In recent years, business aircraft have experienced runway excursions at a higher rate than commercial air transports, says Gerard van Es, senior consultant for flight operations and safety, Air Transport Safety Institute, National Aerospace Laboratory of the Netherlands (NLR).
His study’s starting point was the runway excursion rate per million flights for the last three calendar years (Figure 1). “What I’ve compared here is commercial operations with business operations — business jets, business turboprops worldwide,” he said. “[NLR] looked at accidents, incidents, serious incidents, minor incidents, and tried to be as complete as possible. … In these three years, it’s almost a factor of two as between commercial operations and business operations.”
Looking at the analysis of runway excursions, in business operations versus commercial operations, NLR essentially saw the same causal factors. “We saw them with the same frequency in the major accidents and the serious incidents,” van Es said. Differences in exposure to factors such as unstable approaches, fast landings and high tail wind landings increase the risk of an excursion. NLR found that such causal factors occurred, or occurred more frequently, among business aircraft operated in a business environment than those in airline operations.
Presenting his findings in April during Flight Safety Foundation’s Business Aviation Safety Seminar in Montreal, he acknowledged limitations of data mining from international sources to study off-side excursions (veer-offs) and off-end excursions (overruns) with known causes. Taxiway excursions were not considered, and all events selected for the dataset involved either turbine or turboprop–powered types with more than one engine.
Next he focused on runway-overrun accidents in 2008–2012 for these two industry segments. “Again, you see a similar trend,” van Es said. “You see that the aircraft operated in a business environment have a higher rate.” However, the difference in rates was smaller for the five-year period of excursion accidents than for the different sources in the 2010–2012 dataset.
“The reason for that is not very clear,” he said. “It could be coincidence. It could be due to the fact that the [business aircraft] excursions are mitigated more by all kinds of things like RESAs [runway end safety areas] … runway strips that are much better organized.”
Business aircraft operators want to mitigate runway excursion risks simply because, as in airline operations, such events represent a significant portion of all takeoff and landing accidents, he said. Moreover, damage to aircraft occurred in about half of the excursions studied, and the U.S. National Transportation Safety Board rates the prevention of runway excursions among its top six safety priorities within the risk domain of business aircraft operations, he said.
All told, NLR’s data sample for this analysis comprised about 1,600 occurrences that break down as accidents, serious incidents and incidents — all of them events for which investigations have been completed. The data distribution indicated little difference between the two industry segments related to flight phase or type of runway excursion. “Both [industry segments] have a very similar distribution, and in both, the landing phase is the most critical phase. … [It’s] where the majority of the excursions occur,” van Es said.
Figure 2 shows leading causal factors assigned by the accident investigation boards, civil aviation authority investigators and aircraft manufacturers; none were assigned by NLR. “At the top of the list is what’s on the runway and what it is doing to your tires,” he said. “A wet or contaminated runway in this case means a lower friction than expected … contributed to the accident.”
The figure’s data also showed proportions of excursions among business operators and airlines alike that could be attributed to fast approach/touchdown, crosswind and system failure — in fact, differences in distribution of causal factors were small and similar in frequency. Failure to initiate a go-around also was a common element.
One caveat to this data interpretation, van Es said, citing one example, is the absence of information about standard operating procedures (SOPs) in effect. “We can say ‘Yes, the fast approach contributed to the accident, but maybe there was no SOP for the crew to adhere to,’” he said. “So to them, it was a fast approach but not a necessity to do a go-around. That’s why you don’t see [a direct cause] but you see here elements that justify, in hindsight, a go-around.”
In his effort to overcome this limitation, he turned to data on relative exposure to well-known risk factors. “What I mean by exposed is ‘How often do you do landings in tail winds on wet runways?’ If you do it more, you are exposed more to the risk factor, so there is a probability that operational risks are more related to this factor.”
Data collected by NLR primarily from flight data monitoring (FDM) programs in Europe and from flight operational quality assurance (FOQA) programs in North America enabled deeper analysis of recorded parameters related to the landing phase. Obtaining such data for business aircraft operations proved difficult, however. “We did manage to get data in a reasonable amount, which we could compare to the commercial operations, but it’s not as complete in terms of the amount,” van Es said. “There are not millions of flights. There are tens of thousands of flights.”
Van Es had intended to mine these data partly to identify differences in excursion scenarios for wet and contaminated runway operations. “We have no good data from the flight data or any other data to make an estimate or an analysis of how often business operators are exposed to these kinds of conditions,” he said. “But looking a little bit deeper, we know [from flight data] and we also know from … runway excursion accidents, that business aircraft can be operated at smaller, more remote airports, airports that may be less sophisticated in their surface-monitoring system.” In other words, they may lack systems in which trained observers visit the runway surface and accurately measure and report conditions. Related risk factors are less-than-optimal maintenance checks of runway surface condition, significant exposure to operations with snow on the runway and marginal snow-removal equipment for runways.
