Originally Posted by
visibility3miles
T
The comment in avionics’s post is correct. You should be at least 2,000 feet above a mountain pass at high altitude, as a strong downdraft can make that margin disappear quickly.
Flying at high altitudes is akin to being a glider pilot. You need to pay attention to which way the wind blows and where there will be downdrafts versus updrafts, and plan your path with this in mind.
I cannot agree more. Archer with 3 POB at 9000ft is barely climbing. I fly at an airfield surrounded with 9500ft peaks for the last 30ish years and have plenty of hours in gliders mountain flying and have seen quite some scary moments. My "record" was approx. 3000ft altitude loss in less than a minute - hitting the wrong side of a tremendous rotor under an atmospheric wave, when trying to catch the upside of rotor to climb into the wave. No regular powered piston engine airplane can outclimb that.
Flying in the mountains requires specific knowledge, as there are significant vertical air movements present, limited options for outlanding, traps in the form of "one way street", and sudden local changes of weather, (cloud formation...) that can turn deadly.
In above case, I will risk a prediction that nothing significant was wrong with the engine, but airplane simply could not climb faster than terrain it was approaching and just ran out of air under the wings while attempting to fly thru a col - a giant Venturi tube. A colleague of mine did same thing a few years ago in the same type of airplane - with no survivors. He mostly flew in flat land.
When flying in the mountains, the first thing is to know very well what the wind is doing - not only general wind forecast, but keeping an eye on actual local winds aloft - they can differ significantly from general wind on top, as the air curls around peaks and ridges and turn along ridges, etc. It is a good practice to watch what water is doing when flowing over and between the rocks in a stream/river. I did this for hours on the nearby bridge. It gives you an idea where an updraft might be and which side of the valley to avoid. When it is windy, you often need to change a flight path laterally just for a few hundred meters and you will climb comfortably instead of hanging on all available power to maintain the altitude. Also, think of thermals - they can create tremendous uplifts, but quite often, there is similar downdraft nearby.
Flying thru a col/mountain pass is a special task of mountain flying: not much place to turn around, not exactly sure what is on the other side (you sure the area on the other side is not full of clouds?) and the worst is an invisible local headwind, that might become obvious only in the last seconds before the pass, as Venturi effect comes into working. When this happens, your ground speed decreases and surrounding air starts to settle and it is getting worse when you are getting closer, so crossing the col can become a very scary experience. Did that (too) many times, especially on competition flying, where competition fever might lower safety limits. (that was the reason I stopped flying glider competitions, as I did too many things that were obviously stupid - in the hindsight) Even some of the best pilots were caught in col crossings, like Klaus Holighaus - a world-class glider pilot that died when trying to cross another pass just a little too short and encountered downdraft of about 2m/sec in the last moments.
https://www.sust.admin.ch/inhalte/AV-berichte/1590.pdf Hope I will never run into such event - he was most respected pilot from all of us and yet...
My recipe for mountain pass / col crossing is NEVER reduce speed but instead INCREASE it significantly when aiming for a crossing (means you need sufficient altitude to come to the pass from above, not trying to trundle thru in a straight and level flight, or, God forbid, in a climb). Extra speed should provide energy to use if downdraft is encountered, allowing for U turn without (significant) loss of altitude and not relying on engine power- but must be executed before place (and options) becomes too narrow. If you cannot keep the col crossing on "glide path"+safety margin with this increased speed (and sink rate), do not go closer. Do practice (not too) sharp U turns, becouse if you try it first time when panic settles in, there is a good chance for pulling too much and with added turbulence, etc. AOA might go beyond critical and you will spin into the ground violently.
If you intend to fly in the mountains, I would suggest first take a flight with a twin seat glider on a reasonably windy day with experienced pilot, if there is an option for that - it will be an eye-opener.