So we began to climb. Passing through 3,000 feet, I noted to my chagrin the rate of climb was gradually decreasing. It was taking longer and longer to climb. By the time we had passed through 4,000 feet, to maintain a climb I had greatly increased the nose-up attitude of the aircraft. I only had a one-hour lesson, and time was running out. How soon would we be a mile up?
My instructor now gave me a quick lesson on VY, the airspeed that would result in the greatest rate of climb in the shortest time. But, it was apparent I was going to reach that speed without his suggestion, if I wanted to continue upward. A faster airspeed was clearly not productive of much climb. (VY actually decreases at about 1 percent per 1,000 feet of altitude, but in most light training aircraft this small correction is meaningless.)
Though still a student, I had been flying long enough to know there would be a limit to how far I could raise the aircraft's nose before it would stall. About the time we labored to 5,000 feet the vertical speed indicator (VSI) showed a climb rate of less than 100 feet per minute. I knew we could not make 6,000 feet, and doubted 5,500 feet. Eking out another 100 feet would take a while, and this clearly would not go on forever as the rate of climb was rapidly approaching zero. Controls felt sloppy, and larger deflections of the control surfaces were necessary to obtain the same responses I expected in more dense air at a lower altitude. The aircraft felt sluggish.
It was apparent if I raised the nose further in an attempt to climb, the aircraft would descend, and if I lowered the nose the aircraft would--also descend! I had found the absolute ceiling for the aircraft--the maximum altitude it could attain under those conditions.
The situation reminded me of slow-flight exercises in an earlier lesson at a much lower altitude. Again, either raising or lowering the angle of attack would cause a descent. There were similarly sloppy flight controls, and that uncomfortable feeling of flying on the edge.
While I had demonstrated on that particular day a Cessna 150 could reach 5,000 feet, would that always be the case?
Several factors determine how high you can fly in an aircraft. It should be obvious that greater engine horsepower should carry the same aircraft higher. Specifically it is the excess horsepower above that required to keep an airplane flying straight and level that determines the ability of an aircraft to climb. So if, for example, in a 110-horsepower aircraft it takes 60 hp to keep the airplane flying straight and level at a particular altitude, then 50 hp is available to lift the aircraft. How fast the aircraft climbs on 50 hp depends upon aircraft weight. So the ceiling is also dependent upon a balance between power available and airplane weight. If I had left my instructor on the ground, not only would the airplane have ascended faster, but also I would have been able to climb higher.
Although the effect of weight on ceiling is profound, there are other factors. As a normally aspirated aircraft engine climbs, the volume of air it ingests has fewer gas molecules. This means the same engine produces less and less power the higher it flies. So there is progressively less excess power available to continue the climb. This applies to any engine burning fuel, even jet aircraft. There is also less horsepower available for straight-and-level flight.
So this raises the question, why one would ever want to fly high other than just to say you have been there? The answer is airspeed. As a rule, maximum aircraft cruise airspeed increases with altitude--up to a point. (Other benefits of flying at higher altitudes include better visibility, improved reception of ground-based navigation and communication transmissions, and additional terrain clearance.)
As an aircraft climbs, air density decreases and causes less (parasite) drag. The temperature also decreases, meaning improved engine performance. Engine power output is dependent upon air density, and density (or molecules per unit volume) increases as the temperature decreases. However, as we continue to climb, engine performance eventually begins to decrease because of the lower volume of air, and the increasing angle of attack necessary in the thin air ultimately increases induced drag--and that overwhelms drag benefits from decreased air density.
So all aircraft have a "sweet spot"--an altitude at which airspeed performance is maximized. You can calculate this altitude using the tables and graphs in a pilot's operating handbook (POH). Airplanes typically are fastest at an altitude somewhere above half of their ceiling. As you might expect, this altitude varies depending upon density altitude and aircraft weight. An aircraft becomes lighter as fuel is consumed, and maximum airspeed is attained at progressively higher altitudes the longer the flight. And the airplane is progressively faster as it becomes lighter.
