I have operated my instrument equipped Piper Archer in wave X-C flights in the sense that I was flying far above its operational ceiling (13,750'). In one case at FL 240 for a flight that took me from Palm Springs to Denver non-stop. Above 15,000' feet or so the airplane became a sailplane in the sense that it would descend at about 50:1 at full power without lift. The Archer was IFR equipped with a Mode C transponder and DME and I was instrument rated and current. I had a good oxygen system on board. The procedures followed with ATC would be the same with a sailplane.I air-filed an IFR flight plan once I was in the wave and knew I could reach the flight levels. I asked for, and was given, a direct route and a "cruise" clearance giving me the altitudes from FL240 down to 14,000'. I made sure the controller knew that I would not necessarily be able to hold an altitude above 14,000'. Whenever I encountered a particularly good wave I asked for a "climb in hold". In all but one case, this was granted quickly. On occasion I asked for, and was given, a route diversion to a good looking lenticular. The engine ran incredibly smoothly and quietly at about 2400 RPM producing about 35% power and consuming about 3GPH. The airplane needed to be flown at the Vy speed of 76Kts IAS which is probably the L/D max. The TAS was the normal 120Kts. The experience is very quiet and soothing, almost sailplane-like.The leaning procedure is tricky at FL240. I found that the idle cut-off section of the mixture control allowed for a degree of proportionality but it was hard to get set right. I settled for an EGT reading anywhere on the scale. The crossing of the Rockies was interesting. As the low level jet stream was pushed up over the mountains, the wind speed increased. My GPS groundspeed increased to 240Kts over Vail, CO at which point I realized that I was going to fly east no matter what. Turning around was out of the question since my ground speed would have been zero. The air was very smooth at FL240 but I could look down and see thewind-driven snow boiling off the peaks in what must have been severeturbulence. I elected to continue at FL240 well east of Denver beforestarting the decent to avoid this turbulence. Upon landing, I still had enough fuel for another 2 hours. Not bad for an airplane whose POH says the range is 600NM without reserves. The whole operation was quite legal and rather pleasant. I am inclined to think it could be done with a real sailplane. The controllers seemed to enjoy it too.
Bill Daniels