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Boeing recommends, however, that because of limitations of the equipment that measures verti-cal acceleration, the recorded flight data be used only to cross-check flight crew reports of hard landings.
“There are inherent inaccuracies in using vertical-acceleration-recorder — g-meter or ac-
celerometer — data to identify hard landings,” said
Capt. David Carbaugh, chief pilot for flight opera-
tions safety at Boeing Commercial Airplanes.25 “Vertical-acceleration recorders normally are positioned in the aircraft to sense in-air accelera-tions. They can be used to re-cord touchdown accelerations, also; however, because of their location in the airplane and er-rors due to roll and other factors,

they can be inaccurate up to 0.4 g.”
Recorded vertical accelerations vary both in duration and mag-
nitude and are affected by the airplane’s weight, center of gravity, motion (e.g., sink rate, forward/ side velocity and roll/pitch/yaw attitude), external forces (e.g., gust loads) and structural dynamics (e.g., airframe vibrations and harmonics).
“Using vertical-acceleration values as the sole criterion for initiating unscheduled inspections is generally not advisable because of the location and design considerations of the FDRs and accelerom-eters,” Boeing said.26 “In most instances, there is no absolute way of knowing whether the recorded
accelerations are a minimum [value], maximum
[value] or some intermediate value relative to the
entire airframe structure.”

Boeing recommends that conditional maintenance inspection be performed whenever a flight crew reports a hard landing, even if the recorded verti-
cal acceleration did not exceed the threshold cited
in the AMM.

“Service experience indicates that most flight
crews report a hard landing when the sink rate
exceeds approximately four feet per second,” Boeing said. “Past experience also indicates that
the flight crew’s determination of a hard land-ing is the most reliable criterion because of the difficulty in interpreting recorded acceleration
values.”

Landing ‘Like a Ton of Bricks’
N
either Airbus nor Boeing provides specific guidance for flight crews on identifying a hard landing. Carbaugh said that a landing of suf-ficient impact to cause structural damage should be obvious to the flight crew.
“We don’t spell out exactly what a hard landing is,”
he said. “If I had to define it, I would say that it is a landing that the pilot believes had the potential to cause structural damage and requires a main-tenance inspection.
“When you smash one on, you know it. Nobody on that airplane is not going to know that you had a hard landing. A hard landing is when you land like a ton of bricks. Pilots are supposed to write up hard landings; for the safety of the passengers and the crewmembers to come, they should at least
　
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