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When flying in Mountain Lee Wave we often reach high altitudes. There are a number of challenges that are associated with high altitude flight.
As the pressure is lower at high altitude the volume of gas increases. This is why your drink bottle may swell up and spray fluid when opened. The gas trapped inside is at a higher pressure than the outside, so when the bottle is opened the gas rushes out. It may entrain some of the fluid from the bottle and spray it onto the inside of the canopy. This is why I prefer to use a drink bladder. You can burp the air out of a bladder so it only has liquid inside. The liquid doesn't expand, so no problem.
The gas inside your body has similar issues. It will expand as you climb, potentially causing some discomfort. Burping and farting are not uncommon. If gas is trapped inside the intestine it can become painful and cause real problems. If you have a bad filling in a tooth with air trapped inside the filling the air will expand and may put pressure on the nerve.
The Eustachian tube is a small canal that runs between your nose and your middle ear. It helps equalize the pressure in your middle ear. If there is any restriction here this can become problematic. Do not fly if you have a cold or blocked nose or similar. The pain is nasty and you can get ear infections from the goop being forced along the tube or even burst an eardrum. It's all bad.
As you climb this pressure may build. It can normally be relieved with the Valsalva maneuver. Gently blowing your nose with your mouth closed while pinching your nostrils.
You sinuses are filled with air. As you climb these should equalize with the ambient pressure. If you are young your body will have some elasticity and flex that will help keep up with the pressure change.
The real problem comes as you descend. A rapid descent can exceed the rate your sinuses can adapt to the pressure change. Once the pain starts between your eyes/behind your nose, you know you have a problem. All you can do is reduce the descent to give time for your sinuses to adapt. The first time I overdid a descent it hurt for a week. That's also the last time I descended that rapidly. I generally look for a descent rate of no more than 1,000ft/min sustained. I have found that is good for me. A young pilot may be comfortable with a higher rate.
As we climb the air density is less, so our true airspeed is higher. The VNE of a glider is related to true airspeed. In most gliders flight manual there will be a table showing the reduction of indicated airspeed VNE with altitude. This table should be your primary reference.
Many modern glide computers have an option to show true airspeed. This can be handy to keep track of the VNE reduction too. I get my computer to display true airspeed and I stay under the true airspeed VNE by a healthy margin when at high altitude.
As the air pressure reduces, so does the partial pressure of oxygen available. At some point we need supplemental oxygen to keep enough oxygen available to our body for it to function normally. In particular we need oxygen for our brain to work. Lack of oxygen causes hypoxia.
You will need to know how your oxygen system works, how to operate it, and the limitations of your system. Most glider these days are using the Mountain High system. With a nasal canula these systems are good up to the mid twenty thousand foot area. With a properly fitting mask, good up to the mid thirty thousand foot area.
Your useful time of consciousness starts to drop dramatically above twenty thousand feet, dropping to less than ten minutes. Above thirty thousand it starts being measured in seconds.
Consider that you need to be able to detect an equipment failure, decide upon corrective action, execute your decision, and either fix the problem or descend to a viable altitude. If we assume that we'll be OK below ten thousand feet, and that we can sustain a thousand foot a minute descent safely, getting from twenty thousand feet to ten thousand feet is going to take ten minutes. That is all the margin we have.
Above twenty thousand feet if you have an equipment failure, e.g. the canula comes out of the fitting or the tube gets pinched where it folds, you can expect to pass out before you can descend to a viable altitude. When you pass out you'll likely be in a high speed descent and could easily exceed VNE and suffer structural failure as a result.
This is why I prefer to stay below twenty thousand feet. It gives you a better safety margin.
Temperature drops 2-3 degrees C per thousand feet. You can get into seriously cold air even when the temperature on the ground is comfortable. On a wave flight it is not unusual to be flying around with outside air temperatures of -10 or -20.
These low temperatures can be a problem for the aircraft and the pilot. For the pilot these temperatures can cause hypothermia.
The canopy becomes cold and will shrink. This may cause leaks, particularly with two seat gliders with a large single canopy. The cold canopy will cause the warm moist air exhaled by the pilots to condense and freeze on the inside of the canopy, potentially causing visibility to become a problem. Some gliders have vents blowing air along the inside of the canopy to reduce this problem (DG1000 is one I know of), others will need to keep the front vent open (Duos and most singles) to demist the canopy and avoid icing.
Differential expansion and contraction between different materials used in gliders can cause other problems. Controls may become stiff as control rods shrink more than the fuselage and wings.
Grease and oils used to lubricate linkages may become less viscous (thicker). This may also make the controls more difficult to operate.
As we are likely to be well below the freezing point of water, all the water in (and on) the glider can freeze. If there is moisture trapped in the control seals this is likely to freeze and can cause the controls to lock. This may be from washing the glider before flight, having the glider rained on before flight, even having moisture under the seals from wetsanding the control surfaces or wings recently. I have heard several reports of ailerons freezing in this manner.
I have had the airbrakes freeze shut because of a shower of rain passing through while the glider was sitting on the grid. The water froze in the gaps around the brake caps when I climbed high. I discovered the problem when I tried to open the airbrakes to stay below airspace and couldn't get them open. Most disconserting. Luckily they thawed out on the descent.
The Duo Discus has a water ballast tank in the tail that uses open vent holes to adjust the water level in the tank. Below these holes is the rudder horn, which moves in a slot in the fin with little clearance. I have seen this get wet from water coming out the vent holes, then freeze in flight causing the rudder to lock.
Water ballast in general is another issue. Most glider flight manuals suggest not using water ballast below freezing. Many pilots ignore this advise and don't seem to have many problems. Some care is needed though.
