Comparative Field Lengths
Business jets provide passengers with a level of safety during takeoff that is not found in the typical turboprop or piston-powered aircraft used in Business Aviation- explains David Wyndham.
More than 5-000 airports in the US are not served by scheduled airlines. A business aircraft is productive partly because of its ability to land at many of those locations- which often are significantly closer to your desired destination. When selecting an aircraft- however- it is important that it be able to operate safely from the airport you selected.
Aircraft selection may involve choosing between a business jet and a business turboprop. A selling point in favor of the turboprop may be that its operating manual specifies less distance to take-off than a similar-sized jet. While correct in one sense- that specification can be misleading and confusing.
There are differences in operating regulations governing the two types of aircraft. For aircraft having a gross weight of 12-500 pounds and under- FAR Part 23 applies; Air Transport Category aircraft (i.e.- those with a gross weight of over 12-500 pounds) are certified according to FAR Part 25. In particular- the philosophies behind these two regulations vary significantly.
FAR 25 vs FAR 23
The more stringent rules of FAR 25 provide greater safety margins over those deemed necessary for smaller FAR 23 category aircraft. (Business jets weighing 12-500 pounds or less comply with some of the conditions of FAR Part 25- such as runway length- but not all of the transport category criteria.)
These regulatory differences are the reason for the small turboprops’ reputation for being able to operate from shorter runways than small jets. Take-off distance is the amount of runway required to take-off under full power. The FAR 23 requirement for runway length involves this calculation plus the distance to reach 50-feet (or sometimes 35 feet) in altitude. Required take-off runway length for FAR Part 23 aircraft has no allowance for either being able to stop on the remaining runway or to continue the take-off on one engine after an engine failure.
Most multi-engine turboprops certified under FAR 23 do include an additional calculation called “Accelerate-Stop” distance. This distance is the runway length required to accelerate to the normal take-off speed- identify a problem- abort the take-off and come to a complete stop. This is the runway distance commonly used in discussing turboprop runway length requirements.
By contrast- FAR 25 intended for Air Transport Aircraft such as business jets and large turboprops (i.e. turboprops with a maximum gross weight above 12-500 pounds) assures that in the unlikely event of a problem during take-off- the aircraft can either:
• Stop within the remaining runway length (Accelerate-Stop); or
• Continue to take-off after the loss of one engine and climb on the remaining good engine (‘Accelerate-Go’ distance).
This calculation is called Balanced Field Length (BFL). The limiting factor for the FAR 25 airplane is the distance required to take-off after the loss of one engine. Some FAR 23 airplane flight manuals do include this calculation even though it is not legally required.
Technically- BFL is the distance obtained by determining the “Decision Speed” (V1) at which the ‘accelerate-stop’ distance and the one-engine inoperative take-off distance are essentially equal. Decision speed is the speed of the airplane at which point the pilot decides to either continue with the take-off or slam on the brakes and stop the aircraft.
RUNWAY LENGTH ILLUSTRATED
To illustrate how these regulations work- let’s look at a typical situation involving a small turboprop (FAR 23) and a small business jet (FAR 25)- both seating six passengers. We’ll assume maximum gross weight; sea level; outside air temperature of 59°F and standard pressure of 29-92 in.hg. (i.e.- International Standard Atmospheric conditions); and a dry- level- hard-surface runway. The small FAR Part 23 turboprop can legally take-off from a 2-600-ft runway.
Let’s assume an engine fails at our example turboprop’s published rotation speed (Vr) of 94 knots. Rotation speed is the point when the aircraft starts to lift off the ground. Its distance to accelerate to Vr and stop is 3-400 ft- 800 ft longer than the take-off distance.
If the take-off is continued after engine failure at Vr- then the runway required jumps to 4-750 ft (82% longer than the 2-600 ft take-off distance). Although 4-750 ft is the BFL for the turboprop- it is not a legal requirement under Part 23 for runway length decision. That extra distance required to provide a margin of safety should an engine fail at a critical time during takeoff- however- is significant.
The FAR 25 criteria include the accelerate-stop calculation. The small business jet has a BFL of 4-500 ft runway in the same conditions- 250 ft shorter than the turboprop under the same contingencies and with the same margin of safety. Although the small business jet could take-off or accelerate/stop like the turboprop from a shorter runway- the pilot is not permitted to base the runway length decision on anything other than the BFL data specified in the aircraft approved flight manual.
As you can see- although the FAR 23 turboprops can legally take-off on shorter runways than most jets- FAR 23 doesn’t require the same safety margin as FAR 25. So when comparing turboprops with jets- you should recognize the additional takeoff margins required of aircraft certified in accordance with FAR 25.
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