Page:CAB Aircraft Accident Report, Northwest Airlines Flight 705.pdf/11

 The 1300 Northwest route forecast, which was attached to the dispatch papers provided to the crew, was in agreement with information provided the captain by the incoming pilot of N724US who landed at 1240. The incoming pilot also advised that he had descended after passing the squall line. Because of this weather information, the crew should have been aware that some of the worst weather was still northwest of Miami. This would explain the decision to depart to the south and then reverse course when the continuing climb would "top" the weather. Accordingly, the flight requested and received a south departure.

Transmissions between the pilot and controller disclose a misunderstanding of the intended departure route. The pilot, apparently basing his decision on a belief that the squall line was still northwest of Miami, was requesting an extended southerly climb before reversing course to overfly the weather. The controller, acutely conscious of arriving aircraft descending to the south for approach to Miami, other conflicting traffic which restricted climb capability in that sector, and the proximity of Homestead AFB, envisioned a slight deviation to the south before vectoring the flight through the weather along a departure pattern similar to that which had bee negotiated by a previous flight. Clearly, both were seeking the safest, most expeditious route. The misunderstanding resulted from the pilot's desire to avoid the squall line, and the controller's prime responsibility to provide adequate separation from known IFR traffic.

SIGMET No. 4, valid from 1300-1700 was transmitted via tele-typewriter at approximately 1315, which was 15 minutes late, but 10 minutes prior to the time Flight 705 departed the ramp. It is problematical what effect this latest weather advisory, downgrading the level of turbulence from extreme to severe, would have had on the departure route. However, the crew did not receive this latest information regarding potentially hazardous weather. Since SIGMET No. 3 was no longer valid after 1300, and the crew was not aware of SIGMET No. 4, they might have assumed that potentially hazardous weather conditions were no longer anticipated.

The final and perhaps most important factor bearing on the departure route was the airborne radar. Regardless of other weather information available to the crew, if the airborne radar was operable and being utilized properly, it is difficult to reconcile the flight's progress to the southwest within the confines of the squall line. Apparently, the captain believed that he was southeast of the line and intended to resist the inevitable turn to the north as long as possible, in order to gain more altitude. It is significant to note that the acceleration trace of the flight recorder reflects the worst turbulence while the flight was on a heading selected by the crew.

The flight recorder shows that the flight had experienced varying degrees of turbulence throughout most of the approximate thirteen minute period that it was airborne. The turbulence encountered between 1336 and 1340 while climbing to 5,000 feet appears to be only light turbulence, and the crew transmissions do not indicate that they considered this degree of turbulence unusual in any way. From about 1342.30 to 1346 while climbing from 5,000 to 15,000 feet, the turbulence level indicated by the recorder G trace is moderate to severe, and the crew transmissions confirm this level of turbulence. The airspeed variations during these turbulence encounters did not vary significantly from the recommended 230 to 280 knot penetration range then in use. On several occasions when it appeared the turbulence was heavier, the heading trace showed