Ever wondered why commercial passenger aircraft cannot fly further into space? If you are afraid of heights then this may not be your cup of tea.
Whereas as a passenger or as a pilot it may be thrilling when the aircraft continues to climb higher to many altitude feet, the aircraft is an air vehicle that has performance limitations. One of these is referred to as the ceiling, which is the maximum density altitude that a plane can reach under a defined set of conditions as determined by its flight envelope. The ceiling can be further broken down into service ceiling and absolute ceiling, which we will look at shortly.
A brief explanation: service ceiling and absolute ceiling
Consider most commuter automobiles or even personal cars. You will notice that as the vehicle increases its speed, the vehicle moves faster but it reaches a point when it is not possible to accelerate further because either the vehicle cannot sustain or maintain the maximum continuous power or the topography of the road has changed.
In these circumstances, forcing the vehicle to accelerate beyond the maximum sustainable speed more often leads to mechanical failures such as overheating and eventual breakdown. Notably, these conditions are also sensitive to the serviceability of the car among other considerations. Ultimately, the vehicle has its performance limitations, a replicate of how aircraft equally go about their business, but this time, when they fly into the skies many feet above sea level.
Another way to demystify the service ceiling is by taking it in a literal sense – above your head. With some rooms having higher ceilings than others, then we can say that the ceiling varies with height. However, if you were to jump, you would only go a few inches or feet higher but not further than that. The service ceiling and absolute ceiling similarly demonstrate these limits.
Let’s define and analyze the service ceiling and the absolute ceiling as well as their significance.
Definition of service ceiling
Within the airplane flying handbook, the Federal Aviation Authority, FAA, defines the service ceiling as “the maximum density altitude where the best rate of climb airspeed will produce a climb of 100 feet per minute at maximum weight while in a clean configuration with maximum continuous power.”
This means that for aircraft with piston engines, their speed would begin to drop to as low as 100 feet per minute when they reach the service ceiling. However, for jet aircraft such as a commercial airliner, the threshold of the climb rate is a bit higher at 500 feet per minute under similar flight and environmental conditions.
Note that all reference is made to the standard air conditions, ISA.
Another definition of the service ceiling is that it is a maximum usable altitude of an aircraft. As mentioned earlier, there are detrimental performance limitations for both automobiles and planes primarily because of safety reasons. Therefore, it is common for most pilots not to exceed the service ceiling which is why it is defined as a maximum “usable” altitude.
Difference between service ceiling and absolute ceiling
The absolute ceiling, as defined by the FAA, is “the altitude at which a climb is no longer possible.” That is, at the absolute ceiling, the maximum sustainable rate of climb equals zero. As such, is it possible to sustain level flight beyond the highest altitude?
Simply, no. Therefore, even though the engines operating are at maximum power, the lift generated is only sufficient to maintain level flight since it matches the weight of the aircraft. This balance of thrust and weight significantly means that the only aircraft attitude or orientation that can be achieved is either level flight or a pitch down.
Comparatively, the absolute ceiling is higher than the service ceiling even though it is almost impractical to achieve for reasons discussed in the next section.
Further, at an abstract service ceiling for a plane such as 20,000 feet, the aircraft can climb beyond this rated altitude just that it would climb more slowly than in lower altitudes. However, if the absolute ceiling is defined as 30,000 feet, then for most aircraft, it would be impossible to reach or even go beyond, unless additional thrust-increasing machinery like afterburners is installed.
On the contrary, for some, yes it would be possible to climb even higher, achieving high altitudes called absolute altitude. The absolute altitude is the quantification of the extent to which an aircraft can climb higher after losing its capability for a further climb.
What happens if planes fly too high?
What would happen if you overstretch a rubber band? Beyond its elastic limits, it snaps.
Let’s begin by reminding ourselves of the principles of flight. There are four main aerodynamic forces that act on an aircraft in flight – lift, weight, thrust, and drag. Whereas the lift and weight act perpendicularly on the aircraft, the thrust and drag act along the horizontal axis of the plane.
For a flight to take place, the lift must be greater weight, otherwise, the aircraft would not be able to take off. If it so happens that the aircraft loses lift while airborne, then it would have to come down whichever way, could be a crash landing or ditching. A great example of ditching is the Hudson River incident. Lift is generated by the wings and other lift-augmentation devices such as flaps.
Equally, the thrust must be greater than the drag for a successful flight. Thrust is primarily generated by the aircraft engines whereas drag is the opposing force due to non-streamline air flow around the aircraft, equivalent to friction on the roads.
Because these forces, other than the aircraft weight, rely on atmospheric conditions such as air density, then the absence of ideal atmospheric conditions to sustain flight would affect the aircraft. In the case of higher altitudes, air density reduces. As such, there are fewer oxygen molecules in a given volume of air.
Consequently, because engine combustion is supported by oxygen, the engine will produce less power, perhaps low enough that cannot reach even the service ceiling. This explains why it is almost impractical to reach or go beyond the service ceiling or the absolute ceiling because as the plane approaches the maximum altitudes, the air density decreases significantly such that there will be less power to sustain flight.
For the absolute ceiling, the aircraft becomes unable to climb higher because of the deprivation of enough power to meet the required rate of climb. To counter this, which is not common in all aircraft, turbochargers or superchargers are installed in some aircraft, which compensate for the lost power.
Similarly, at higher altitudes, the planes would be operating on smaller envelopes such that the pilots would have to balance between flying at higher speeds not to stall and flying slow enough to avoid supersonic flow from building up over the wings. Every aircraft has a given critical Mach number, which is the maximum speed beyond which supersonic flow develops, which is an onset for further flight problems.
Shock waves develop over the wings, reducing the effect of the lift generated.
There is increased air temperature because of the air friction over the surfaces, which is many times more than the surrounding atmospheric conditions. Ultimately, thermodynamic principles add to the aerodynamic principles, which becomes ambiguous to solve while flying.
Eventually, the aircraft loses lift and stalling occurs – another reason why it is not always a matter of competition which aircraft reaches either the service ceiling or absolute ceiling ahead of the other.
Even more, cabin pressurization limits would hinder such high flights if they must happen. Because the atmosphere is divided into many segments, the air density is equally non-uniformly distributed across the layers. Similarly, the higher the plane flies, the thinner the oxygen becomes hence the supplementation by oxygen bottles.
In an economical sense, unless the aircraft operators manufacture the oxygen bottles themselves, it would be very expensive to provide sufficient oxygen for both the flight crew and passengers for such high-altitude flights. Even on a broader scale, the flight crew would have to compensate for the reduced cabin pressure at higher altitudes by installing better pressurization systems, which is expensive.
Planes have different performance capabilities and whereas there is no harm in knowing both the aircraft’s service ceiling, the decision to try to reach such maximum operating altitude other than the cruising altitude, should, to a broader extent, be motivated by any other factor except proof of heroism.
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Jet pilot @NASA
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