The Falcon 7X in August received FAA approval for steep approaches, after a process that involved testing the Dassault large-cabin business jet at slopes up to 8.65 degrees. EASA approved the trijet for steep approaches last year and the UK CAA approved the 7X for approaches to London City Airport (LCY) in February of this year.
“The idea is to demonstrate that the flight features and performances remain unchanged,” during a steep approach, said chief test pilot Philippe Deleume. “To be greenlighted for a given slope, you have to prove pilot inaccuracy does not compromise safety.” The airplane must therefore be able to tolerate slope errors of up to two degrees.
For London City’s 5.5-degree approach, this translates into a 3.5- to 7.5-degree range. For Lugano, Switzerland, which has a 6.65-degree approach, it means a maximum of 8.65 degrees. Steep-approach airports are more common in Europe than in the U.S., although Aspen, Colo., is a well-known American example.
Meanwhile, variations of five knots around the nominal approach speed (Vref) must be accepted. On any approach, speed must be kept constant. This gets trickier on a steep approach, because the aircraft tends to accelerate more. To counter this on the Falcon 7X, the airbrakes are partially extended. This also degrades lift, however, so Vref is increased by five knots.
Some aircraft make steep approaches with the engines at idle, creating a challenge for manufacturers, who have to show that the engine de-icing system–which typically uses engine bleed air–still functions properly. By using the airbrakes on the
approach, enough engine power must be maintained on the 7X that the de-icing system remains functioning, Deleume claimed.
For steep approaches, the 7X’s landing weight is limited to 45,000 pounds, far below its maximum landing weight of 62,400 pounds. The reduced weight lessens the chance of damage to the aircraft and injury to its occupants in the event of inadvertent slat retraction during the flare. The sudden loss of lift from this could cause a too-heavy aircraft to hit the ground so hard that it exceeds the limits of the landing gear.
The most difficult aspects of flying steep approaches are “keeping the approach speed constant and executing the flare,” Deleume said. Visual cues that indicate the beginning of the flare are different in a steep approach. Flare should not start higher than the usual 50 feet in the 7X. This makes the maneuver swifter–at 1.2g–on a steep approach.
However, he said the 7X’s fly-by-wire controls help make flying it easier than other jets. “If you set the HUD’s flightpath cue to 5.5 degrees [for LCY, for example] and have the correct flap configuration, the approach is very stable,” he said. Moreover, because the sidestick’s position commands a constant pitch rate, executing the flare is relatively simplified.
FAA pilots began familiarization with steep approaches in CAE simulators at Morristown, N.J. A good portion of the flight tests took place at Dassault’s Istres flight-test center because “abuse testing cannot be conducted on a real airport,” Deleume said. FAA pilots also went to Lugano.