Aircraft Performance for “Dummies.”

Today's modern business jets are at the leading edge of aerodynamic design. These aircraft fly faster, further and consume less fuel than their first generation predecessors. What they are capable of is amazing. Within aviation, we tend to focus on and discuss all the maximum performance capabilities. However, when we are dealing with the non-aviation person, these limits of the aircraft’s capabilities can lead to much confusion, and sometime to acquiring the wrong aircraft for the job. Consider this to be in the vein of those wonderful “Dummies” books!

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: 8 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 466 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 family sedan to get 28 MPG at 130 MPH! Most non-aviators can understand that analogy.

Following are some explanations of salient terms for those who don’t share in our world.

Aircraft can carry people, baggage, and fuel, but not the maximum of all three. Fill up the tanks with fuel for that maximum 2,350nm range trip, 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.

Best economy speeds are slower than the maximum speeds. Our aircraft is capable of traveling at very high speeds. But the best fuel economy is found at a much lower speeds. 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. 

Runway length required for takeoff 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

 

With a relatively short runway, at altitude, on a warm day, we may need to reduce the weight of the aircraft below its "maximum" weight in order to safely depart on the runway. Come back after dark when it is cooler and you may be able to add more 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.

Regarding the performance of aircraft, 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. We pride ourselves in knowing the maximum capabilities of our aircraft. We also need to pride opurselves in our ability communicate to non-aviators the trade-offs inherent in our aircraft.