Even intercontinental airliners begin cruise at one altitude, and then as the airplane burns off fuel and becomes lighter, climb to an altitude it could not have attained earlier--and flies faster. The same principles apply to the airplanes we fly, although in some cases the differences in speed are miniscule.
When a manufacturer touts how fast its aircraft can fly, it usually specifies the most efficient altitude and mid weight, meaning a weight halfway between maximum gross weight and a (favorably chosen) empty weight. Propitious parameters may also be chosen to maximize the touted ceiling. The parameters found in POHs are optimistic and represent the best a test pilot has accomplished after several trials under ideal circumstances. They must be taken with a grain of salt.
While I explored absolute ceiling, a more practical service ceiling is typically found in POHs. It is the altitude an airplane can achieve and still have a climb rate of 100 feet per minute.
That means there still is some performance remaining, and a pilot should reasonably be able to maintain that altitude. Of course, an airplane can climb somewhat higher--to the absolute ceiling--but that is an impractical altitude for flying, as I already proved. (Those flying light twin-engine aircraft will note that the POH offers the "service ceiling with one engine inoperative." This is defined as the altitude at which the rate of climb is only 50 feet per minute.)
The service ceiling in your POH is not a single number, but a calculated variable taking into account weight and atmospheric conditions. If only a single number is mentioned, it applies to "standard conditions." This is defined as atmospheric pressure 29.92 inches of mercury at sea level, with the pressure dropping evenly at 1 inch of mercury per thousand feet. It is a day when the temperature is 15 degrees Celsius (59 degrees Fahrenheit) at sea level, and decreases at 2 degrees Celsius per thousand feet. Of course these exact conditions are the exception; atmospheric conditions are not usually standard.
The failure to understand how ceiling and altitude above the ground may not coincide accounts for several accidents each year. Pilots attempt to clear a mountain close to their aircraft's published service ceiling. We meet a hapless pilot expecting to cross a 4,500-foot mountain ridgeline in a plane with a touted 5,000-foot ceiling. The airplane might be a little heavier than the pilot expected, the air might be hotter than standard, and the barometric pressure lower than standard. As the mountain is approached, there may be a little turbulence or perhaps a downdraft. Soon it looks too close for comfort. In what appears to be a timely decision, the pilot begins a turn to retreat. However, the turn decreases the lift vector--as occurs in any turn--and the airplane descends into the trees. This is the same physics principle that explains why an airplane can fly on the edge of a stall while straight and level, but will stall at the same airspeed when in a turn. However, when you practiced such turning stalls, you were not close to a mountain and had room to recover.
Considering these vagaries, flying near the published service ceiling to clear clouds is a questionable strategy, and clearing mountains close to the service ceiling is a gamble.
It is important for pilots to explore the ceiling of their aircraft. This can be simulated at a lower altitude, saving the time necessary to climb to the absolute ceiling. Simply choose a comfortable altitude, and decrease the throttle while maintaining level flight. The airplane will slow and reach VY, the best rate of climb airspeed. At that point do not further reduce power or touch the throttle. Now attempt to climb. At not much higher, the aircraft will reach its absolute ceiling for the set horsepower, aircraft weight, and atmospheric conditions.
You can then experiment. Any attempt to fly higher by raising the nose of the aircraft will lower the airspeed and result in a descent. And lowering the nose of the aircraft will increase airspeed, but will also result in a descent. Try a turn, and convince yourself that no matter what you do the aircraft is going to descend. Manhandling controls will also assure a descent, and you can understand how turbulence would also upset the apple cart. So we have found the absolute ceiling for that set of conditions.
So the question "How high can I fly?" does not have a simple answer. In depends upon the performance of the aircraft, but also on weight, temperature, barometric pressure, turbulence, and--last but not least--pilot proficiency. Under most circumstances an aircraft ceiling will not be as high as the optimistic figure published in the POH.
Dr. Ian Blair Fries is a CFI, senior aviation medical examiner, and ATP, and holds a Lear 35 type rating. He serves on the AOPA Air Safety Foundation Board of Visitors and is cochairman of the AOPA Board of Medical Advisors.
Want to know more? Links to additional resources about the topics discussed in this article are available at AOPA Flight Training Online.
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