June 1998 Volume 41 / Number 6
Landmark Accidents: Cleared for the Approach
One accident led to many changes
Controlled flight into terrain, or CFIT, in instrument conditions is a leading cause of accidents for both the airlines and general aviation. By definition, the pilot believes the flight not to be in jeopardy or has an uneasy feeling that something is not right — but is not successful in resolving it before the impact. His-tory continues to repeat itself, so perhaps a review is appropriate.
A classic CFIT accident that occurred 24 years ago changed the way we handle IFR flights, resulted in better charting, and added a glossary to the Aeronautical Information Manual. Some aspects of the accident were so disturbing that, to this day, pilot examiners and flight instructors drill new IFR pilots on what the phrase "cleared for approach" means.
On December 1, 1974, TWA Flight 514, a Boeing 727-231 (stretch), was en route from Indianapolis to Washington's National Airport. An intense early winter storm system was moving up the East Coast. Crosswinds at National were too much for the north-south runway, and flights were being diverted to Washington Dulles International Airport's Runway 12. Weather observations at National showed a 1,200-foot overcast, with visibility better than five miles in light rain. Surface winds were from 070 degrees at 25 to 28 knots, with gusts as high as 49 knots. Winds at Dulles were substantially the same as at National, with a ceiling of 900 feet and visibility of three to seven miles in rain.
Weather radar showed large areas of precipitation, including some widely scattered embedded thunderstorms. Cloud tops at the accident site were 24,000 feet. There were sigmets for thunderstorms with tops to 40,000 and moderate to severe mixed icing in the clouds. Numerous pilot reports had been received, although none were specifically for the accident area. It was a grungy day to fly.
At 10:42 a.m., Cleveland Center cleared Flight 514 to Dulles via the Front Royal VOR and to maintain Flight Level 290. At 10:43 the flight was cleared down to FL 230 and to cross 40 miles west of Front Royal at that altitude. The flight was handed off to Washington Center at 10:48. During this time the captain turned over control of the flight to the first officer and they discussed the Runway 12 VOR/DME instrument approach into Dulles. The cockpit voice recorder reveals that the crew discussed the various routings they might receive from ATC, such as via the Front Royal VOR, Martinsburg VOR, or proceeding on a "straight-in" clearance.
At 10:51 the center controller gave the flight a heading of 090 to intercept the 300-degree radial of the Armel VOR, to cross a point 25 miles west of Armel at 8,000, and "...the three-zero-zero radial will be for the VOR approach to Runway One-Two at Dulles, altimeter two-niner-point-seven-four." The crew acknowledged. Cockpit voice recordings (CVR) showed that the VOR was tuned and altimeters properly set.
At 10:57 the crew again discussed the approach, including Round Hill intersection, the final approach fix, VASI, runway lights, and the airport diagram.
At 11:01 the flight was cleared to 7,000 feet and handed off to Dulles Approach Control. Dulles cleared it to proceed inbound to Armel VOR and to expect the VOR/DME approach to Runway 12. At 11:04 the flight reported level at 7,000, and five seconds later the controller said, "TWA Five-Fourteen, you're cleared for a VOR/DME approach to Runway One-Two." The captain acknowledged this.
The following is the CVR transcript. Voices are identified thus: Capt — Captain, FO — First Officer, and FE — Flight Engineer. The National Transportation Safety Board did not quote all items verbatim except what is considered pertinent. The "nonpertinent" comments here are copied directly from the report.
11:04 Capt: "Eighteen hundred [feet] is the bottom."
FO: "Start down. We're out here quite a ways. I better turn the heat down [probably referring to cabin heat]."
11:06:15 FO: "I hate the altitude jumping around." Then he commented that the instrument panel was bouncing around.
11:06:15 Capt: "We have a discrepancy in our VORs, a little but not much. Fly yours, not mine."
11:06:27 Capt discusses the last reported ceiling and minimum descent altitude and concluded: "...should break out."
