Mountain Turbulence

If there's one thing a survey pilot learns about first hand more than probably any other kind of pilot its turbulence associated with mountains.  The diagram below pretty much explains it better than I can.  Note there is turbulent air on both sides of the mountain, but the windward side tends to be updrafts, whereas the leeward side tends to be downdrafts.

The only time MOST pilots would experience this phenomenon is when flying over large mountains in strong winds, simply because there is usually no reason for the flight profile to cross over hills/mountains at a height of less than 1000 ft, and it takes a larger mountain and stronger winds for the turbulence to be felt that high above the peak.

For survey pilots, often our flight profile takes us over the peak as low as 100 ft above the surface.  In fact high peaks are often when we fly the lowest.  It doesn't take a large mountain to generate severe turbulence and down-drafts that close to the terrain, and that height leaves little margin to react to the down-draft or manoeuvre out of it, so its important that we always remain very aware of wind conditions, especially on the leeward side.  It wouldn't take much for a down-draft to drive us into the hill-side when we're climbing up the leeward side of a mountain.  Climbing up the windward side can also create different problems of an illusionary nature, simply because climbing the windward side means you're flying downwind, which can decrease your climb angle and carry you into the mountain even though it looks like you should clear it - but that's a discussion for another day.

I remember I cautioned one of our pilots (who shall remain nameless) about mountain flying once.  He was not new to being a pilot, in fact has more hours then me, but completely green to low level survey.  He dismissed my words confidently saying that he would be concerned about weather conditions, but not about terrain that doesn't move.  His statement exposed his inexperience with flying close to the ground and revealed a little of his ego in general, but it wouldn't have been fruitful for me to do anything but leave it at that.

This mountain turbulence has been a reoccurring issue for us throughout our time in Newfoundland because the terrain rises sharply to almost 1500 ft ASL right out of the ocean, and even though our survey block is mostly offshore, it overlaps land by about 3-5 km, so every line we fly involves either climbing up over the shoreline hills, or descending over it.  Today was no exception.  The weather was kind of a scuzzy overcast of about 4-5000 ft, isolated rain-showers, and a strong but mainly steady wind (at least over the water).  We're at a section of the block where the terrain rises out of a valley very steeply - almost vertically for 1800 ft.  Its quite a stunning vista actually, but not so much when you're bouncing around trying to keep the airplane level and on line.  It was once of those days where you pull your seatbelt as tight as it can go.

I did not take the pictures below, but I found them from a google search on a hiking website - they are the exact area we flew over today, right down to the canyon lake (bottom photo) which we ended up flying right over.  Anyone who can guess the exact area based on the pictures gets 100 points, which are redeemable towards recognition as a Canadian traveller and bragging rights.

 

So back to the story.  As we near the end of our line the mountain ridge looms in our windscreen.  Keeping the airplane on a laser-accurate flight line doesn't take the concentration that it used to when I first started flying survey, which is good because I can devote more of my cognitive abilities to deciding how to handle the approaching mountain.  Our grid actually ends about 3 km before the mountains in the top picture begin, which means we're tasked with either 1) making our turn-around inside a 3 km area, 2) climbing above the mountains and making our turn-around above them, or 3) crashing into the side of them.  Option 3 would make for a bad day, option 1 would be possible but very risky due to requiring high bank angles close to a wall of mountains, and in turbulence even, which is exactly when you DON'T want to be banking hard, which really only leaves option 2.

So today we were left only with the option to climb over the mountains to make our turn.  Under normal conditions I could have continued a straight over climb and clear the peak by a couple hundred feet, but in today's conditions that's not much margin to leave in case we experienced a heavy down-draft.  I elect to turn left and climb parallel to the hills until we're well clear of the peak, and then begin our right turn over the ridge and follow it back to re-intercept our line descending down over the ridge.  Its not the most time-efficient turn, but its the safest action.  Sure enough as I was paralleling the ridge on my way back to the line we experienced a small but steady down-draft right at the crest of the ridge.  In level flight at higher airspeeds it didn't end up being a factor and the airplane could handle it no problem, but encountered during the climb when airspeed is slow and the airplane is already approaching the maximum climb performance, it might have been a little hairier.

Luckily we were only faced with that condition for only the last 3 lines of the flight.  These ones in particular are short lines, and making efficient turns can really cut down on unproductive flight time.  Hopefully the winds on our next flight will be a little calmer, or in a different direction and we won't have to go out of our way so much.