June 2001 Volume 44 / Number 6
Landmark Accidents: Bowling Alley Blues
Caught between a thunderstorm and a hard place
In the course of investigating the crash of American Airlines Flight 1420, the NTSB created an animation of the final approach and landing. See it on the NTSB Web site.
Thunderstorms are one of flight's great challenges. There are relatively few accidents, but the storms frequently disrupt schedules and cause flight cancellations. Most pilots have seen what a severe thunderstorm can do to an aircraft, and a lucky few who have been soundly thumped resolve not to get caught again.
The airlines have a huge logistical problem with convective weather because of the interlocking nature of the hub-and-spoke system, which requires aircraft to be at certain places at the appointed hour. The strategy of flying thunderstorms is simple—don't be there when the punch comes. For general aviation, that is easier advice to follow than for the airlines. It's not just the pilot and a few passengers who are inconvenienced, but a whole planeload of customers, company teammates who are depending on you to get the job done, and the need to deliver the aircraft to the gate for the next leg. The captain is under significant pressure, much of it internal, to deliver the goods—safely.
In the high flight levels it is a rare day that a safe hole cannot be found to slide through or around a line. But during takeoff and landing there is no maneuvering room, altitude, or airspeed to trade with a storm.
On June 1, 1999, at 10:40 p.m. Central Daylight Time (CDT), a McDonnell Douglas MD-82 operating as American Airlines (AA) Flight 1420, with 144 passengers and crew, departed Dallas on a routine flight to Little Rock, Arkansas. The flight was delayed for about two hours awaiting the inbound aircraft. The flight crew had been on duty since about 10:30 a.m. and had flown two legs, from Chicago to Salt Lake City and then to Dallas.
The National Weather Service surface analysis chart at 0600Z showed a low-pressure system on the Illinois-Wisconsin border with the cold front stretching across western Illinois southward into southeast Missouri and northern Arkansas, and then southwestward across west-central Arkansas and southeastern Oklahoma into north-central Texas. A squall line extended from Michigan through Indiana and into southern Illinois. A second line was depicted over Arkansas.
Convective sigmets and severe thunderstorm warnings covered the Little Rock area at the time of Flight 1420's arrival. At 10:54 p.m. the AA flight dispatcher sent the following printed message to the crew in the cockpit: "Right now on radar there is a large slot to Little Rock. Thunderstorms are on the left and right, and Little Rock is in the clear. Sort of like a bowling alley approach. Thunderstorms are moving east-northeastward towards Little Rock and they may be a factor for our arrival. I suggest expediting our arrival in order to beat the thunderstorms to Little Rock, if possible." At 11:11 p.m., flight dispatch advised the crew of revised fuel burn. Additional fuel had been added before departure for en route weather deviations and diversion to Nashville or a return to Dallas if needed.
The cockpit voice recorder shows that the crew was clearly focused on the weather as they watched the storms moving in from the west and northwest. The captain was flying the aircraft. CAM-1 (cockpit area microphone) is the captain; CAM-2 is the first officer; RDO is a radio transmission from the aircraft; and APR is Little Rock Approach or the tower controller. An asterisk (*) indicates an unintelligible word and # denotes an expletive. Nonpertinent remarks have been edited out and my comments are in italics.
11:25:43 CAM-1 We got to get over there quick.
11:25:48 CAM-2 I don't like that...that's lightning.
11:25:56 CAM-1 Sure is.
11:26:48 CAM-1 This is the bowling alley right here.
11:26:50 CAM-2 Yeah, I know.
11:26:55 CAM-1 In fact, those are the city lights straight out there.
11:26:57 CAM-2 That's it.
11:29:44 CAM-2 Yeah, that alley's getting big...closing to the west.
11:29:48 CAM-1 Yeah, it is.
11:29:49 CAM-2 * be OK.
11:29:52 CAM-2 I say we get down as soon as we can.
11:31:52 CAM-2 Whoa. Looks like it's movin' this way, though.
11:31:54 CAM-1 Yeah *.
11:32:05 CAM-1 Just some lightning straight ahead.
11:32:11 CAM-2 Think we're gonna be OK. Right there.
11:32:28 CAM-1 Down the bowling alley.
11:34:09 APR American 1420, Little Rock Approach. We have a thunderstorm just northwest of the airport moving through the area now. Wind is 280 at 28, gusts 44, and I'll have new weather for you in just a moment, I'm sure. [The crew acknowledged and said they could see lightning.]
11:34:36 CAM-1 Right near the limit. [The crew then discussed the crosswind limits of the aircraft and decided that 20 knots was the maximum allowable on a wet runway.]
11:39:05 APR American 1420, your equipment's a lot better than what I have. How's the final for [Runway] 22 Left lookin'? [The controller is referring to the aircraft's radar being much better for seeing weather than ATC radar.]
