Aircraft Performance For Executives
Today's modern business jets are at the leading edge of aerodynamic design- says David Wyndham. These aircraft fly faster- further and consume less fuel than their first generation predecessors. What they are capable of is amazing.
Regardless of their abilities- today’s aircraft are nevertheless still bound by the laws of physics and aerodynamics- meaning there are limits to what they can do. Consider the following article a primer in aircraft performance designed to enable you to better understand the limits of these magnificent machines.
The confusion will often start with the sales brochures which list the maximum capabilities of the aircraft. Typically they focus on range- speed- and payload (or seats). It is important to realize that when the brochure states “Maximum Range: 2-350 nm; Executive Seating: eight passengers; Maximum Speed: 470 knots-” it does not mean that this aircraft is capable of taking eight people 2-350 nautical miles at a speed of 470 knots! It simply states that the aircraft can fly 2-350 nautical miles- it can fly with eight people on board- and it is capable of reaching speeds of up to 470 knots - just not all at once.
Think of it like you would your car. You wouldn't expect your Sedan to get 28 MPG at 130 MPH! Following are some important parameters to consider:
Aircraft useful load: This is the amount of weight that can be carried- and includes people- baggage and fuel. Fill up the tanks with fuel for that maximum 2-350nm range- and you may only have enough useful payload left for three or four people. Conversely- fill up the seats and baggage capacity of the same aircraft- and you may have enough useful payload left to fly a 1-800nm trip.
Chart A depicts a typical ‘Payload versus Range’ curve showing passengers carried and the effect on the range the aircraft can fly. Bear in mind that these payload-range curves are unique for each individual aircraft model.
Speed: Our aircraft- like our sample Sedan (above)- is capable of traveling at very high speeds. But the best fuel economy is found at a much lower speed. With our car- to get 28 MPG we may need to drive at 40 MPH as opposed to 130 MPH. With the aircraft- we may need to slow to 430 knots as opposed to 470. Best economy speeds are slower than the maximum speeds.
Runway length required for takeoff: This will vary depending on many parameters. Again- the brochure may list a runway length of 5-000 feet. But that is with very specific parameters. Can you remember that time you drove to the mountains? Pulling onto the highway with a car full or people and bags it took a while to accelerate to the required speed. With the aircraft- it is similar. In a nutshell:
• Heavier weights = more runway length
• Hot days = more runway length
• High-altitude airport = more runway length
In addition- aircraft safety planning also requires that the aircraft be able to maintain a safe rate of climb in the unlikely event of a loss of power. Aircraft climb faster when they weigh less because they have more power compared to weight.
Our aircraft may only require 5-000 feet of runway at sea-level on a normal day- but if you were to fly from Denver- Colorado on a warm day it may need over 8-000 feet for take-off. At some point- the runway length and the ability of the aircraft to climb will require that the weight of the aircraft be kept below its 'maximum' weight. So the extreme may be an aircraft that can take-off at sea-level from a 5-000 foot runway with four people and fly 2-350nm- but may only be able to manage a trip of 850nm from an 8-000 foot long runway in the mountains on a warm day.
Tankering: One last consideration is what value you place on the aircraft's flexibility to carry fuel and passengers. In general- if you buy a significant volume of fuel- you will get a price discount. This is commonly available at the aircraft's home base- where you buy fuel on a regular basis.
If you can carry extra fuel in your tanks purchased at home for $4.75 per gallon- you save money by avoiding the purchase of fuel on the road at $6.00 per gallon. This is known as 'tankering.'
This carriage of additional fuel purchased at a lower price needs to be balanced with the runway and climb requirements of the airports being used for any particular trip. Also- the heavier the aircraft- the higher the fuel consumption (which means higher cost of operation). Nevertheless- when all factors are considered- tankering may reduce total trip costs- especially when fuel purchased at the destination airport is particularly expensive.
The more you consider the performance of aircraft- the more you will realize that they are a series of compromises. They can offer speed- range and payload but often require trade-offs in two of those areas to maximize the third.
Do you have any questions or opinions on the above topic? Get it answered/published in World Aircraft Sales Magazine. Email feedback to: Jack@avbuyer.com