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Off Roading in a 747

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This article originally appeared in IFR magazine, Mar. 2005.

Safety

Accident reports can make enlightening reading, but some of the NTSB reports from prior to 1983 are a bit cryptic. Like this one, from a Boeing 747 accident at Chicago O'Hare:


DCA79AA009

TYPE OF ACCIDENT: COLLISION WITH AIRCRAFT, BOTH ON GROUND

PHASE OF OPERATION: LANDING ROLL

PROBABLE CAUSE(S): TRAFFIC CONTROL PERSONNEL ISSUED IMPROPER OR CONFLICTING INSTRUCTIONS

FACTOR(S): TRAFFIC PERSONNEL. INADEQUATE SPACING OF AIRCRAFT

MISCELLANEOUS: EVASIVE MANEUVER TO AVOID COLLISION

REMARKS: NEAR COLLISION. APCH CONTROLLER FAILED TO EFFECT ADEQUATE SPACING BTN THIS ACFT & PRECEDING ACFT.

Confused? I would be, if I hadn't been there that day. The "type of accident" entry notwithstanding, there was no collision; as is noted later on, it was an evasive maneuver to avoid collision that caused the accident. In other words, this was a runway incursion accident -- which begs the question: Why did an Approach controller take the hit for an incident that occurred on the ground? Aren't the Local (tower) and Ground controllers responsible at that point? Here's the inside story; you decide.

The view from the OíHare tower on a good day.

It was an RVR day, visibility varying to less than a mile in fog, and Approach Control was busy, vectoring steady streams of arrivals to Runways 9L and 9R. Controllers in the tower were busy, too, as dozens of airplanes milled around the airport waiting for gate space, while others slogged through the fog on their way out to a runway for departure.

One of those departures was a Delta Air Lines Boeing 727, taxiing on the 14R/32L parallel taxiway (now known as taxiway T). In accordance with their taxi clearance, its crew (joined by a Chicago Center controller hitching a ride in the cockpit jumpseat) stopped short of Runway 9R and awaited instructions to cross the runway. The Ground controller, squinting through the tower windows, was barely able to make out a DC-10 as it rolled out on 9R through the fog; he then glanced at the BRITE (radar) scope and saw that the next arrival was an L-1011 at the outer marker. As he watched the target of the DC-10 roll past Taxiway T on the ASDE (ground radar), he cleared the Delta 727 to cross Runway 9R.

Based on the information available and the procedures in place at the time, this was a perfectly reasonable action. Ground controllers were authorized to cross active runways using an ATC variant of see-and-avoid, referred to as "look and go." Look and go required that the controller use all his tools -- the windows, the BRITE, and the ASDE -- to ascertain that the runway was clear before issuing a runway-crossing clearance. This Ground controller not only did all that; but had further reason to believe the decision sound, based on knowledge of the rules of his trade: A "heavy" jet had just landed, and there was another heavy less than five miles from the runway threshold. Since the minimum in-trail separation between heavy jets is four miles, there could be no airplanes between the DC-10 and the L-1011.

Except, of course, there was. Unbeknownst to the Ground controller, the Approach controller had stuffed a Flying Tiger Boeing 747 between the DC-10 and the L-1011. The 747 was, of course, far too close behind the DC-10, and should have been pulled off the final or issued an early go-around by Local. He obviously wasn't, although it had been considered. The Approach controller and the Local controller had a discussion on the landline about the situation, but ultimately decided that (in the best O'Hare tradition) they could "make it work." They did, too, in that the DC-10 was clear of the runway when the 747 crossed the threshold. Unfortunately, just as it was doing so, the crossing 727 was just entering the runway from the left. (The taxiway/runway intersection is configured in such a way that the 747 was approaching from a blind spot 50 degrees off the 727's right rear, in fog). The 747 touched down and its crew immediately found themselves about to T-bone the 727. Too fast to stop, too slow to fly, the captain exercised the only option available -- he swerved hard right, and took the jumbo jet off-roading.



Swerving to the right to miss the 727, the 747 stops in the mud, shedding gear and engines along the way.

Chicago was in the midst of a spring thaw, and as soon as the nosewheel left the pavement, it sank into the mud and snapped off. Shedding the right main gear and both right side engines as it slid, the behemoth stopped not far short of Lake O'Hare, without hurting a hair on the heads of any occupants of either airplane. The Center controller in the 727 cockpit (no longer nearly as impressed with the skills of his O'Hare colleagues as he once had been) mentioned later that he was able to see the individual rivets in the cowling on the #1 engine of the 747, as it passed mere feet in front of the windshield and the wing passed directly overhead.

Why hadn't the 747 appeared on the BRITE radar when the Ground controller checked? Because the antenna for the radar system is located on the western edge of the airport, creating what pilots might call a cone of silence (and controllers call the main bang) that was later documented to extend almost two miles beyond the end of runway 9R. The ASDE was similarly blind as it detects aircraft on the surface and within the airport boundary. The 747 was still in the air and outside the boundary at the time the Ground controller checked. The windows, of course, were nearly useless due to the fog, so a system that had once been thought to have triple redundancy turned out to have no safety net at all. The gargantuan 747 was, at the critical moment, invisible to both the human eye and current technology.

The end result was multiple millions of dollars in damage and, of course, changes to air traffic procedures. The immediate reaction was to prohibit runway crossings by Ground controllers, for any reason, ever. That quickly proved to be unworkable (due to the increased workload on Local controllers) and in due course the rule was amended to allow crossings by Ground controllers, if the runway is closed, or if the Ground controller had specifically requested and received permission to cross from the involved Local controller. As noted in the NTSB report, the Approach controller was officially blamed for this accident. He went through the standard de- and re-certification exercises, and was back at work (undoubtedly with a much firmer understanding of the importance of conforming to separation standards) within days.

You might expect that, since it was controller error that lead to this accident, the pilots would be absolved of responsibility. Not quite -- the 727 side of the NTSB report has an additional line in the Probable Cause section, one not found on the 747 report:

PILOT-FAILED TO MAINTAIN VIGIL FOR LANDING TRAFFIC.

Fair? You decide.


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