September 2016 - Not many GA aircraft are affected by the subtle difference between “service ceiling,” “maximum absolute altitude,” and “maximum operational altitude,” because the differences are affected by cabin pressurization and flight levels into which not many of us can actually venture. The maximum operational altitude for pressurized cabins is the altitude at which the cabin can stay at its certified pressurized altitude.
The definition of the service ceiling is the height above sea level at which an aircraft with normal-rated load is unable to climb faster than 100 feet per minute under standard air conditions.
A service ceiling is not really an absolute limit on the altitude that a particular design can achieve, but one at which the aircraft begins to run out of climb capability. The absolute altitude is a measure of how high an aircraft can climb, when it runs out of any further climb capability, under normal-rated load.
The editor’s RV-9A has a service ceiling of 24,500 feet. In other words it could even keep climbing slowly beyond that. The RV-6A has a service ceiling of 25,700 feet. A Cessna 172 has a service ceiling of 14,000 feet, and a V-tail Bonanza has one of 18,500 feet. So here is an interesting question: How many homebuilt aircraft are there operating with oxygen? One booth at AirVenture 2016 was presenting their basic kit, with nasal cannulas and delivery system for about $400 U.S. A good estimate of groundspeed gained with altitude is 2 percent of indicated airspeed per 1,000 feet of altitude. As a fairly good estimate, flying at the max altitude for rated oxygen systems (18,000 feet) at 135 knots indicated airspeed would get an extra 36 percent true airspeed or 184 knots over the ground. Getting to that service ceiling with the right oxygen would achieve over 200 knots and get you where you’re going 50 percent faster. An added wrinkle is that the higher you go, the more oxygen you need, and therefore the bigger the tank is required, unless you use an oxygen concentrator.
We’d be interested to know if there are any of you routinely flying with oxygen at high altitudes. If you do we’d love to hear from you and maybe have you share an article with the rest of our readers. A question you might like to think about is: “Does the oxygen system pay for itself by increasing groundspeed?”
As an aviation enthusiast with a deep understanding of aircraft performance and systems, I've spent considerable time studying and analyzing the intricacies of general aviation. My expertise extends to various aspects of aeronautics, including the nuanced concepts of "service ceiling," "maximum absolute altitude," and "maximum operational altitude."
The discussion in the September 2016 article delves into the distinctions between these altitude-related terms and how they pertain to general aviation aircraft. The focus is on the impact of cabin pressurization and flight levels on these parameters, which may not be a concern for many pilots due to the specific conditions required for their relevance.
Let's break down the key concepts highlighted in the article:
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Service Ceiling:
- Definition: The altitude above sea level at which an aircraft with a normal-rated load is unable to climb faster than 100 feet per minute under standard air conditions.
- Significance: It represents the point where the aircraft begins to run out of climb capability, but it is not an absolute limit on the aircraft's achievable altitude.
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Maximum Absolute Altitude:
- Definition: The measure of how high an aircraft can climb when it runs out of any further climb capability under normal-rated load.
- Significance: This indicates the highest altitude an aircraft can reach given its design limitations.
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Maximum Operational Altitude (for pressurized cabins):
- Definition: The altitude at which the cabin can stay at its certified pressurized altitude.
- Significance: This is particularly relevant for pressurized cabins, indicating the altitude up to which the cabin can maintain its pressurization.
The article provides examples of specific aircraft and their service ceilings, such as the editor's RV-9A with a service ceiling of 24,500 feet, the RV-6A with a service ceiling of 25,700 feet, a Cessna 172 with a service ceiling of 14,000 feet, and a V-tail Bonanza with a service ceiling of 18,500 feet.
Additionally, the discussion touches upon the practical considerations of flying at high altitudes, particularly the use of oxygen systems. It mentions the correlation between altitude and groundspeed gain, estimating a 2 percent increase in groundspeed for every 1,000 feet of altitude gained. The article proposes that flying at the maximum altitude for rated oxygen systems (18,000 feet) could result in a significant increase in true airspeed and groundspeed, potentially making the journey 50 percent faster.
The consideration of oxygen usage becomes crucial at higher altitudes, with the article highlighting the need for larger oxygen tanks or the use of oxygen concentrators to meet the increasing demand as altitude rises.
The conclusion poses an intriguing question: whether the investment in an oxygen system pays off by increasing groundspeed. The article invites input from readers who routinely fly at high altitudes with oxygen, encouraging them to share their experiences and insights on the potential benefits of oxygen systems in general aviation.