11:39:11 RDO-2 OK, we can see the airport from here. We can barely make it out but we should be able to make [Runway] 22. That storm is moving this way like your radar says it is but a little bit farther off than you thought.
11:39:22 APR American 1420, roger, would you just want to shoot a visual approach? [The captain declined the visual because of poor visual conditions. The controller advised of a wind shear alert at Little Rock and reported the current winds as "center field wind is 340 at 10, north boundary wind is 330 at 25, northwest boundary wind 010 at 15."]
11:42:26 APR American 1420, it appears we have the second part of this storm moving through. The winds now, 340 at 16, gusts 34. [The crew acknowledged. The gust factor exceeded the wet runway crosswind limitation. Because of the wind shift, the crew asked to land on Runway 4 Right, which was approved. They then decided that they could see the airport well enough to land visually, which would be faster than a full instrument approach. Strong gusts and wind shifts indicate the airport was under the influence of the storm. For those of us flying light aircraft, the game is over — we must divert.]
11:43:59 APR American 1420, you can monitor 118.7, Runway 4 Right, cleared to land. The wind right now, 330 at 21.
11:44:05 RDO-2 18.7, we'll monitor, American 1420, thanks. Cleared to land Runway 4.
11:44:13 CAM-2 If you look at....
11:44:14 CAM-1 Those red lights out there. Where, where's that in relation to...?
11:44:18 CAM-2 There's another, there's two runways here. There's three runways.
11:44:19 CAM-1 Yeah, I know. See? We're losing it. I don't think we can maintain visual. [The captain concedes that he can't see the airport.]
11:44:22 CAM-2 ** yeah.
11:44:28 RDO-2 Approach, American 1420.
11:44:29 APR American 1420, yes sir.
11:44:30 RDO-2 There's a cloud between us and the airport. We just lost the field and I'm on this vector here. I have basically the last vector you gave us. We're on kind of a dog leg it looks like.
11:44:39 APR American 1420, can you fly heading 220? I'll take you out for the ILS. [There is some additional discussion about the visual approach.]
11:45:15 CAM-16I hate droning around visual at night in weather without...having some clue where I am.
11:45:23 CAM-2 Yeah, but the longer we go out here the....
11:45:24 CAM-1 Yeah, I know. [The captain acknowledges that he was unsure of the flight's location, and the FO is keenly aware that the storms are very close to the airport and may compromise their arrival if there is any delay, such as accepting an ILS approach.]
11:45:29 CAM-2 See how we're going right into this crap.
11:45:31 CAM-1 Right.
11:45:47 RDO-2 Approach, American 1420, I know you're doing your best, sir. We're getting pretty close to this storm. We'll keep it tight if we have to. [During the next minute, approach control vectors the aircraft to just outside the final approach fix and then advises that the runway visual range for Runway 4 Right has dropped to just over one-half mile.]
11:46:39 APR American 1420 is three miles from the marker. Turn right heading 020. Maintain 2,300 'til established on the localizer. Cleared ILS Runway 4 Right approach.
11:46:47 RDO-2 020 'til established, American 1420, cleared 4 Left approach.
11:46:52 CAM-1 Aw, we're goin' right into this.
11:46:52 APR American 1420, right now we have heavy rain on the airport. The current weather on the ATIS is not correct. I don't have new weather for ya, but the visibility is less than a mile. Runway 4 Right RVR is 3,000.
11:47:04 CAM-1 Three thousand.
11:47:04 RDO-2 Roger that, 3,000, American 1420. This is 4 Right, correct?
11:47:08 APR American 1420, that's correct, sir. Runway 4 Right, cleared to land. The wind 350 at 30, gusts 45.
11:47:10 CAM-1 Can we land? [The captain voiced some uncertainty and probably wanted the FO to check the crosswind chart or RVR.]
11:47:16 RDO-2 030 at 45, American 1420. [The FO misinterpreted the winds, thinking they were aligned with the runway when they were actually 50 degrees off runway alignment. This exceeded the crosswind limitation. The crew then discussed the visibility minimums.]
11:47:19 CAM-2 ** zero forecast right down the runway.
11:47:22 CAM-1 3,000 RVR. We can't land on that. [The captain voices more uncertainty. Actually, the minimum RVR for Runway 4R is 2,400.]
11:47:24 CAM-2 3,000, if you look at uh....
11:47:27 CAM-1 What do we need?
11:47:28 CAM-2 No, it's 2,400 RVR.
11:47:29 CAM-1 OK, fine.
11:47:30 CAM-2 Yeah, we're doing fine.
11:47:31 CAM-1 All right.
11:47:34 CAM-1 Fifteen. [Captain calls for flaps.]
