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Assume you enter a cloud with no visibility of the horizon. Your glider is not equipped with an artificial horizon, turn speed indicator or the like.

You must fly within safe limits, using air speed, ball, yarn, compass, variometer, altimeter and what basic instruments are available.

Is this possible, and how difficult is it? Can most "normally skilled" pilots do that without extensive previous training?

For example, if you just correct the yarn with the ailerons, the speed with the elevator and keep rudder neutral or fly a fixed compass heading, is that likely to keep you flying safely until you're out of the cloud?

Edit: I now remember an old "story". I am not sure I remember it correctly though. What is certain is that in the "old days" some instruments were sometimes sealed before a competition, in order to (try to) make it impossible to go into clouds. I think it was the turn speed indicator and also the compass? But some pilots were caught cheating by bringing a small compass in their pocket. OK, I am not sure I am telling this story correctly. (Nowadays they check GPS logs and compare to cloud base altitude.) If I remember correctly, these pilots were willing to fly in clouds and also curve using only primary instruments.

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    $\begingroup$ Flying in IMC for non-rated pilots, even with all the instruments available, is very dangerous and has killed many people. I can't see why gliders would be an exception. $\endgroup$ – Simon Apr 10 '16 at 14:45
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    $\begingroup$ @Simon: This opinion is shared on this PPRuNe thread: "it rates with storm chasing and hand feeding sharks on the common sense scale". $\endgroup$ – mins Apr 10 '16 at 14:52
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    $\begingroup$ There seems to be strong emotions and funny expressions involved. Sometimes this hides the true situation. It is not advisable, but is it likely to keep you alive (or at least not having to see your precious glider in parts while you land in a parachute). // I definitely discourage anyone from trying, I have not tried myself. But even so, I try to keep an open mind on the subject. $\endgroup$ – ycc_swe Apr 10 '16 at 14:58
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    $\begingroup$ Why would you try to keep an open mind on something that usually kills people? The fact that you are required to wear a parachute, even when trained, should answer your question. $\endgroup$ – Simon Apr 10 '16 at 15:04
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    $\begingroup$ I did cloud flying or "Blindflug" as passenger in a glider with my father in the 80s, who actually had a special license for that at the time (those are not issued any more). We entered a nice Cu and came out on top a few minutes later with a Blanik with a turn-and-slip indicator. Pretty cool, but: After a few seconds, you loose any orientation whatsoever. I could not even tell which way we were turning. So: No way, you need at least a turn-and-slip indicator, and then proper training to interpret and react correctly. $\endgroup$ – yankeekilo Apr 11 '16 at 12:05
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With basic instruments (altimeter, speed and climb indicator and compass): Forget it!

A glider can enter a cloud when thermalling into it from below. Below the cloud the rising air was governed by dry adiabatic laws. Once it crosses into the cloud, the air is fully saturated and humidity starts to condense - after all, this is how clouds form. Now the rising air gets additional energy from condensation (its lapse rate goes down from the usual degree per 100 m of the dry adiabatic lapse rate to half a degree per 100 m) and the thermals inside the cloud become stronger. This is to say that turbulence will be more violent and the airplane will be tossed around. It will bank and pitch, and there is little chance that it will stay level for long. Since the spiral dive motion has very little damping, the glider will soon enter a spiral dive and overspeed.

The advice I got for the case that you are "sucked" into the cloud always was: Let go of the stick and open the speed brakes. Then wait until it gets light again.

In the 1930's some competition pilots tried to gain additional altitude by entering clouds. The results were catastrophic. In some cases the cover of the wings was torn to shreds by hail, and even bailing out would not be safe: One pilot froze to death when strong thermals inside the cloud lifted him to more than 8000m while he hung on his parachute.

I have done it once on a calm day when returning from the Alps. In the mountains the cloud base is higher, so it is entirely possible to arrive at the clouds over the alpine upland above their base height. I had a GPS on board that told me that I would reach the glider port safely when flying a straight line. It was late in the day, the thermals had died down and the clouds were fuzzy already, so little turbulence could be expected. One was in my way, and I went into it and flew out less than a minute later. The whole thing was completely uneventful, but I would not had done it with active thermals.

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    $\begingroup$ +1 But letting go of the stick and hitting the speed brakes in a cloud only works if you are high enough in altitude and not near a mountain. $\endgroup$ – Robusto Apr 11 '16 at 2:34
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    $\begingroup$ @Robusto: Certainly. If the clouds touch the ground we speak of fog. I was speaking of proper cumulus clouds on a sunny day. Remember, just before you entered the cloud your sink speed was not high enough to prevent being sucked in from below. $\endgroup$ – Peter Kämpf Apr 11 '16 at 6:15
  • $\begingroup$ @PeterKämpf what aircraft types do you fly? $\endgroup$ – Albin Stigo Jan 30 at 14:49
  • $\begingroup$ @AlbinStigo: Gliders, Motorgliders and single engine GA aircraft. $\endgroup$ – Peter Kämpf Feb 1 at 9:24
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Yes it is possible to fly in cloud in a glider. In fact there is an endorsement for it for your licence with an exam. (In the UK anyway.)