11:06:42 FO: "Gives you a headache after awhile, watching this jumping around like that."
11:07:27 FO: "...you can feel that wind down here now."
Capt: "You know, according to this dumb sheet [referring to the instrument approach chart] it says thirty-four hundred to Round Hill — is our minimum altitude." The FE asked where the captain saw that, and the captain replied, "Well, here. Round Hill is eleven-and-a-half DME."
FO: "Well, but...."
Capt: "When he clears you, that means you can go to your...."
Unidentified: "Initial approach."
Capt: "Initial approach altitude."
FE: "We're out of twenty-eight for eighteen."
Unidentified: "Right; one to go." [meaning 1,000 feet more before reaching the supposed level-off altitude of 1,800]
11:08:14 FE: "Dark in here."
FO: "And bumpy too."
11:08:25 Altitude alert horn sounds.
Capt: "I had ground contact a minute ago."
FO: "Yeah, I did too."
11:08:29 FO: "...power on this [expletive]."
Capt: "Yeah, you got a high sink rate."
Unidentified: "We're going uphill."
FE: "We're right there, we're on course."
Two voices: "Yeah."
Capt: "You ought to see ground outside in just a minute. Hang in there, boy."
FE: "We're getting seasick."
11:08:57 Altitude alert horn sounds.
FO: "Boy, it was — wanted to go right down through there, man."
FO: "Must have had a [expletive] of a downdraft."
11:09:14 Radio altimeter warning horn sounds and stops.
11:09:20 Capt: "Get some power on."
Radio altimeter warning horn sounds again and stops.
At 11:09:22 TWA 514 struck the west slope of Mount Weather, about 25 nautical miles from Dulles at an elevation of about 1,670 feet. Seven crewmembers and 85 passengers perished in the crash. There were no survivors.
The flight data recorder showed a continuous descent with little rate variation from 7,000 feet until about 1,750 feet. There were minor altitude excursions from 100 to 200 feet. Airspeed was fairly stable at 230 knots, with fluctuations between 222 knots and 248 knots. Gear, leading-edge devices, and flaps were up at the time of impact. The G-load was constant, ranging from plus or minus 0.2 G to 0.5 G until impact.
At the accident site, witnesses on the ground reported low ceilings with visibilities of 50 to 100 feet, drizzle, and wind estimated at 40 knots with stronger gusts. Possible altimeter errors caused by high wind speeds were calculated, with localized pressure changes, and the National Weather Service estimated that a worst-case scenario with 80-knot winds could result in an altitude indication 218 feet higher than the actual aircraft altitude.
Testimony following the accident indicated that ATC frequently vectored aircraft off published routes and cleared them to descend below altitudes published on the charts. Pilots and controllers both had available published minimum sector altitudes (MSA) within 25 miles of the airport. MSAs provide 1,000-foot obstacle clearance within 25 miles of an airport and are considered emergency altitudes since they do not assure navigation signal coverage. Controllers also made use of minimum vectoring altitudes (MVA), which were not normally available to pilots.
The testimony also indicated that pilots had become so accustomed to receiving assistance from controllers that, unless advised by the controller, they did not know what type of ATC service they were receiving. Often they were unsure of their position relative to terrain. There was considerable debate as to whether the flight was being handled as "radar arrival," which could have put the burden for terrain separation on ATC. The term was not well defined, although in the controller's manual under "radar arrivals," the following guidance was provided: "Issue approach clearance, except when conducting a radar approach. If terrain or traffic does not permit unrestricted descent to the lowest published altitude specified in the approach, prior to final approach descent, controllers shall: 1) Defer issuance of an approach clearance until there are no restrictions or, 2) Issue altitude restrictions with approach clearance specifying when or at what point unrestricted descent can be made...."
FAA witnesses testified that Flight 514 was inbound to Armel by means of the pilot's own navigation, thereby relieving the controller of the responsibility cited above. Judging by the cockpit conversation, it was obvious that the crew thought otherwise, although the captain apparently had a nagging feeling that something wasn't quite right.
