Probable Cause #14: The "Dirty Harry" Vector

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This article originally appeared in Aviation Safety, June 2004.

Probable Cause

When we're not accustomed to operating in high-density areas, flying into a busy terminal in a single or light twin can be unnerving. It's busy airspace, and everyone expects a certain level of proficiency. Planning ahead and refusing to be intimidated into doing something with which we're not comfortable are keys to success. Sometimes, though, the willingness to project a "can-do" attitude for the controllers and others on the frequency can have drastic results.

Whether a result of peer pressure, a desire to demonstrate that we have the "right stuff," or overconfidence in one's abilities -- or all three -- we often try to "make ends meet" when the idea of nice, stable, constant-rate decent to a pattern or initial approach altitude gets all FUBARed by weather, traffic, a late handoff or a controller's overoptimistic idea of our steed's abilities.

Clint Eastwood, acting as Harry Callahan, said it best: "A man's got to know his limitations." It's also helpful to know the aircraft's limitations. Even then, knowing them and willingly pushing them -- or exceeding them -- can put us in an aerodynamic no-man's land, an area where we become test pilots and the results are unknown.

Flight History

Shortly before noon on March 31, 2002, a Piper Seneca II experienced an in-flight break-up while being vectored for the ILS Runway 5R approach at Raleigh-Durham International Airport, N.C., (RDU). Both aboard the Piper twin, including the 619-hour instrument-rated private pilot with 410 hours in multi-engine airplanes, were killed.

The flight had originated in Brunswick, Ga., about two hours earlier. Weather at RDU was reported as 500 feet broken, 5,500 feet overcast, with visibility of 1-1/2 miles in mist. Surface winds were from the northeast at 7 knots.

Timeline

At 11:42:50, the Seneca was approximately 30 miles south-southwest of RDU at 9,000 feet. Less than three minutes later, at 11:45:29, it was approximately 22 miles south-southwest with a groundspeed of 186 knots when the controller cleared the pilot down to 4,000 feet with a 10-degree left turn. Eighty-nine seconds later, when the controller instructed the pilot to fly a 230-degree heading, the Piper's Mode C indicated 8,000 feet and a groundspeed of 203 knots.

At 11:48:49, with the Piper approximately 13 miles southwest of RDU, the controller asked, "[A]re you gonna make the descent? I'm turning you into the final in about 3 to 4 miles." At this point, the accident flight's Mode C indicated 7,100 feet at a groundspeed of 164 knots. The pilot acknowledged that he would expedite his descent. A scant 50 seconds later, the airplane was approximately 12 miles southwest of RDU at 5,400 feet with a groundspeed of 191 knots when the controller cleared the pilot for the approach, "4 miles from the final approach fix."

Predictably, at 11:50:24, the controller observed the Seneca going through the localizer and instructed the pilot to turn left to a heading of 90 degrees to rejoin it. The pilot acknowledged the transmission. At this point, Mode C indicated 4,100 feet at a speed of 211 knots. However, by 11:50:29, there was no Mode C readout; at 11:50:47, the data block went into "coast" status, indicating ATC radar was no longer receiving the transponder.

Aftermath

Wreckage was scattered over three-quarters of a mile through a wooded and residential area. What failed first is not clear from the NTSB factual report, but some of its verbiage seems to indicate the stabilator as the culprit. For example, portions of the stabilator were not found, including balance weights, and both the vertical stabilizer and the aft tiedown area had paint marks on them, presumably from contact with the stabilator.

The cabin and empennage sections of the fuselage and the vertical fin were found upside down with both engines nearby; the nose cone was found 530 feet northwest of the fuselage. The flap handle was found in the flaps-up position. The landing gear lever was found in the down position; the main gear was in the down and locked position.

In the Cockpit

It's not hard to figure out what caused the airframe to fail: exceeding the airplane's airspeed limitations, namely Vne, the do-not-exceed speed, represented by the red radial line on the airspeed indicator. It's also clear that the pilot knew he was well into the yellow arc and flirting with the red line.

Remember the radar data? At 11:45:29, the flight was level at 9,000 feet, trucking along at 186 knots over the ground, only 22 nm from RDU and only some 12 nm from the point at which the controller attempted to turn it onto the localizer. Exactly five minutes later, at 11;50:29, the Mode C readout had ceased. Five seconds before the break-up, the Seneca had been at 4,100 feet, doing 211 knots. In other words, for the preceding five minutes, the Seneca had averaged a 1,000-fpm descent. But that was the average. Probably realizing he was high and hot, the pilot had slowed long enough to drop the gear -- this occurred around 11:48:49, with the Seneca doing only 164 knots over the ground at 7,100 feet. During the next 118 seconds, with the gear down, things began to happen even faster: The flight accelerated to as fast as 211 knots and had descended from 7,100 feet to 4,100, for a descent rate of at least 1,500 fpm.

The NTSB found it significant to note the airplane's operations manual stated the Vne speed for this airplane was 195 knots. In addition was the following: "Do not exceed this speed in any operation."

Because we can't determine the winds and temperatures aloft from the NTSB report -- nor can we determine if there was any turbulence or wake vortices -- it's not a certainty the Seneca's pilot pushed it beyond these limits. But, we have the gear down, a high rate of descent and the airspeed well up into the yellow arc. Throw in a turn to intercept the localizer, and what happens next puts us in test-pilot territory and calls to mind that immortal line from Harry Callahan: "Do ya feel lucky? Well, do ya?"


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