Page:CAB Aircraft Accident Report, Northwest Airlines Flight 705.pdf/4

 turbulence in heavier thunderstorms. This advisory was called to the attention of the crew of Flight 705 by the operations agent at Miami, and was attached to their dispatch papers. SIGMET No. 4, valid from 1300-1700, was not received until approximately 1315, after the crew of Flight 705 had left the operations office. It forecasted moderate to severe turbulence, but deleted the reference to extreme turbulence indicated in SIGMET No. 3. Since the dispatcher for this flight is stationed in Minneapolis, the physical limitations involved made it difficult to appraise the crew of this latest advisory prior to their taxi time of 1325.

Northwest Airlines route forecast for Chicago south, valid at 1300, indicated a cold front at Fort Myers, Florida, moving eastward at 20 knots, with a line of thunderstorms 100 miles east of the front. The Macon to Miami portion of the en route weather forecast indicated the tops of clouds would be 25,000 feet, with a few thunderstorms to 40,000 feet in the Miami area. There was no specific reference to turbulence. However, the company meteorologist who prepared the route forecast for Flight 705 stated that turbulence was indicated in his forecast by the presence of convective clouds. The company Flight Operations Manual states that if cumulus clouds are forecasted to exceed 10,000 feet severe turbulence may be expected.

The captain of Flight 705 also obtained weather information from the pilot who arrived in N724US at 1240. He stated that the weather extended from LaBelle, approximately 70 miles northwest of Miami, to the Miami VORTAC. The tops of the clouds were estimated to be at 27,000 to 30,000 feet. He also stated that "... I simply explained to him the weather as I saw it approaching the front, and I explained to him how we had been cleared over the weather and made our letdown to the east side of the frontal area."

The aircraft was equipped with a Fairchild flight recorder which scribes oscillographic traces of in-flight pressure altitude, indicated airspeed, magnetic heading, and vertical acceleration as a function of time. The readout of the flight recorder tape from Flight 705 (See Attachment B) indicates that following lift-off at 1335:22, a series of turns to headings of south, southwest, west, and northwest were accomplished while climbing to 5,000 feet in light turbulence. At 1342:46, as a climb was begun, heavier turbulence was encountered for approximately three minutes, until a left turn to 200 degrees was accomplished just prior to the cessation of the large acceleration excursion. The indicated airspeed fluctuated from 320 knots to 210 knots, and the altitude increased from 5,000 feet to 15,000 feet. The aircraft continued climbing from 15,000 feet to 17,250 feet in a right turn which continued through 320 degrees while the climb ceased and altitude remained constant for about 12 seconds. At 1347:25 the altitude began increasing again and the rate of climb gradually increased to approximately 9,000 feet per minute at 1347:38. Following this the rate of climb decreased through zero at 1347:47 when the altitude peaked momentarily at 19,285. During this climb the airspeed decreased from 270 to 215 knots and as the peak altitude was approached the vertical acceleration changed rapidly from +1G to about -2G. In the next seven seconds the negative acceleration continued to increase at a slower rate, with rapid fluctuations, to a mean value of about -2.8G, while altitude was lost at an increasing rate. As the descent continued with rapidly increasing airspeed, the acceleration trace went from the high negative peak to +1.5G, where it reversed again. In the last