The NTSB also reviewed the handling of other arriving IFR flights at Dulles on December 1, 1974. A flight arriving about half an hour before the accident was cleared for the same approach. Because the aircraft was a considerable distance from the airport and was not given an altitude restriction, the pilot requested the MVA, which was provided. The controller then offered a surveillance approach, which the captain accepted.
Six hours after the accident another flight inbound from the southwest asked the controller its position relative to Round Hill intersection. The controller replied that he did not have that on his scope — and the captain, familiar with the terrain west of Dulles, stated that he did not descend until DME indicated 17.6 miles (Round Hill) and the aircraft was established on the final approach course.
The concept of crew resource management had not been formalized at that time, and the discussion between the captain and the flight engineer would probably raise some warning flags on today's flight decks. As noted above, several crews had some confusion and asked for clarification from ATC. This is excellent guidance for GA pilots — when in doubt, start asking lots of questions and be very conservative regarding terrain clearance. Feelings of uncertainty are frequently a premonition of disaster.
As the NTSB conducted research on how the pilot community interpreted the concept of radar arrivals and when pilots were responsible for terrain separation, the only thing that was clear was that there were multiple interpretations. Ironically, as early as 1970, TWA personnel were concerned about what they saw as conflicting information between the Airman's Information Manual and the controller's manual. Both the U.S. Air Force and TWA pointed out to the FAA that "Cleared for the approach" terminology could be misinterpreted unless a specific altitude restriction was included in the clearance.
In its findings the NTSB had plenty of blame for all parties. The Board noted that procedures in the FAA's terminal control handbook were not clear and resulted in TWA 514's being classified as a "nonradar arrival," but the terms "radar arrival" and "nonradar arrival" were not defined. Depiction of Round Hill intersection was not shown in the profile view nor did it contain all of the minimum altitudes associated with the approach procedure. The ATC system was not clear as to what services controllers should provide under the circumstances of this flight. Both military and civilian aviation organizations had shown repeated concern for the possibility of misunderstanding clearances that could lead to premature descent, but the FAA had taken no action.
The NTSB found the probable cause of the accident to be the flight crew's decision to descend to 1,800 feet before the aircraft had reached the approach segment where that minimum altitude applied. However, two of the five-member board dissented, identifying the probable cause to be the failure of the controller to issue altitude restrictions in accordance with the terminal controller's handbook. The pilot was also cited for failure to adhere to the minimum sector altitude depicted on the instrument approach chart and to request a clarification of the clearance.
Several things changed in the aftermath — the FAA directed that all air carrier aircraft be equipped with ground proximity warning systems (GPWS), and the FAA published a change to FAR Part 91, clarifying pilot responsibilities regarding operations on unpublished routes. An incident reporting system was established (NASA's Aviation Safety Reporting System), a pilot-controller glossary was published in the AIM, and chart depictions were improved. Procedures regarding ATC responsibilities during approaches were defined.
The legacy that grew out of TWA 514's loss is a staple of all IFR training and appears in the Aeronautical Information Manual: "When operating on an unpublished route or while being radar vectored, the pilot, when an approach clearance is received, shall, in addition to complying with the minimum altitudes for IFR operations, maintain the last assigned altitude unless a different altitude is assigned by ATC, or until the aircraft is established on a segment of a published route or IAP." It also appears in FAR Part 91.175(I).
Most GA aircraft don't have GPWS, and many pilots are not all that cozy with Part 91 or the AIM. Regardless of rules or procedural protocol, no one should have a greater interest in the height of terrain than the pilot. ATC will be quick to absolve itself, and rightfully so. It is a PIC responsibility that cannot be delegated — ever.
See also the index of "Safety Pilot" articles, organized by subject. Bruce Landsberg is executive director of the AOPA Air Safety Foundation.