Aftermath: Unforeseen Circumstances

Having multiple modifications made to the same aircraft can often have unintended, and in these cases, fatal, consequences.

Airplane Crash

Airplane Crash

In February 2010, a CESSNA T337G Skymaster made a low pass over the runway at Farmingdale, New Jersey. As it pulled up, a six-foot piece of the right wing broke away; the ensuing crash, before the eyes of friends and relatives for whose benefit the pass was being made, killed all five aboard.

In August of the same year, in Saltsburg, Pennsylvania, a Beech Baron crashed into a house. The pilot-owner of the Baron and a flight instructor died, and the house was destroyed by fire.

Both airplanes had been modified under multiple STCs, and the NTSB concluded that interactions between various modifications could have played parts in both crashes.

The pilot of the Baron, a 1982 Model 58, had owned the same airplane 22 years earlier. He had sold it, moved up to a Cessna Citation 510, and four days before the accident purchased his old Baron again. In the interval, the airplane had been modified with more powerful engines — 300 hp rather than the original 285 — four-blade props, winglets, and vortex generators on the wings and vertical stabilizer.

The purpose of the vortex generators was to lower the minimum control speed by delaying the stall and improving the effectiveness of the rudder. Normally, the minimum control speed of the 58 is 81 knots; the VGs dropped it to 74, and the modification included replacing the original airspeed indicator with one marked to reflect the new, lower Vmc.

The more powerful engines were installed later. One of the consequences of the added power was an increase in Vmc, which rose from 81 to 87 kias on the stock airplane. In the absence of flight test data, the NTSB noted that the Vmc of the VG-equipped airplane was likely to have been similarly increased by the added engine power; but the holder of the engine-­replace-ment STC was not required to determine a new Vmc for every Baron variant, only for the standard model. The STC instructions did include a direction to “mark instruments in accordance with Flight Manual Supplements for the appropriate model,” but it was probably unclear to the installer what the instruction meant in the particular case of the VG-equipped Baron, and the FAA provided no guidance; so the airspeed indicator with the red Vmc line at 74 knots remained unchanged.

The Baron’s radar track, as well as the statements of witnesses and the evident purpose of the flight, suggested that it was performing a Vmc demonstration at around 4,000 feet when the spin occurred. The maneuver would involve reducing power on the left engine to idle, bringing the right engine to full throttle, and gradually reducing speed until it was no longer possible to hold heading.

As far as NTSB investigators could determine, the pilot may not have flown a reciprocating-engine airplane for three years prior to the accident. They noted too that he was required to have a second pilot with him in the Citation, which is eligible to be flown single-pilot, because he did not meet single-pilot proficiency requirements. There were indications that his shortcomings were related to speed control, which could, of course, be involved in a Vmc-related mishap.

A possible contributing factor was the airplane’s throw-over control column, which might have impaired the instructor’s ability to prevent the spin or recover from it. Another contributing factor was “the lack of guidance by the Federal Aviation Administration for an installer of a supplemental type certificate (STC) modification to determine the interrelationship between all STCs incorporated into an aircraft.” At the end of 2011, the FAA partially remedied the omission with an airworthiness directive (AD) mandating that, in the absence of other information, the Vmc for the VG-plus-engines mod would be the VG-equipped Vmc multiplied by the ratio between the re-engined Vmc and the stock Vmc — in other words, 74 x (87/81), or 79.5 kias.

It should be noted that Vmc-related spins are by no means unheard of in the stock Baron or in its precursor the Travel Air, and so the fact that this Baron had a misleadingly marked airspeed indicator, while suggestive, cannot be absolutely linked to the loss of control.

The Baron had been modified with only two STCs (the winglets and props came with the engines). The ­Skymaster whose wing failed after the low pass in New Jersey had 22. Many of them were unrelated to the accident, but those that the NTSB thought might be related included span-extending tip tanks, winglets and a STOL kit that included stall fences and drooped leading edges on the inboard wing panels.

There were five people in the airplane, and it had just taken on 90 gallons of fuel. NTSB investigators calculated that the weight of the airplane was over gross, most likely by several hundred pounds, and that its CG was significantly behind the aft limit. The overload was concentrated in the fuselage, increasing the bending stresses in the wing.

Again, the NTSB identified airspeed indicator markings as a factor. The wingtip extension STC required a placard reducing the maneuvering speed to 117 kias when the tip tanks were empty. How much fuel, if any, was in the tip tanks is unknown, but at any rate that placard was absent; the placard in the airplane instead gave a maneuvering speed of 135 kias. The low pass was conducted at about 160 knots. The NTSB said that the airspeed had “exceeded this operating limitation,” a judgment that seems to betray a misunderstanding of the concept of maneuvering speed. The maneuvering speed is the speed below which a rapid, full application of the pitch control should not overload the structure. There is nothing in it to suggest that you cannot legally and safely make a pass over a runway at any speed right up to Vne. The critical issue is the nature of the pull-up, not the speed of the pass.

In principle, the wing should have withstood its design ultimate load factor of 5.7 G’s for at least several seconds without breaking, or somewhat less because of the cabin overload. If the pilot really pulled that much G, the aft CG location could have had a role; control forces get lighter as the CG moves aft, and dangerously light when it’s well beyond the aft limit.

A witness said that the pilot had made low passes “followed by rocking the wings or pitch-up maneuvers” on several previous occasions — behavior which the NTSB characterized as “consistent with an ostentatious display.” Now, many pilots make low, fast passes over runways, and while their purpose may be “ostentatious display,” it is certainly not tantamount to “reckless flying.”

There was more to the story. Investigators found indications of abnormal skin fatigue in the wing and attributed them to “vibratory stresses” related to unspecified interactions among the airplane’s many wing modifications. Field reports on similarly modified airplanes revealed other instances of wrinkled skins, cracks, loosened rivets and “loose wingtips.” The FAA advised operators to have their wings inspected but did not consider that the situation warranted an AD.

STC certificates include a piece of boilerplate that is supposed to deal with interactions of aftermarket modifications: This approval should not be extended to other aircraft of these models on which other previously approved modifications are incorporated unless it is determined by the installer that the interrelationship between this change and any of those previously approved modifications will cause no adverse effects on the airworthiness of the aircraft. How the installer is supposed to make such determinations is not explained.

Postscript
In the April issue, this column discussed an accident involving a stall-spin following a steep climb performed in a gusty wind. The NTSB report (CEN11FA304) cited witnesses who said that they had seen the pilot, Pete Vinton, flying in a reckless manner — specifically, performing "downwind departure stalls at 200 feet" — in similar conditions the day before.

Several friends and associates of the pilot have petitioned the NTSB to amend the report. In particular, Dagmar Kress, a former member of the German national aerobatic team, and Brian DeVandry, a retired United Airlines 747 captain, state that they were in the airplane with Vinton during the flights the day before, that no departure stalls took place, that no maneuvers were performed at inappropriate altitudes, and that there was nothing reckless about the pilot’s flying then or, to their knowledge, at any other time.

If the NTSB had interviewed these witnesses, the general impression created by its report would have been different.

This article is based on the NTSB’s report of the accidents and is intended to bring the issues raised to our readers’ attention. It is not intended to judge or to reach any definitive conclusions about the ability or capacity of any person, living or dead, or any aircraft or accessory.