Recent accidents highlight value of runway threshold margin

 - February 8, 2008, 4:38 AM

Two accidents late last year involving Bombardier Global 5000s landing short of the runway have provoked scrutiny of safety issues associated with the landing phase, specifically the information available to pilots about clearance above the runway when following lighted approach slope guidance systems.

The first accident occurred on Nov. 11, 2007, when a three-week-old Global 5000 carrying eight passengers landed short at Fox Harb’r Resort’s 4,885- by 75-foot runway in Nova Scotia. One of the passengers was Ron Joyce, cofounder of the Tim Hortons doughnut shop chain and developer of the resort. At the time of the landing, heavy snow and strong winds were reported, and the Global 5000’s nosegear hit the lip of the runway and broke off. No one was seriously injured in the accident.

On December 12, a Global 5000 struck approach lights and a fence 56 feet before reaching the landing threshold at Vance W. Amory Airport in Charlestown, St. Kitts and Nevis. No one was injured and the pilots were able to taxi the damaged jet to the parking area, according to a preliminary NTSB report on the accident. The Global 5000’s rear fuselage, right flap and right main landing gear were damaged.

The airport’s runway is 4,026 feet long, yet the NTSB preliminary report writes that the runway is 13,110 feet long: “According to preliminary information from the Eastern Caribbean Civil Aviation Authority, the airplane was approaching Runway 10, a 13,110-foot-long, 321-foot-wide, asphalt runway.” None of the public information published about the airport mentions a 13,110-foot-long runway.

A discussion on an aviation forum about the two accidents prompted corporate pilot Richard Boll, an expert on airport and aircraft performance issues, to point out some features of airport infrastructure that might lead pilots to think they have more space above the runway during landing than they actually have. Boll’s research doesn’t necessarily apply to the two airports involved in these two Global 5000 accidents.

Misleading Cues
What Boll found is that when pilots are following a visual approach slope indicator (VASI) or precision approach path indicator (PAPI), there is no guarantee of an assumed 50-foot threshold crossing height (TCH) when crossing the end of the runway. In other words, a pilot following a PAPI or VASI could think that at the threshold there is plenty of room between the airplane’s wheels and the end of the runway, but there might not be.

Boll cited examples that any pilot can easily find, at least for U.S. airports, in the FAA’s Airport Facility Directory (available free at At Georgetown, Del., for example, when landing on Runway 4 or 22, the TCH for an airplane using the PAPI is just 30 feet. In a large airplane such as a Boeing Business Jet or Embraer Lineage 1000, both of which are completed at Georgetown, a pilot’s eyes could be at 15 feet as the wheels are about to touch the runway, leaving little margin at the threshold. “Following a VASI or PAPI on-path indication at out-of-the-way airports may lead to a touchdown before the 1,000-foot fixed distance markers,” Boll explained, “and in the case of long-bodied aircraft, a premature touchdown prior to the threshold.”

TCH numbers at Teterboro offer more margin, ranging from 43 feet on the Runway 6 PAPI to 58 feet on the Runway 1 VASI. Another airport that Boll found with marginal TCH space is Woodward Field in Camden, N.C. That airport’s 5,000-foot Runway 24 has a 28-foot TCH using the PAPI and 31 feet on Runway 6. At Jackson Hole Airport in Wyoming, the PAPI TCH for Runway 19 is 38 feet.

There are criteria for the visual indicator TCH, which Boll found listed in FAA Order 6850.2A, published in 1988. The order notes that “the minimum allowable TCH varies according to the height group of the aircraft that use the runway. The PAPI approach path must provide the proper TCH for the most demanding height group that uses the runway.”

Unfortunately, as Boll pointed out, the order doesn’t take into account large business jets like the hundreds of Gulfstreams and Globals that pilots are now flying all over the world. The order splits the representative aircraft types into four height groups, three of which include airline-type airplanes ranging from the Fokker F-28 (Height Group 2) to the Boeing 747 (Height Group 4). “Corporate turbojets” are assigned to Height Group 1, along with “small commuters” and general aviation airplanes. Clearly, Order 6850.2A is long overdue for updating.

In Height Group 1, the approximate cockpit-to-wheel height is assumed to be 10 feet or less. For that group, the visual TCH must be 40 feet, plus five or minus 20. Even in Height Group 2, which includes the Boeing 737, the cockpit-to-wheel height is 15 feet and the allowable 45-foot visual TCH can still be 45 feet plus five or minus 20 (so, as low as 25 feet). This doesn’t leave much threshold margin for a pilot following the visual indicator.

“The Airport Facility Directory has information about a runway that is not always included on the Jeppesen 10-9 chart,” Boll noted. “I have found it useful to have a copy of the A/FD entry with me in the cockpit if I’m going to an unfamiliar airport.”