11:47:44 CAM-1 Landing gear down.
11:47:49 CAM-1 And lights ** please.
11:47:53 APR Wind shear alert, center field wind, 350 at 32, gusts 45. North boundary wind 310 at 29. Northeast boundary wind 320 at 32. [These reported winds exceed the wet crosswind limitation of 20 knots.]
11:48:04 CAM-2 Flaps twenty-eight?
11:48:11 CAM-1 Add twenty.
11:48:13 CAM-2 Right.
11:48:13 CAM-1 Add 20 knots. [The captain is carrying additional speed to prepare for a possible wind shear encounter.]
11:48:15 CAM-2 OK.
11:48:13 APR American 1420, the Runway 4 Right RVR now is 1,600. [This is below landing minimums but the regulations allow the approach to continue if the flight is established inside the final approach fix. They can't land if the visibility stays down but in a dynamic situation the weather can improve as quickly as it can deteriorate.]
11:48:18 CAM-2 Aw, #.
11:48:19 CAM-1 Well, we're established on the final.
11:48:21 CAM-2 We're established, we're inbound, right.
11:48:25 RDO-2 OK, American 1420, we're established inbound.
11:48:27 APR American 1420, roger, Runway 4 Right, cleared to land, and the wind, 340 at 31. North wind, boundary wind is 300 at 26, northeast boundary wind 320 at 25, and the 4 Right RVR is 1,600.
11:48:42 RDO-2 American 1420, thanks.
11:48:48 CAM-2 Keep the speed.
11:48:51 CAM-2 Thousand feet.
11:48:55 CAM-1 I don't see anything. Lookin' for 460. [Referring to the msl altitude that is the decision height for landing.]
11:49:01 CAM-2 It's there.
11:49:03 CAM-2 Want 40 flaps?
11:49:05 CAM-1 Oh yeah, thought I called it.
11:49:06 CAM-2 Forty now. 1,000 feet. 20, 40, 40, land. [Probably referring to extra speed margin and flap indication on each wing.]
11:49:11 APR Wind is 330 at 28.
11:49:13 CAM-1 This is, this is a can of worms.
11:49:25 CAM-1 I'm gonna stay above it a little. [Probably referring to glideslope.]
11:49:25 CAM-2 There's the runway off to your right, got it?
11:49:27 CAM-1 No.
11:49:28 CAM-2 I got the right runway in sight.
11:49:31 CAM-2 You're right on course. Stay where you're at.
11:49:32 CAM-1 I got it, I got it.
11:49:33 APR Wind 330 at 25.
11:49:38.6 CAM-? Wipers.
11:49:42.3 CAM [Sound similar to windshield wiper motion.]
11:49:47.3 CAM-2 Five hundred feet.
11:49:54.6 CAM-1 Plus twenty. [Referring to extra 20 knots of speed.]
11:49:54 APR Wind 320 at 23.
11:49:57.5 CAM-? Aw, #, we're off course.
11:50:01.4 CAM-2 We're way off. [Referring to the localizer.]
11:50:02.5 CAM-1 I can't see it.
11:50:05.4 CAM-2 Got it? [Probably referring to the airport.]
11:50:06.1 CAM-1 Yeah, I got it. [There are some altitude callouts just prior to landing.]
11:50:21.2 CAM [Sound of two thuds similar to aircraft touching down on runway concurrent with unidentified squeaking sound.]
11:50:23.2 CAM-2 We're down.
11:50:25.4 CAM-2 We're sliding.
11:50:27.1 CAM-1 #...#.
11:50:32.9 CAM-? On the brakes.
11:50:34.2 CAM-? Oh, #....
11:50:34.6 CAM [Sound similar to increase in engine rpm.]
11:50:36.2 CAM-? Other one, other one, other one.
11:50:42.0 CAM-? Aw, #.
11:50:42.7 CAM-? ##.
11:50:44.9 CAM [Sound of impact.]
11:50:45.4 CAM-? ##.
11:50:48.0 CAM [Sound of several impacts.]
11:50:49.1 End of recording.
The driver of a large tractor-trailer passing the airport at about 25 mph reported torrential rain when a jetliner crossed in front of his truck from left to right at an estimated altitude "of no more than 10 to 20 feet" above the top of his truck. The truck in front of him was blown from the center lane into the left lane just prior to the aircraft's passing overhead. The driver also reported intermittent golf ball-size hail and "almost continuous lightning."
At 11:51 p.m. AA1420 overran the end of Runway 4R and collided with the approach light stanchion. The captain and 10 passengers died; the remaining 134 passengers and crewmembers sustained various injuries.
