After reiterating my aircraft type, the controller asked me how I got up that high. I shared his incredulity, but mine had more to do with the fact that he witnessed me in a climb. After all, I was sure that when I reached 15,800 feet, he had time to run down the street for a cup coffee and return to his screen before I hit 15,900.
Each phase of flight places its own demands for the climb capability of an aircraft. A high climb rate is desirable on departing an airport because altitude affords options in case of an engine failure or other emergency. To gain altitude quickly, the best rate of climb airspeed VY is perfect for the job. A pilot who takes off from a short runway with obstacles at the end may slow to the airspeed for the best angle of climb VX until clear of them. Once obstacles no longer present a problem, many pilots choose a cruise climb airspeed higher than VY to allow for better engine cooling and visibility over the aircraft nose. The graph on the facing page demonstrates how these airspeeds might vary with wind effects.
For aircraft with normally aspirated engines, the ability to climb degrades with increased altitude and even turbocharged aircraft have their limits. In cross-country flight planning, care must be exercised in selecting the route and altitude. Controllers expect aircraft to be able to climb at 500 fpm for traffic avoidance, and flying under instrument flight rules requires the ability to meet any climb gradients required by departure procedures, minimum en route altitude increases, or missed approach procedures. The climb/descent table in the FAA Terminal Procedures Publication provides a quick way to calculate the climb and descent rates (in feet per minute) necessary to meet the associated gradients (in feet per nautical mile).
It turns out that my anemic climb rate that day wasn’t exactly problematic; rather, it was expected. I had informed Huntsville Approach of my intention to perform a 70-turn spin in Wilbur, my Cessna 152 Aerobat, but it seems Memphis Center didn’t get the memo. Based on my calculations, I needed to climb to an altitude of 16,000 feet msl to perform the spin and execute the recovery above the FAA minimum altitude for aerobatic flight. The previous year I had completed a 60-turn spin by starting at 14,000 feet msl.
The ability of an aircraft to climb depends on myriad factors such as density altitude, propeller efficiency, and the health of the engine. The only one of those factors I could readily control was density altitude, so I waited for a cold day in January for which the density altitude at 16,000 feet fell below my absolute ceiling. Since a Cessna 152 isn’t exactly a rocket, I brought along my music so the long climb to altitude would be enjoyable. It turned out that 16,000 feet was my absolute ceiling because I had just one available airspeed, about 60 knots. If I held an airspeed either above or below 60 knots, the aircraft began to descend.
Once the controllers understood my mission, I commenced my long spin and they even communicated with me during it. Given my descent rate of 6,000 fpm and nose sweeping full circle every two seconds, I politely informed them that “Traffic, two o’clock, ten miles, same altitude” wasn’t particularly useful to me. But after about 35 turns the engine quit, the propeller came to a halt, and the nose pitched down enough so that Wilbur’s spin converted to a spiral. While muttering a few words from my occasional expanded vocabulary, I recovered to straight and level flight before the airspeed became dangerously high. The propeller stopping during an extended spin in Wilbur happens regularly with less than 18 gallons of fuel. I anticipated and had tried to prevent it by filling the tanks to the very top at the airport. Alas, it took me an hour to get to 16,000 feet and I burned off too much fuel in the process.
The next day I tried it again with another long climb to Wilbur’s absolute ceiling followed by similar results. It turns out that I have reached the limits of his ability to climb in order to go beyond my 60-turn spin. Short of getting a lift up to a higher altitude using another aircraft, I suppose I will have to content myself with it.