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On March 19, 2011, about 1140 Pacific daylight time, a Beech G35, N4211D, was substantially damaged during a recovery from an unintended unusual attitude near Whidbey Island, Washington. The certificated private pilot/owner was not injured. The personal flight was operated under the provisions of Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and no flight plan was filed for the flight.
According to the pilot, he was on a personal flight, cruising at an altitude of 4,500 feet, and was engaged in correlating the indications of two panel-mounted navigation receivers in the cockpit. He also had a third navigation receiver, a handheld global positioning system (GPS) mounted on the flight control yoke assembly, in the lateral center of the cockpit. During the correlation effort, the pilot noticed that he was about to intrude into the restricted airspace for Whidbey Island, designated R-6701. He initiated a turn to the right, with a bank angle that he estimated to be about 45 degrees, in order to avoid the restricted airspace. During the turn, the pilot referred to the GPS to ensure that he would clear the restricted area. When he returned his attention to the airplane's attitude, he noticed that the bank angle had increased to about 75 degrees right wing down, and the pitch attitude had decreased to about 20 degrees airplane nose down. At that point, the pilot noted that the airspeed was about 190 mph, which was in the yellow (caution) range of the airspeed indicator scale.
The pilot stated that he leveled the wings, and then initiated a pull-up. During the pull-up, he heard three or four "thumps" in rapid succession. After recovery to level flight, the airplane continued to "fly fine," but the pilot was concerned about the thumps, since he had never heard noises like them in that airplane. The pilot then flew the airplane "gingerly" back to his home field, where he landed uneventfully. After shutdown, the pilot examined the airplane, and noticed that some aft fuselage skins were wrinkled and/or torn.
The airplane and airplane maintenance records were retained for additional examination.
Federal Aviation Administration (FAA) records indicated that the pilot held a private pilot certificate with airplane single-engine, airplane multi-engine, and instrument airplane ratings. According to information provided by the pilot, he had approximately 1,188 total hours of flight experience, which included approximately 592 hours in the accident airplane make and model. His most recent flight review was completed in June 2010, and his most recent FAA third-class medical certificate was also issued in June 2010.
According to FAA information, the airplane was manufactured in 1955, and was first registered to the pilot in 1990.
The airplane was equipped with a V-tail configuration instead of the more common cruciform arrangement. The airplane was equipped with the post-delivery capture fairings known as "cuffs" which attached the stabilizers' inboard leading edges to the fuselage. Each of the two stabilizers was equipped with a moveable control surface called a ruddervator, and each ruddervator was equipped with a cockpit adjustable trim tab. Each ruddervator functioned as part rudder, part elevator. Ruddervator inputs were via conventional cockpit controls (rudder pedals and control wheel/column), and actuation was via control cables and a mixer unit known as the differential mechanism. One cockpit control operated both trim tabs.
Review of maintenance records indicate that the ruddervators were reskinned and rebalanced in January 1997. The logbook entry for that activity did not explicitly state whether the control system was rerigged, or whether the control cable tensions were checked or adjusted. The records indicated that Airworthiness Directive (AD) 94-20-04-R2 was accomplished in February 2004, when the airplane had accrued a total time in service (TT) of about 2,271 hours. That AD required verification of ruddervator balance, visual inspection of the empennage, aft fuselage, and ruddervator control system for damage, and proper adjustment of the ruddervator system surface travel, cable tensions, and rigging. There were no logbook entries that documented any subsequent cable tension checks or adjustments, control surface travel checks, or control system rigging.
As of its most recent annual inspection in August 2010, the airplane had a TT about 4,257 hours. As of the accident, the airplane had a TT of about 4,261 hours.
The 1153 automated weather observation at the pilot's home airport, located about 10 miles south-southeast of the accident location, included winds from 180 degrees at 7 knots; visibility 10 miles; scattered clouds at 3,200 feet; temperature 8 degrees C; dew point 1 degree C; and an altimeter setting of 29.75 inches of mercury.
AIDS TO NAVIGATION
The Sectional Aeronautical Chart for Seattle graphically depicted R-6701. The tabular chart data indicated that the area extended up to an altitude of 5,000 feet above mean sea level (msl), and that it's "Time of Use" was "intermittent by NOTAM" with 2 hours' advance notice. The chart indicated that the "Controlling Facility" was the Whidbey Island Naval Air Station air traffic control tower (NUW ATCT), and provided the appropriate communications frequency for the NUW ATCT.
The airplane was equipped with two panel-mounted very high frequency omni-range (VOR) instrument heads, which the pilot was trying to correlate with one another in flight. Those instruments do not enable a pilot to readily determine the boundaries of the restricted area R-6701. The pilot's handheld GPS did depict the boundaries of R-6701. The boundaries of R-6701 could also be approximated by visual dead reckoning when cross-referenced with the Sectional Aeronautical Chart for Seattle, provided the landforms were not obscured by clouds.
According to the pilot, Restricted Area R-6701 was "normally not active on Saturday" but he had not confirmed the operational status of R-6701 prior to or during the accident flight. There was no evidence to indicate that the pilot checked NOTAMs prior to the flight, or that he was monitoring or in communication with an air traffic control facility at the time of the event. The investigation did not determine whether R-6701 was active at the time of the event.