According to the NTSB Operations Report, the FO gave his recollection of the last few seconds on the approach, the touchdown, and the rollout. The approach was stabilized until about 400 feet agl, when the aircraft drifted to the right of course, and the FO estimated that they were displaced "about a runway width" to the right. "We were going right of course and I thought it's getting more and more difficult to handle. So I thought it would be safer to go around." The FO said that he thought he said "Go around" at that time. He looked at the captain to see if he had heard him, but the captain was intent on flying and was doing a "good job," except "his azimuth was off." The FO thought to himself that "the captain knows what he is doing and he was flying the airplane."
There was a significant difference in experience level between the pilots. The captain was a check airman on the MD-80 and the chief pilot at the Chicago base. He had total flying time of 10,234 hours with MD-80 pilot-in-command time of 5,518 hours. In contrast, the FO had total flying time of 4,292 hours with second-in-command MD-80 time of 182 hours. This may have accounted for his previous comment about the captain's being on top of things. The FO was a relatively new hire at American and may have been reluctant about voicing concerns to a senior captain, although other pilots described the captain as "someone who does the right thing when no one's looking" and not intimidating to fly with.
The captain elected to use manual brakes for landing in place of autobrakes. The touchdown was on the centerline and "not that far down the runway." At touchdown, the main gear was to the right of the runway centerline and the nose of the airplane was pointed left. The FO said the touchdown was "sort of flat, sideways, and it was violent."
The captain selected reverse thrust immediately and the FO said that "he really honked on it," with engine pressure ratios (EPRs, or power output) of 1.6 to 1.8. The AA DC-9 (MD-80) operating manual stated in part: "The application of reverse thrust tends to blank out the rudder. The effectiveness of the rudder starts decreasing with the application of reverse thrust and at 90 knots, at 1.6 EPR (in reverse) it is almost completely ineffective.
"One of the worst situations occurs when there is a crosswind and sufficient water to produce total tire hydroplaning," the manual continued. "Reverse thrust tends to disrupt airflow across the rudder and increases the tendency of the airplane to drift downwind, especially if a crab or yaw is present. As reverse thrust increases above 1.3 EPR, rudder effectiveness decreases until it provides no control at about 1.6 EPR. Use aggressive manual braking or maximum autobrakes and auto spoilers. Apply reverse thrust as soon as possible after nosewheel touchdown. Do not exceed 1.3 EPR reverse thrust on the slippery portions of the runway, except in an emergency. When reversing, be alert for yaw from asymmetric thrust. If directional control is lost, bring engines out of reverse until control is regained. Do not come out of reverse at a high RPM [revolutions per minute]. Sudden transition of reversers before engines spool down will cause a forward acceleration."
The FO did not remember if the spoilers extended after touchdown. According to the flight data recorder, the spoilers did not deploy and there was no indication that they had been armed. Immediately after touchdown, the FO stated that they had no control of the airplane and it did not feel like they had ground contact. The FO recalled that the airplane was skidding "right off the bat" in a straight line sideways to the right.
"We then started to drift to the left across the runway," and he described the sensation as "like a roller coaster." The aircraft went to the left but came back toward the center of the runway, and it felt like they had it under control. The main concern was the speed and the hydroplaning. At one point, the FO said the airplane was "fishtailing and it felt like we might ground loop."
The captain brought the engines out of reverse and "it looked like he was either going to do a go-around or to regain directional control." They "kind of drifted" on the runway and seemed under control but "going fast." He said the captain then went back into reverse thrust, but it wasn't working. They slid sideways down the runway and the main gear slipped off the left side while the nose gear remained on the runway.
As the slide continued, the crew saw the alternating red-and-white centerline lights that mark the last 3,000 feet of the runway. Nearing the end of the runway, at about 80 knots, the captain said, "Brakes," and the FO got on the brakes with him.
The aircraft was carrying an extra 20 knots, and touched down at about 150 knots as a precaution against the reported wind shear. This solved one problem and created another. With the water on the runway, this probably caused the tires to hydroplane, providing little or no ability to stop or steer. The wind was near or beyond the crosswind limits of the aircraft under these conditions. The spoilers did not deploy, which would have helped slow the aircraft, and the design of the MD-80 caused rudder blanking when the engines went above 1.3 EPR in reverse—which further compromised steering ability.
The information provided here comes from the NTSB's factual report; a probable cause has not yet been determined. It appears that the crew was aware of the weather. Every airline captain and many general aviation pilots have been in similar circumstances before. As a flight gets close to the airport and it looks like the landing could work, it's tough to make the divert decision. The mantra of the Monday-morning quarterback is if the landing is successful, the pilot is a hero—and if it failed, then one should have known better. Remember that every bowling alley has a gutter.
See also the index of "Safety Pilot" articles, organized by subject. Bruce Landsberg is executive director of the AOPA Air Safety Foundation.