Because the tail water ballast is a small volume it can freeze more quickly than the wing ballast. The wing is generally a foam sandwhich which insulates the wing ballast better than the tail which is generally just glass or carbon skin. So, depending on the specific tail ballast system, this requires special care. Generally though, the worst problem is that the dump valve is likely to freeze and you may not be able to dump the tail water. If you dump the ballast and the wing ballast goes but not the tail you can get into a seriously aft C of G situation.
Wing ballast is less likely to freeze, because the volume is greater and it takes a longer time to freeze a larger volume, plus it's likely to be better insulated. The dump valves on many gliders are on the wing skin (Schempp Hirth I'm looking at you) which means they will freeze and stick if there is any moisture on the valve seat. Best to clean and grease the seat before each use to reduce the chance of moisture in the seat of the valve. Others have the valve inside the wing and protected by a flap, which helps keep the dump valve from freezing.
With both wing and tail water I have heard horror stories about the water freezing and bursting the wing/fin. I'm sure this can happen, but I've seen plenty of wave flying with water ballast and the only problems I've seen have been leaks causing icing problems and dump valves freezing causing inability to dump some or all of the ballast.
I suspect these structural horror stories are more likely to happen if you leave a glider overnight with ballast so it freezes. In flight the movement of the glider helps stop freezing, and overnight is longer most flight times, even long flights.
Ballast freezing problems with the dumps are enough though. Landing with one wing 100kg heavier than the other or with an aft C of G is something to avoid.
Cool air holds less water vapor than warm air, so when we get high and things cool down the air becomes very dry. Our warm moist breath takes water away from out body as we breathe, and the cool dry air coming in dries us out.
Because of this we must be careful to avoid dehydration. In the heat most people increase their water intake to stay hydrated, but in the cool it is less obvious that we need to keep the water coming. It is still important.
In the wave we often encounter high windspeeds. I generally consider 80 knots to be the upper limit for flying in the wave, and maybe 50 knots if you want to make serious progress somewhere.
Flying in these winds you need to allow a lot of offset for drift to avoid getting blown downwind. You also need to fly fast if you want to make any progress. This is something that it is common to see in pilots who aren't used to flying in the wave. Cruising at 100 knots and having the heading significantly different to track can take a bit of getting used to.
The instructors often joke that once established in the wave they could be replaced by a box that just repeats two commands. "Fly faster", and "more upwind".
At high altitude our true airspeed becomes significantly different to our indicated airspeed. Refer to Low Air Density for more about that.
Because of the high winds and strong lift we often cruise at high speed. Keep a gentle touch on the controls. Many gliders become more pitch sensitive at high speeds.
Conflicting traffic will close quickly. With both gliders travelling at 100 knots or more, closing speeds over 200 knots are common. That's about 1km every 10 seconds. Keep a good lookout, and remember that the nose isn't pointing along the path you're travelling. This makes alerts from the Flarm misleading. If you're tracking directly at conflicting traffic the Flarm will alert you to traffic directly ahead. If you look directly ahead over the nose for traffic you will not see it. It is along your track, not your heading.
Sometimes we need to stay below airspace or just stay low for some other reason. Once we're going as fast as we are able the next step is either to fly out of the lift or to open the airbrakes. Opening the airbrakes at VNE might not be a good idea. Make sure you know what your glider flight manual has to say about maximum brake opening speed.
If you are going to open your airbrakes at high speed be very careful. The overcenter on the airbrakes might require a strong pull on the airbrake handle to unlock the airbrakes. At high speed the wings bend down at the tips due to washout and this may tension the airbrakes a bit. When the brakes unlock you do not want them to slam fully open.
So, a firm pull to get the brakes unlocked, then a controlled movement to progressively open the airbrakes.
How well this works differs considerably from glider to glider. I'd start by slowing down before opening the airbrakes, then as I got the feel for it try it at higher speeds until I found the limit I was happy with.
Some gliders I am familiar with (DG1000, early Duos) I slow below 100 knots to open the brakes, and am very careful to control them when they unlock to avoid slamming them open. Later model Duos with the flaps linked to the brakes resist opening at speed, so they are much easier to open in a controlled manner. I will open them at any speed, still careful at higher speeds, but nowhere near the drama of the other types.
Your milage may vary. The can be a large variation between gliders of the same type as well as different types. Get to know what your glider does and what you are comfortable doing with it.
We often fly above cloudbase. You will need to know what the rules are for clearance from cloud. Care is required. It is easy to fly into a cloud, but can be very difficult to get out.
When flying above cloud you must always have a clear escape route available. Descending through cloud cover in the mountains is not something you want to do. Your escape route should take you somewhere useful, somewhere that offers a good landing option and perhaps also some chance of soaring away under the cloud.
Just having a gap visible to go through is only part of the solution. Gaps can close, so you need to be close enough to your gap to get through it in case it starts closing. A gap 10,000 feet below you has a good five or ten minutes to close before you can get to it. A gap upwind may be hard to reach as you have to push into wind. A gap downwind is much easier to reach, but you'll be facing the other way most of the time so it will be harder to keep an eye on it.
If I'm not in a hurry I will occasionally do an orbit to have a look behind, maybe even do a few complete turns to give the glide computer a chance to update the wind speed and direction. In strong winds this can be tricky, you get blown downwind quickly.
Often in wave we want to move to the upwind wave line. This generally involves pushing upwind over a wave cloud. Allow plenty of margin for this. Everything is working against you. You are pushing into the wind, often very strong, so you have to fly fast to make progress. This increases your sink rate. You are flying into the sink behind the upwind wave bar. Still more increase of sink rate. Always have a clear escape back to where you came from.
Briefing prepared by Phil Plane.
This work is licensed under a Creative Commons Attribution 4.0 International License