Yesterday the weather decided to cooperate and we where able to head up the Andes mountains again. For this trip we set up a portable oxygen system on the cargo area right behind the pilots seat.
The setup allowed for access to the oxygen regulator valve from the cabin (a bit ackwardly) so we could adjust the delivery when needed up in the air.
For the first part of the trip we kept the engine at 5200 rmp and 70kts. With these settings the climb went uneventful at about 500 fpm all the way to 9'000 feet.
Although the day was clear, we barely found any workable thermals on our way up, even considering that the rise of altitude on the area of the Andes where we fly is quite steep, so even we climbed at 500fpm, we always had terrain less than 2000 feet below us. By the time we reached 12'000 feet, we where barely 30 miles east from our airport which sits right next to the Pacific ocean.
September in Peru is the end of the winter on the coast, the seasons are not to defined here in the coastal areas, but today the temperature clearly was warmer that our previous flight where we reached 16'000 feet. Today the climb rate from 12'000 to 16'000 was barely 400fpm and it started lowering as we got above 16'000.
Once above 16'000 feet, I kept in close mind a couple of things that the guys at Pipistrel mentioned to me on the owners internet forum:
1. Watch your speed. Most experienced pilots understand that because air at high altitude is thinner, the ram air hitting the pitot tube will feel "softer" so the indicated airspeed will show lower than the real speed. The neat thing with the instrument is that because of the same thin air reason the wing of the plane also needs more speed to generate lift. In regular airplanes it seems to work out fine with instrument reporting the right speed to fly but, on a long laminar carbon fiber wing like the Sinus, another factor takes place: Flutter.
2. Slowly reduce the power setting as you reach altitudes above 18'000 as the barometric compensator on the Bing carburetor, no longer can reduce the fuel/air mixture going to the cylinder.
On the first point, we were *very* careful. Before the flight I had already read the manual *and* the actual incident reports of cases where the wings of the sinus "came apart" in flight due to flutter on the wing induced by unawarely exceeding TAS on the plane. At 16'000ft an indicated airspeed of 105kts is actually a true speed of 135kts (already exceding VNE!) and At 19'000 feet you only need to reach 100kts to exceed VNE. Knowing the smoothness of the airfoils will prevent any "speed" noise that might alert about high speed, and know how fast the Sinus like to accelerate when asked to, We made a point to keep a close eye on the speed both on the mechanical instrument as on the Dynon flight computer.
On the second point, the guys at the board where right on. Close to 18'000 the engine felt a bit rougher a bit after the low CHT alert came up (Because of mixture too rich!) .. we lowered the power setting to 4300 and the engine started sounding good again.
With the power back, our climb rate took a big drop barely climbing more than 100fpm until we reached 19000 feet on the altimeter (GPS reported 20'180)
So, 40 miles to the east of the sea, 19'000 feet high, the terrain directly below above 16'000 feet and further east the majestic Pariacaca Nevado with snow-caps going over 19'000 feet what did we do?
We turned off the engine, feather the prop and set up for a long glide back home.




