Fast and High
The NLR analyst also searched for evidence of unstabilized approaches. Looking at unstabilized approaches and excursions, the key issues are fast and high approaches, van Es said (ASW, 5/13, p. 34; fast means crossing the runway threshold more than 15 kt in excess of target approach speed). “We found that it was three to five times more likely, when compared to the commercial operations,” that the fast speed condition existed in business aircraft operations.
“In commercial operations, we saw roughly between 1 percent and 8 percent of all their approaches were unstable. In business operations we saw a range … as low as 1 percent, but it can also be as high as 14 percent. … There’s a very low go-around rate … after an unstabilized approach. We see roughly numbers in the 1 to 2 percent range for go-arounds in the business operations.
“What often happens is that the approach is unstable at 1,000 ft, but the crew manages — by speed brakes or whatever — to get it back on line at 500 ft, and for the pilot, there is no reason to abort the landing, then go into a go-around. It’s still an unstabilized approach, and if it was in IMC [instrument meteorological conditions], it would have justified a go-around.”
In NLR’s methodology for this analysis, a long landing was defined as a touchdown more than 2,400 ft (732 m) from the runway landing threshold, and flight crews of business aircraft conducted about eight times the number of long landings (Figure 3) compared with their airline counterparts, he said.
“What we don’t know in these data … at least from the business operations, is how long the runways were,” van Es said. “Pilots can say, ‘Well, I have a longer runway; that justifies maybe a longer landing.’ They also have to keep in mind that if you operate a small aircraft like a Cessna Citation on a runway that’s set up for a [Boeing] 747, and you fly the PAPI [precision approach path indicator lights], you always land longer because the PAPI is set up for the 747. Many pilots don’t realize that and they are landing much longer than performance calculations assume.”
Tail Wind Realities
Figure 4 shows, as a percentage of total FDM/FOQA flights, a relatively higher incidence of tail wind operations involving business aircraft flight crews than airline crews. “Particularly, if you look at the very high ones, more than 10 kt [of tail wind, the difference is] significant, and to operate beyond 10 kt means that you have to have a separate certificate … for a Part 25–certified aircraft because you get 10 kt as a standard,” van Es said. “I’m not aware if there are many business aircraft that have this certificate or operators that use this [exception to the rule].”
NLR also studied runway length as a variable in takeoffs and landings, but found it was not significant in this part of the issue analysis. A similar question — whether the width of the runway was causally relevant in veer-off events — involved looking at data that showed business aircraft flight crews typically operating on runways narrower than those used by airline flight crews. The important factor here was airplane wheel track, the distance between the outer edges of the main gear, relative to runway width.
“If you have a commercial aircraft, the wheel track runs typically between 5 and 14 m [16 and 46 ft],” he said. “Typical aircraft wheel track that they’re using in business operation is between 2.5 and 6.0 m [8.2 and 19.7 ft].”
Citing an indirectly relevant regulation, which sets a maximum limit of 9-m [29.5-ft] for landing gear deviation from the runway centerline after an engine failure, he said that “a typical business aircraft doesn’t need that much of a wider runway to comply with this.” He cited Australia as a state with strict regulations applicable to business aircraft operating on runways defined as narrow, while most states only specify corrections to minimum control speeds and maximum allowable crosswind conditions applicable to transport category aircraft.
No Crosswind Surprises
Lacking any risk-exposure data about business aircraft encounters with strong gusty crosswind conditions during business operations (ASW, 5/13), NLR compared conditions during actual runway veer-offs and overruns with the corresponding fleet-type data for maximum demonstrated crosswinds on a dry runway as a function of the year of aircraft certification.
Looking at the average over all these years, event rates are a little bit higher on the data from airline passenger aircraft, but there is no huge difference, van Es said. “They come up in the same numbers: [operating within] what they are demonstrated to be capable to handle,” he said.
What business aircraft operators often will find, however, is that a combination of strong gusty crosswinds with a contaminated runway sets up an unacceptable risk. “That combination is a very tricky one. … It’s not part of the official certification,” he said, leaving operators and pilots with only advisory material to make correct judgments about the risk level. “Sometimes there are statements in the operating manual saying, ‘Extreme care should be taken when landing in crosswind on a contaminated runway.’ That doesn’t help me. … I need some guidance, I need a number. Which crosswinds can this aircraft handle under these conditions?”