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Aircraft Technical Analysis Tips

by David Wyndham 25. July 2010 13:31
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When evaluating aircraft, we tend to focus in on the things we know best. Once we get a feel for those, we then hope the other information we turn up is OK as well. When I’m working with a financial type, she’s interested not in how fast the aircraft flies, but in the costs, how we account for major maintenance, what we show for the acquisition price, etc. With the pilots, we focus in on the specs and performance, the technical analysis.
With the pilot, we focus in on size, features, range, and performance. The mission drives these requirements. Many of the aircraft acquisition plans we do focus on requirements such as passenger seating, cabin size and range. The general way we approach this is to:
* Determine the most (likely) demanding payload, range, cabin size and/or passenger seating requirement as defined by your key mission.
* Compare those mandatory requirements against the capabilities of a range of aircraft from the sources of information you have gathered.
* Eliminate all those that do not meet the requirements.
* Eliminate those aircraft that are vastly more capable than required. The cost of acquisition and ownership does up dramatically as size, range and speed increase.
Here is where it can get tough. Just how are the numbers derived? I’ve had pilots distrust our data initially until we’ve discussed the ground rules used.
If you need a range of 1,450 nautical miles (NM) with four passengers, what exactly do you mean? For the range, do you mean VFR range? IFR range? IFR range with what sort of alternate airport? 100 NM alternate, 200 NM alternate, something else? In general, literature on turboprops and very light and light jets refer to ranges with a 100 NM alternate that follows the NBAA IFR Fuel Reserve format. Somewhere in the light jet category, the 200 NM alternate becomes “standard.”  Our numbers for an aircraft such as the PC 12 are with a 200 NM NBAA IFR Fuel Reserves. Very different than IFR 45-minutes that an owner-pilot may be thinking about.
Passengers are passengers, right? No. Most published data assumes each passenger (with bags) weights 200 lbs. But some data may refer to 170 lb passengers, while the FAA and airline data suggest the average American airline passenger with bags runs well over 200 lbs. So when we discuss how far with how many passengers, I like to make sure that we are talking about 800 lbs instead of four passengers.
The same passenger weight comment applies to the Basic Operating Weight (BOW) of the aircraft as well. BOW includes crew. Is your aircraft to be flown single pilot or with two pilots? That 200 lb difference in weight can, when carrying near full loads, mean 200 lbs plus or minus on the fuel load, or almost 30 gallons. If your aircraft burns 120 gallons/hour at 240 knots, 30 gallons is 20 minutes or 80 NM. That may be enough to move the aircraft from acceptable to not acceptable due to its range.
The last area where things can be confusing is that much of the published data on aircraft are “maximums” and may not be achievable under most conditions. As an example, the Certified Ceiling is the maximum ceiling the aircraft is certified to be able to operate safety. That does not mean that the aircraft can climb that high on an everyday basis. Just because the aircraft has a Service Ceiling of 51,000 feet does not mean that the aircraft routinely flies there.
We look at Service Ceiling at max take-off weight. How high can the aircraft initially climb? The 51,000 certified jet may initially climb to 43,000 feet, where it sits until it can step climb the FL450, then FL470. That initial figure is a good basis for comparison, one I favor over an absolute maximum.
When evaluating aircraft performance and technical specifications, you need to understand the assumptions that went into the number. Especially as they relate to your aircraft requirements. When comparing data for different aircraft, you need to have the data based on the same assumptions – the old “apples-to-apples” comparison.

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David Wyndham


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