• 01 Jul 2023 07:18 PM
• 0

Coming to Terms: Runway Independence

By Dan Newman
Vertiflite, Jul/Aug 2023

This series addresses the uses of terminology that threaten to become routine expressions or idioms — or already are — but are misleading, unfavorable, erroneous or prejudicial. This installment addresses terminology for vertical takeoff and landing capability derived from a fixed-wing perspective.

For aviation, runways are conventional, or normal. The image that comes to mind for most people when they hear “aircraft” is a fixed-wing airplane that uses a runway to take off and land. This is understandable, as piloted aircraft have been flying for 120 years, and passenger air travel is commonplace.

Airplanes (aka “fixed-wing aircraft”) require forward velocity to create airflow over the wings and generate lift. The thrust to fly forward is typically only a fraction of the aircraft’s weight. A runway is used for takeoff and landing, and this procedure is typically labeled “conventional takeoff and landing” (CTOL). The term “conventional” stems from the fact that airplanes came first.

The required runway length can be reduced by increasing the installed power and thrust for more rapid acceleration and/or by increasing the wing lift. These are generally referred to (progressively) as short takeoff and landing (STOL), super short (SSTOL) and extreme short (ESTOL) — or ultra short (USTOL) operations, respectively.

Enabling the takeoff roll to be reduced to zero eliminates the need for a runway altogether. It compresses the ground infrastructure from a very long, expensive, prepared one-dimensional runway to a zero-dimensional point — vertical takeoff and landing (VTOL) aircraft can land virtually anywhere. It is not just the end of a runway length continuum, but it represents a nonlinear change in capability, as it provides unique freedom from surface conditions such as slope, positive obstacles (stumps) and negative (holes), and in surface quality. Takeoff and landing with no forward velocity also offers some independence from wind, providing real-time freedom in the direction of approach and departure, unavailable with prepared runways.

The need for a runway can be eliminated for a fixed-wing aircraft by launching and recovering using off-board a launch and recovery system (LARS) as does the InSitu ScanEagle and AAI Pioneer drones, such as using rocket-assisted takeoff or pneumatic rails and landing by flying into a catch net. This significantly reduces the space required for terminal operations at some weight impact on the aircraft for increased power and structure, but the aircraft can only operate from/to locations with that GSE, increasing the logistics burden. Vertical flight aircraft can fly without any aircraft velocity. This VTOL capability reduces the size of the infrastructure, and so it dramatically increases the number of available locations to operate from and to (basing flexibility), both planned and on-demand. The organic (on-board) capability for vertical launch and recovery also enables airborne loitering in place (hovering). As vertical lift requires that the aircraft thrust exceeds its weight, there are power, weight and complexity costs far greater than for STOL, but they are warranted for applications that require the capabilities.

“It’s far easier to stop and land … than to land and try to stop.”

— Harrier Test Pilot Bill Bedford

In many aircraft design trades, there is a decision between whether something should be included on board or kept off board, such as for airstairs, maintenance platforms or support equipment. The choice is between paying the price of carrying it with you always, or being dependent on it being at every airfield where the aircraft may land. The airplane modifications to reduce runway length can be considered a choice to provide on-board provisions to reduce off-board needs, i.e., taking the runway with you.

A general term used for the capability to operate without a takeoff or landing roll is “runway independence,” performed by “runway independent aircraft” (RIA). It was first used formally by NASA in program plans and budget documents in the late 1990s.

While “runway independence” is accurate, it seems unfortunate, as it is a derivative term to express differentiation from normal or conventional (CTOL) airplane operations. This term, as well as CTOL, presumes that the audience worldview is that of fixed-wing aircraft, and the runway-independent world is an extension. This is entirely understandable as runway independence came after, but it doesn’t represent how VTOL is an evolution beyond conventional fixed-wing capabilities.

While “runway independence” honors removing the runway constraint, it misses the significant value of VTOL for flexible operations in expanding the usable 3D airspace. CTOL not only requires the constant, fixed direction while rolling on the ground, it also limits airspace use to only a few degrees to each side of the runway azimuth for miles in both directions for the airplanes’ approach and departure, as well as missed-approach paths. The majority of the “inverted wedding cake” that is Class A airspace around airports mostly goes unused.

VTOL offers operations in all the unused airport airspace, to almost any point from almost any direction, and (unlike airplanes) can just stop upon a missed approach, if they could even occur. VTOL can vastly increase air infrastructure capacity… and complexity. And so, runway independence is only a small part of the story.

CTOL should be replaced with a descriptive term, such as rolling takeoff and landing (RTOL). Interestingly, the term rolling takeoff (RTO) is used, not to refer to fixed-wing aircraft, but for VTOL aircraft using forward velocity to takeoff at increased weight when space is available. It seems that RTOL should be used for fixed-wing runway operations, in place of CTOL.

“Runway independence” is broadly used, and there appear no other terms in common use for this capability. Perhaps there should be. For the aircraft, how about fixed takeoff and landing (FTOL)? And for the capability, how about airspace expansion aircraft (AEA) or Agile Air Access Point Aircraft (A3PA)?