B767-300 flight crew reported exceeding 250 kts below 10;000 ft when their unloaded aircraft accelerated more quickly than anticipated. A wake turbulence encounter was cited as contributing to the incident.
Synopsis
B767-300 flight crew reported exceeding 250 kts below 10;000 ft when their unloaded aircraft accelerated more quickly than anticipated. A wake turbulence encounter was cited as contributing to the incident.
Narrative
I was the pilot flying; and the Captain was the pilot monitoring. The 767-300 aircraft was completely unloaded. Our aircraft was cleared to 5000 feet on the DOTSS2 RNAV departure out of LAX but there was an intermediate restriction at HIPPR to remain below 3000 feet. Preflight; a threat was identified that the light loaded aircraft would have difficulty making the 3000 foot intermediate restriction on departure. A mitigation of deselecting the auto pilot was pre-briefed to account for this threat.Takeoff proceeded normally. Autopilot was selected at 200 feet per the brief. LNAV was already active at 50. PF selected VNAV at 1000 feet according to the brief. The light aircraft accelerated and climbed rapidly. The thrust management computer had a target of 250 kts indicated airspeed and it rapidly accelerated to that as the aircrew got the gear up and flaps up. Approaching the intermediate restriction of 3000 feet; it became obvious the auto pilot would not level off to make the restriction. The pilot flying deselected auto pilot; and manipulated the controls to make the 3000 foot restriction; it was during this level off the aircraft finished cleaning up. As the 3000 foot restriction way point passed; the thrust management computer failed to compensate for the level off and the airspeed accelerated to 275 kts indicated airspeed; though 250 kts was the selected airspeed. The pilot monitoring noticed the airspeed deviation and directed a correction. The pilot flying corrected the airspeed back to 250 kts and continued climbing without further incident. A complicating factor was the traffic in front of our aircraft by 5 miles created significant wake turbulence that we encountered at the 3000 foot level off.In the future; a better way of handling this would be for the pilot flying to manually reduce the throttles at level off to slow the acceleration. Additionally; a lower airspeed than 250 kts could be selected such as 230 to give a greater margin.
Second reporter narrative
We were assigned to ferrying an empty aircraft from LAX to ZZZ1 departing at night; in VMC; and calm winds. Our pre-brief covered many threats and contingencies for our flight including light aircraft gross weight and departure restrictions. We took off on Runway XXL and assigned the DOTTS2 departure with an altitude restriction of 3000 ft. or below at HIPPR and a 5000 ft. or below altitude at ADORE. I was Pilot Monitoring (PM) and the Pilot Flying (PF) elected to engage the autopilot at 200 ft. for a normal LNAV/VNAV departure.While retracting the flaps to up; I didn't believe VNAV was going to respect the altitude restriction of 3000 ft. I pointed it out to the PF at approximately 2700 ft. and he disengaged the autopilot and lowered the nose to level off. At that point we also entered moderate wake turbulence from a heavy that departed prior to our departure. During the wake turbulence and the level off; I thought we may have exceeded the airspeed limit for flaps 1. It was only after clearing the turbulence I noticed we were quickly accelerating and got up to approximately 275 knots on the departure. I pointed out the accelerating airspeed to the PF and he then stabilized the aircraft. The autopilot was then engaged and we continued climbing on the departure. ATC was notified of the moderate wake turbulence event and they tried to de-conflict us with a continued climb above the heavy in front of us. At cruise altitude; I queried Dispatch to ask Maintenance if any flap limits were exceeded. We were told no flap limits were exceeded.We briefed and planned for this departure; but I believe our execution was too slow. Passing 2000 ft. we should have been more aggressive in downgrading the automation and hand flying the aircraft. From my experience I have noted the autopilot cant keep up with the performance of the aircraft at light weights. The wake turbulence then just exacerbated this event.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.