At the end of February, the National Transportation Safety Board released the findings of its investigation of the crash of a Bell 206 JetRanger into New York’s East River in October 2011. The helicopter, with five aboard, had just taken off from the riverside East 34th Street Heliport when it began to yaw out of control and fell into the water. It floated for some time inverted, its skids above the surface. The pilot and front-seat passenger escaped, swam to the surface, and then made repeated efforts to dive down, open the rear doors and extricate the three women still in the back seat. The pilot had succeeded in pulling two of the women out when the helicopter sank in 35 feet of water. The two, who were in critical condition after the rescue, died subsequently of the consequences of oxygen deprivation, the second one 33 days after the accident.
The surviving passenger, 71, recalled an erratic motion, an exclamation of dismay from the pilot and the nose of the helicopter striking the water. The inverted cabin — because of the weight of the rotor, helicopters that crash in water always flip over — rapidly filled with murky, turbulent water. He struggled to release his seat belt. He had no recollection of opening the door (the pilot reported that he had pushed him out), but he found himself swimming up toward the light. When he tried to help the women, he felt a limp arm through an open window, but he could not open the door.
The NTSB’s final report on the accident, with its finding of probable cause, was discussed in New York papers because the crash itself had been such a conspicuous event. News accounts emphasized the fact that the helicopter was overloaded, probably because, of the several findings in the report, it was the one most comprehensible to a lay reader. The excess weight was not, in fact, so great that it would have doomed the helicopter, had not more abstruse aerodynamic factors come into play.
The NTSB’s probable cause was as follows:
The pilot’s failure to anticipate and correct for conditions (high gross weight, low indicated airspeed, and a right downwind turn) conducive to loss of tail rotor effectiveness (LTE), which resulted in LTE and an uncontrolled spin. Contributing to the accident was the pilot’s inadequate preflight planning, which resulted in the helicopter being in excess of its maximum allowable gross weight at takeoff.
The pilot, the surviving passenger and the latter’s wife were old friends. They were visiting New York from Portugal on the occasion of their daughter’s birthday, and the pilot had offered to take them on a sight-seeing flight in his helicopter. He learned only after arriving at the heliport that his friends’ daughter and her wife, who had initially been reluctant, had been persuaded to come along. The three women got into the back seat, and a heliport employee assisted them in buckling up and putting on headsets.
The pilot said that the three women reported their weights to him once they had gotten aboard, and that they were around 155 pounds each. He said that he calculated the weight and balance, using weights of 190 and 210 pounds for himself and the male passenger, and that the total load, with fuel, of 1,131 pounds was within limits for the helicopter, whose last weight-and-balance check had given its empty weight, with rather pedantic precision, as 1,915.52 pounds. Its maximum allowable gross weight was 3,200 pounds.
The surviving passenger did not recall the pilot asking the women their weights. They added up, in reality, to considerably more than 465 pounds. Based on information from various sources, the safety board estimated the weight of the front-seat passenger to have been 225 pounds and the total weight of the rear-seat passengers 675.
To complicate the question of the helicopter’s actual weight at takeoff, it was weighed after being lifted out of the river, and the result, which included an unknown amount of water retained in cushions, carpets, and various voids in the structure and power plant, was 232 pounds greater than the weight recorded in the helicopter’s paperwork. Thus, the weight at takeoff could have been as little as 28 pounds, and as much as 260, over the maximum permissible.
A helicopter, like an airplane, can operate above gross weight, but it may not. To put the overload in perspective, the 400 hp 206 is approved for a gross weight of 3,350 pounds for external load work. The back seat is on the center of gravity, so balance would not have been an issue, although the NTSB demurely declined to perform the simple weight-and-balance computation on the grounds that the takeoff weight was not within the weight-and-balance envelope.
The important point in the NTSB finding is not that the helicopter was overloaded, although that may have been a contributing factor; it is that it went out of control because of loss of tail rotor effectiveness, or LTE.
The function of a helicopter’s tail rotor is to neutralize the torque reaction of the main rotor blades; without a tail rotor, the fuselage of a single-rotor helicopter would spin in the opposite direction to the rotor. In addition, the tail rotor is used by the pilot to control the helicopter’s azimuth — the direction it is pointing — and overcomes its natural tendency to weather-vane into a crosswind or tailwind. Unlike an airplane, which can maneuver only in relation to the air mass surrounding it, a helicopter can maneuver in relation to the ground, a moving ship or any other frame of reference it chooses, and so it is subject to a much wider range of wind effects than a fixed-wing airplane is. In a helicopter, a downwind turn, particularly to the right, actually may be dangerous.
LTE occurs in hover or at low airspeed, particularly out of ground effect, when the demand on the engine is highest, and it is related to the direction from which the wind is striking the helicopter. It can be caused by interference from the tip vortices of the main rotor, by the weather-vaning tendency or by a steady left crosswind that produces a doughnut-shaped vortex at the tail rotor — similar to the main rotor condition of “settling with power” — and keeps it from continuously delivering its maximum thrust. The typical sequence of events is that the tail rotor loses authority, the helicopter begins to yaw to the right, and the pilot either does not react quickly enough or does not find sufficient tail rotor power to arrest the rotating momentum. The uncontrolled yaw induces a banking and pitching gyration, and the usual result, since these incidents take place at low altitude, is the helicopter hitting the ground or water.
LTE is not an uncommon NTSB finding. A search for the phrase “tail rotor effectiveness” in a decade’s worth of accident reports yields 72 results. Only five resulted in fatalities, and, of those, four involved Bell 206 helicopters. (The nonfatal accidents showed no such preference as to type.)
Because the East River crash was obviously survivable, the NTSB commented on several safety-related issues that had no particular bearing on it. The pilot said there were five life vests aboard, but investigators found only four — not that it mattered, since all four were in the front compartment and there was no time to don them anyway. Placards were supposed to be affixed alongside the door handles illustrating their positions when open and closed; these had not been installed, but it is by no means certain that the passengers would have noticed them if they had been. The fire extinguisher had last been serviced five years earlier and was almost fully discharged.
The survival factors specialist noted that the exterior door handles were small, flush and unmarked, but did not explicitly associate these characteristics with the failure of the front-seat passenger to open the back door and assist the other passengers.
The NTSB stated that the pilot did not conduct a safety briefing; the pilot said that he did. Whatever the case may be, the outcome of the accident underscores the importance of the preflight briefing of passengers, however tedious we find it when it is delivered to us in an airliner. If the women in the back seat had had fresh in their minds the location of the door handles and how to operate them, perhaps they might have escaped. Perhaps not; but the briefing can help, cannot hurt, and at the very least may focus the pilot’s own attention on the possibility of a takeoff mishap.
_This article is based on the NTSB’s report of the accident 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. _