It does only require primary instruments, the one used for cloud flying is called a turn and slip, showing bank and yaw. Along with the yaw string. These are found in almost all gliders I have come across.

It is generally an endorsement only held by otherwise experienced pilots, and isn't done for prolonged periods of time. But, you are then licensed to do so.

It is easy to become disorientated, and be in odd angles of attack etc. So you must learn not to trust your feeling. I have heard of goggles used in training to cloud your vision.

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    $\begingroup$ This now depends on whether a turn-and-slip indicator is considered part of the standard instrumentation. In Germany, this would not be considered a "primary instrument" as stated by the OP. $\endgroup$ – yankeekilo Apr 11 '16 at 11:59
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It would be difficult to stay stable for any length of time because those basic instruments give you almost no information on your angle of bank.

If you could manage to keep your spirit compass on a constant heading and keep the yarn straight, your wings would be level.

So while it would be possible, I would think you would need an experienced glider pilot and relatively calm conditions to maintain wings level for any usable length of time.

It would be interesting to hear from someone who has actually done it.

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    $\begingroup$ "none of those basic instruments give you any information on your angle of bank" / If yarn is straight and compass heading fixed, wings are level $\endgroup$ – ycc_swe Apr 10 '16 at 14:40
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    $\begingroup$ Although @ycc_swe is right up to a point, I would not include that in your answer. Those instruments don't give a pilot enough information to correct problems. $\endgroup$ – GdD Apr 11 '16 at 8:15
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    $\begingroup$ Why would it be so different to read the compass instead of the turn-speed-indicator? We are not discussing precision here, just flying within safe limits. Mostly flying straight forward or making slow turns. / I am absolutely not advising anyone to try. I haven't tried myself. We are discussing if you can save your life/glider in an emergency, even if it naturally is better and very possible never to enter a cloud under these premises. $\endgroup$ – ycc_swe Apr 11 '16 at 18:58
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    $\begingroup$ @ycc_swe The magnetic compass isn't that steady an instrument. I can't imagine a pilot being able to prevent a spiral dive using only the magnetic compass and yaw string. Even with a gyro compass, that seems like a tall order. $\endgroup$ – Wayne Conrad Jan 31 '18 at 9:48
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    $\begingroup$ @Sean Not normally, as far as I know. Maintaining attitude in IMC is normally done with an artificial horizon, or if that fails, a combination of turn/bank indicator and ASI. The gyrocompass is used for maintaining heading, of course. But I don't know how many gliders have any gyro instruments at all. Things might have changed, but they used to be kind of power hungry instruments, and gliders didn't used to have much of an electrical system. $\endgroup$ – Wayne Conrad Jan 28 at 8:37
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I'm offering this answer more for amusement to readers than anything; everything is factual first-person observation by me.

No. I cannot fly a glider in a cloud on primary instruments.

When I was a young man of 23 years old I flew hang gliders before I obtained my Private Pilot's license in the USA about 1986. My flight instruments in my hang glider were an aircraft altimeter with 20 foot resolution, a variometer that was sensitive to about 50 foot per minute climb/descent rate, and very sensitive airspeed indication by human ear. I lacked any turn or bank or artificial horizon.

In my flying club it was common knowledge that intentionally flying into a cloud like a cumulonimbus was tantamount to suicide. This was about 1986 and already there were a number of reports of hang-glider's deaths from hypoxia and hypothermia from thermallinig up into cumulonimbus clouds.

Once in a competition flying from Crested Butte Colorado USA I was in such a strong thermal that it took me over one minute to exit a thermal and I luckily did exit it at 17000 feet MSL. Imagine this; usually glider pilots seek lift. In this case I identified it was dangerous and dove a straight line to exit the thermal and just escaped zero visibility within the cloud deck of 17000 feet MSL.

My total flight time that day was 4 hours and 20 minutes with a maximum altitude of 17,000 feet above sea level.

In my years in the flying club I knew of one individual that fought being lifted into a cumulonimbus cloud by performing wing-over maneuvers which is an easy way to waste energy meaning to waste altitude. He eventually broke the spar of his hang-glider with excessive G forces, then spiraled like a maple seed under his busted glider and saved himself at lower altitudes with his emergency parachute. He's the only hang-glider friend I know to have deployed a parachute.

I hope this was fun to read. Comments or questions are welcome.

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I was taught to fly in cloud using basic instruments in 1976. The problem was one of staying in the cloud without exiting through the side. In the event of losing control, the drill was to open the air brakes and wallow out of the bottom. This worked well enough in my Olly 2B until one day in 1978 it didn't. My cosmim indicated a maximum climb rate and my altimeter needle moved round the dial at an alarming rate. I had entered cloud over Parham with a light northerly breeze drifting me south. With my air speed overtaking maximum rough air speed I decided to raise my nose and deploy air brakes. I have no idea how long I remained in cloud but when I finally escaped the murk it was near the top, very high and out over the channel. I never flew in cloud again.