WRECKAGE AND IMPACT INFORMATION
Three days after the event, Federal Aviation Administration (FAA) inspectors examined the airplane in the pilot's hangar. The inspectors noted three primary damage sites. On the aft right fuselage, a diagonal wrinkle about 2 feet long extended up and forward from the juncture of the fuselage side and bottom; the wrinkle intercepted the juncture at about the second bulkhead/former forward of the tailcone. On the opposite side of the aft fuselage, the skin was crumpled and dented in the same general region as on the right side, but the deformation did not exhibit the linear pattern observed on the right side. On the lower aft fuselage, the forward bottom skin was separated from its lap joint with the aft bottom skin at the aforementioned bulkhead/former; the skin was torn from the fasteners, which remained in the bulkhead. The FAA inspectors did not observe any indications of pre-event damage or corrosion in the affected areas. A cockpit g-meter that was operational during the event registered a maximum of about 2.5g and a minimum of about minus 0.7g, but the accuracy of the meter was not determined.
About 6 weeks after the event, the airplane flight control system and damage sites were examined in detail. One additional damage site was observed, in the form of a small buckle in the lower outboard skin of the left ruddervator. There was no visible damage to any other portion of the aft stabilizers or control surfaces, and no damage to the internal bulkheads was discovered (contrary to the pilot's initial report). No discrepancies with the routing, condition, or operation of the aft control surface cables or associated hardware were observed.
The control surface (ruddervator and tabs) travel deflections (as activated by cockpit controls) were measured, and those values were compared to the design specifications. Most measured control surface travels were within manufacturer's limits. Discrepant values included the following:
- Up elevator travel less than specification (left 2.1 degrees less; right 2.9 degrees less)
- Down elevator travel greater than specification (right 1.1 greater)
- Down elevator tab greater than specification (left 1.5 degrees greater; right 0.1 degrees greater)
In addition, the control surface travel ranges were not symmetric between the left and right sides. Refer to the accident docket for additional information.
Airplane Certification Basis and Limit Loads
The airplane certification basis was Civil Air Regulations (CAR) Part 3 effective November 1, 1949, and including paragraph 3.112C of Amendment 3-4. According to CAR 3, the "Normal" category maneuver limit load factor requirement was 2.1, with a factor of safety of 1.5, for an ultimate limit load factor of 3.15. The "Utility" category maneuver limit load factor requirement was 4.4, with a factor of safety of 1.5, for an ultimate limit load factor capability of 6.6.
V-N or V-G Diagram
The FAA Pilot's Handbook of Aeronautical Knowledge (FAA- H- 8083-25) provided explanatory information regarding aircraft structural strength and the flight envelope. Significant highlights are presented in the following paragraphs.
The flight operating strength of an airplane is typically presented on a graph called a V-N or V-G diagram; airspeed (velocity, "V") is the abscissa and load factor ("N" or "G") is the ordinate. Each airplane model has a unique V-N diagram which is defined by the certification criteria and the airplane design. Certain points on the V-N diagram correspond to or define key operating airspeeds, which are intended to enable pilots to avoid structural damage to the airplane due to flight loads.
The intersection of the Normal category positive limit load factor (in this case 3.15) and the line of maximum positive lift capability yields an airspeed at which the limit load can be developed aerodynamically; operations at airspeeds greater than that provide the capability to damage the airplane. That speed is "maneuvering speed" or "Va." An airplane operating below Va cannot produce a damaging positive flight load. Conversely, an airplane operating above Va can develop aerodynamic loads sufficient to induce structural damage or failure; those loads can be introduced by control surface deflections (flight control inputs), turbulence/gusts, or some combination of pilot and atmospheric inputs. Va for this airplane was 130 mph.
The maximum structural cruising speed (Vno) is the rightmost (highest speed) boundary of the normal flight envelope that has the positive limit load factor as its upper boundary. Vno is depicted as the boundary of the green and yellow arc on the airspeed indicator, and the yellow range is referred to as the "caution range." Operations at speeds above Vno can induce structural damage at loads below the positive limit load. Vno for this airplane was 175 mph.
The FAA Handbook stated that "The airplane must be operated within this 'envelope' to prevent structural damage and ensure that the anticipated service lift of the airplane is obtained. The pilot must appreciate the Vg diagram as describing the allowable combination of airspeeds and load factors for safe operation. Any maneuver, gust, or gust plus maneuver outside the structural envelope can cause structural damage…"
According to the pilot's written report of the event, he estimated that he applied about 2g to the airplane at an airspeed of about 190 mph, which the pilot stated was above the maximum structural cruising speed of 175 mph; this was about 0.5g less than the value registered by the cockpit-installed g-meter. No information regarding the pilot's ability to accurately estimate g-loads was presented. Correlation of that information with the V-N envelope indicated that structural damage due to flight loads was possible or likely.
Pilot's Planning, Decision-Making, and Prioritization
The FAA Pilot's Handbook of Aeronautical Knowledge provided explanatory information regarding workload management and situational awareness. Significant highlights are presented in the following paragraphs.
The Handbook stated that "effective workload management ensures that essential operations are accomplished by planning, prioritizing, and sequencing tasks to avoid work overload." The Handbook stated that in order for pilots to manage workload, "items should be prioritized. During any situation, and especially in an emergency, remember the phrase 'aviate, navigate, and communicate.' This means that the first thing the pilot should do is to make sure the airplane is under control" before attention is diverted to other tasks.
The Handbook defined situational awareness as "the accurate perception of the operational and environmental factors that affect the airplane, pilot, and passengers during a specific period of time." It then expounded with "Maintaining situational awareness requires an understanding of the relative significance of these factors and their future impact on the flight. When situationally aware, the pilot has an overview of the total operation and is not fixated on one perceived significant factor. Some of the elements inside the airplane to be considered are the status of airplane systems, and also the pilot and passengers. In addition, an awareness of the environmental conditions of the flight, such as spatial orientation of the airplane, and its relationship to terrain, traffic, weather, and airspace must be maintained."