation?  Aren't there forecasts, altitudes, and procedures to avoid this scenario?  The answer is yes to all of the above but the interesting part is that we followed all of them. 

For those of you who have had the privilege of flying in the great North you are aware of the extensive mountainous terrain that surrounds Anchorage.  Rare is the day when there isn’t icing near the hills and it’s quite normal for the rocks to be surrounded by clouds, particularly in the winter months.

My student was near the end of his instrument training and had only to finish a couple of cross countries before he could take his check ride.  We waited out the weather and finally had a VFR day to complete his long cross country.  We planned from Anchorage (PANC) to Talkeetna (PTKA) to Fairbanks (PAAF) and back again on V436.  This is a particularly beautiful flight that spends a good portion of its time over the Alaska range, the home of Mt. Denali.  Our MEA for the trip was 10,000 and we were flying a C172S, with a six pack and a Bendix GPS. 

No booing!  Remember, this is a University with a limited budget so we had to make do with Cessnas. 

The weather was forecast VMC for the entire day and there were no AIRMET's or SIGMET's to speak of so we felt comfortable in our Go decision.  The first hour and a half of our flight went as planned and we even had the opportunity to shoot a genuine NDB approach into Talkeetna!  I love Alaska…  After that approach we were back on our way to Fairbanks. 

We got about 80NM north of Talkeetna and were just starting to reach some of the higher peaks along our route when the weather started to deteriorate.  Our forward visibility dropped to IMC but we could still see the ground below us and we were on an IFR flight plan so we presumed no problem and continued as planned. 

At this point I was watching the instruments a little more intently and I noticed that my student had lost a couple hundred feet of altitude and about 10kts airspeed.   This was very unusual for this particular student as he was one of my sharper pilots.  He corrected and we determined we must have hit a small down draft.  Approximately 20 miles later, I noticed we were losing altitude again but this time at a greater rate.  Between the time I checked his airspeed and asked him what he was doing, we were at around 80kts and still losing altitude! 

It was at this point I took the flight controls added full power and pitched for Vx.  At this point we had zero forward visibility and limited visibility below us.  With full power and Vx we were still losing 500fpm. 

I called ATC to inform them of our situation and requested a lower altitude.  This was an interesting call because at first ATC informed me of the MEA for our airway (as if I didn’t know!) and that they would not be able to give me lower.

I restated our problem and requested a 180 back to Talkeetna.  During these first two calls I had turned the plane around anyway and established us direct to Talkeetna.  I wasn’t going to wait for the controller to figure out what was going on… his sticking to procedure would put us in the rocks, eventually.

I made one more call to inform them that I could see the ground below us and I was direct to Talkeetna.  By now I had decided that if they still didn't grant what we needed I would declare and emergency.  It wasn’t needed in this case because upon hearing that I could see the ground they cleared us as requested.  In the end we lost 1,500ft in the span of only a few minutes.  We figured the downdraft we encountered was approximately 1500fpm to 2000fpm based on our normal performance as we climbed through the downdraft but still lost altitude.

What is interesting about this experience is that we followed the rules and still got caught.  We checked the weather, the advisories, winds aloft, filed on a victor airway, and flew at an MEA and still ended up in a wave out of our control. 

What did we take away from this? 

One: pay attention to early warning signs.  We did encounter a smaller downdraft earlier in our flight. Was it enough to make us turn around?  No, but we should have treated it as a warning sign. A downdraft like that while over the smaller hills should have made us aware that something bigger could be coming.

Two: the changing weather.  None of our reports called for IMC but there it was right in front of us.  Hey we were on an IFR flight plan so nothing to worry about, right?  Wrong!  Something had changed and we needed to consider that part.  Why did it become IMC?  Can anybody guess what we were in that would limit our visibility like that?  Take a look at this picture and make some comments if you think you know what it was.  It should have been clear to me that a wave was out there, thinking back.  This is a dead giveaway:

Three: an MEA is not a guaranty of anything weather related.  In mountainous terrain we have 2000 ft of clearance on the terrain but mountain waves can happen as high as 5000 ft above a mountain top.  That 2,000 feet clearance at the MEA is a lot smaller than you think.

Although I did not have time to check our ground speed I know that it had changed drastically.  That would have been another clue had I been paying attention to it.

This was an eye opening experience and although we were in a Cessna, a Cirrus (turbo or not) would not have fared much better.  I think what was most nerve-wracking about this experience was the idea of something like this happening under true IMC conditions where VMC is not just a 180 and a few minutes away. 

Something to keep in mind when planning a flight over mountainous terrain!  Stay safe out there!    

Austin

Flight Instructor, NW Region

www.theflightacademy.com