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I've done it many times to climb slightly into the cloud (in situations when the risk to meet with another fellow in the cloud is low, i.e. far from any airfield and there are quite lot of useful clouds nearby). Immediately after losing visual to the ground I level wings and quickly find my way out of the cloud. Punching a hole on the side of the cloud and scooting into sunlight with 200km/h is something what I really like in flying gliders :) But this is not real IMC flight just biting a bit into the cloud. I once tried that how long I can maintain spiraling in a cloud just by pants (so w/o turn indicator). And even a slight turbulence was enough to confuse my senses, so I opened speedbreak and left the cloud. What surprised me was that I entered into cloud in a left spiral and descended out of it in a right spiral - without even noticing this direction change. Important fact that I done this experiment in a glider which has speedbrake effective enough the keep the speed below Vne even in vertical dive (fortunately I didn't have to use this capability in this situation). So I'd guess that even with significant IR experience it is very difficult if not impossible to maintain a steady spiral in a cloud without proper instrumentation (like a turn indicator).

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    $\begingroup$ Thanks for sharing an interesting experience. I hope you really consider safety aspects. // If I "got caught in a cloud with only primary instruments" I would control elevator after airspeed, ailerons after yarn and keep rudder neutral (or fix a compass heading). My guess is I would survive (unless extreme turbulence). Could I save myself a parachute jump that way? $\endgroup$ – ycc_swe Apr 11 '16 at 12:23
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    $\begingroup$ It's extremely dangerous to do this @hunglider, you are taking a big risk every time you go into cloud. One of the big lessons in IR training is that you cannot trust your senses - your inner ear is easily fooled. $\endgroup$ – GdD Apr 11 '16 at 12:58
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    $\begingroup$ ycc_swe: w/o turn indicator you are short of information. The whiskey compass is not for determining the turn rate. In an unsteady condition it will just tumble w/o providing any useful information. If things went out of control, relying on the inherent bank stability of the plane could be much better than try to control based on false info. So the second best strategy in case you sucked into a cloud accidentally is to open speedbrake and release everything. The best strategy is not to caught in IMC accidentally especially if you are not trained for IR or don't have proper instrumentation. $\endgroup$ – hunglider Apr 11 '16 at 15:14
  • $\begingroup$ @hungglider Yes, using brakes must be right, unless very low clouds/fog. I don't think that will worsen anything. // Inherent bank stability is a very important factor. // Regarding the Edit I made in the question. I am not sure I remember correctly but I wonder if the oldtimers curved in clouds with only a compass they brought in their pocket :) "Just radio the other guys, they must wait 2 min before the next guy goes into the same cloud" ;) $\endgroup$ – ycc_swe Apr 11 '16 at 16:07
  • $\begingroup$ And pitch I think you should definitely control reading air speed. Maybe just using trim. $\endgroup$ – ycc_swe Apr 11 '16 at 16:16
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In some gliders "the cloud procedure" is you trim it at around 1.3*Vstall and let go of the controls with air brakes wide open. It spirals down in a controlled fashion and then you hope to exit the cloud and take the yoke back. I used this to get down twice in waves when the hole closed up, every time the decent took longer than I thought with eyes fixed on the altimeter :)

Someone may argue that hands-off the controls is not considered IMC flying, but it's definitely better than trying something fancy. Gliders have a tendency to over-speed beyond their VNE as they have less drag, so keep your hand on air brakes during cloud entry!

Some gliders are not certified for cloud flying due to their instability in spiral dives, you can check this while in visual conditions or refer to the POH.

PS: in the UK it is legal to fly glider in IMC in class G airspace as long as the glider has a turn indicator and the pilot has a cloud rating. Mine did not have any useful IFR instruments when it happened but I did complete glider cloud training.

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The VMC criteria define legally whether you have the choice to fly VFR (in the UK and Europe).

In the UK, you can only fly either VFR or IFR*.

If you are flying close to (or within) a cloud, you must be flying in IMC which logically means you must be flying IFR because the VMC criteria cannot be met.
The fact you have a cloud flying endorsement / IMC rating means you must be flying IFR whenever you are less than the legal VMC.

If I'm wrong, please let me know how, I've been trying to reconcile this in my head for ages.

*Excluding Special VFR, and excluding clear of cloud in sight of surface at or below 3000ft, not relevant here.

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I have heard from high hour wave pilots where the clouds close in beneath them before noticing (or enough time to get back down to the holes) that if they just point the glider away from the mountain, level their wings, begin their descent, then just hold that position as they enter the cloud by keeping the compass fixed, yarn straight and airspeed constant their glider will live for another day (assuming far enough away from heavy rotor). In theory the same should work if one is sucked up into a cloud. Not legal.

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    $\begingroup$ Yes, wave clouds have nice, steady airflow. Most clouds a glider pilot will encounter, however, are cumulus clouds with a high level of turbulence inside. That makes a big difference! $\endgroup$ – Peter Kämpf Jan 30 '18 at 22:41

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