The 737 is robust but not indestructible. We have all done a "firm" landing but when should it be reported as "heavy" and entered into the Tech Log. Hopefully this page will help...
The 737 has been designed to withstand landings at 600fpm, reducing to 360fpm at MLW before a hard landing inspection is required. Most pilots report a hard landing when the sink rate exceeds approximately 240fpm. On-board accelerometers are notoriously unreliable indicators of heavy landings because of their low sampling rates (8 or 16 samples/sec) and because they are located near the CofG and may not represent the peak loads in other parts of the aircraft (eg the tail). Also any roll rate may dramatically increase the load on the downgoing landing gear. If the acceleration values are recorded during a hard nose landing or accompanied by more than 2 deg of roll at the time of main landing gear impact, a hard landing may be experienced at significantly smaller vertical acceleration values.
*** Updated 23 Nov 2020 ***
A 737-400F after a hard landing (details here)
Boeing has intentionally not published specific values for either rate of descent or vertical acceleration which quantify a hard landing. This position was established after careful review of available flight/aids recorder data and flight test experience.
Accelerations registered on flight data recorders are not considered to be adequate indicators for hard landings for several reasons. Accelerometers measure G forces at their installed location only. There is no way of knowing whether the forces are a minimum, a maximum or some intermediate value due to the data sample rates. These devices can provide reliable acceleration data during inflight maneuvers where accelerations are steady or slowly varying. However, Boeing flight tests have shown that recorded acceleration data could be a significantly inaccurate indictor of a hard landing. This results from the system response during the short time when the wheels are contacting the runway. During this interval the recorded vertical acceleration component can become highly erratic. In those cases where apparently valid data is readable, data sampling rates or recording time intervals are such that the peak value is usually not discernible.
Additionally, several accelerometers placed throughout the airplane have revealed significant variations in both time and magnitude of G forces (structural loads) due to the airplanes weight, CG, motion (sink rate, forward and side velocity, roll, pitch and yaw angles and rates), external forces (gust loads, ground effect, runway contact loads) and structural dynamics (vibrations, harmonics). Because of the number of complex factors which must be analytically combined and correlated to an equivalent G force factor, Boeing has concluded that a reliable method is impractical. Quoting a G level low enough to assure the airplane has exceeded the design sink speed, the appropriate criteria would lead to frequent and unnecessary inspections. Quoting a medium or high G level for the inspection threshold would result in some high sink speed occurrences without an inspection.
Boeing believes pilot judgment and reports describing the landing remain the best source of information for ascertaining if a hard landing has occurred. Pilots ordinarily land the airplane well within the allowable limits and become accustomed to the sensation. Translation of service experience reports indicate that airplane flight and cabin staff typically report a hard landing when sink rates approach 4 feet per second (240fpm). All Boeing model airplanes have been designed for a 10 feet per second (600fpm) sink rate at the maximum designed landing weight and 6 feet per second (360fpm) at the maximum designed takeoff weight. These values are considered when designing both the main landing gear and nose landing gear assemblies as well as the wing and fuselage support structure.
Note that if the acceleration values are recorded during a hard nose landing or accompanied by more than 2 deg of roll at the time of main landing gear impact, a hard landing may be experienced at significantly smaller vertical acceleration values.
From the latest 737 NG AMM 05-51
When a hard landing has occurred or is suspected by the flight crew or cabin staff, Boeing recommend the hard landing inspection be performed.
The hard landing inspection is conducted in two phases:
Phase I. A close visual inspection of various structural components, especially those most vulnerable to damage, to determine whether further inspections are warranted. Phase I inspections are kept as simple as possible with minimal access and disassembly requirements.
During the phase I inspections of the main landing gear, operators should check for shock strut leakage and examine the inside diameter of the fuse pins of the drag strut and the outboard end of the main landing gear beam for distortion. This involves checking for visible damage to the specific component without removing it. Operators also should examine the main landing gear beam to the inboard rear spar stabilizing link for damage to the link or the crank shafting of the forward and aft attach bolts. This is accomplished by loosening the nut on the stabilizing link bolt and turning the bolt to determine whether it is deformed or crank-shafted. Damage at this location will warrant further action during phase II inspections; specifically, the trunnion link should be removed in accordance with the AMM and the forward trunnion fuse bolt inspected. On the outboard attach fuse pin for the main landing gear beam, the retention bolt should be removed and the pin rotated to check for crank shafting.
Phase 2. A second phase of inspections is conducted if any damage is found during phase I.
During phase II inspections of the main and nose landing gear, operators should ensure proper hydraulic fluid levels are in the shock struts by performing a two-point service check, or by completely servicing the shock struts in accordance with the AMM. Operators also should remove the landing gear inner cylinders if shock strut servicing was found to be incorrect or if both a hard and a high-drag-load or side-load landing occurred at the same time. The barrel of the inner cylinders and axles also should be dimensionally checked for distortion or bending and examined for cracking.
Airline technical and operational staff may be consulted following phase I and II inspections, depending on inspection findings. Boeing is often requested to provide technical assistance during such reviews.
Additional information can be found in the Aircraft Maintenance Manual, Chapter 05, Section 05-51, Hard Landing or High Drag/Side Load Landing, and in the AERO magazine, Issue 14, there is an article titled Updates to 737 Conditional Maintenance Inspection Procedures dated April 2001.
These include high-drag-load, and side-load landings, as well as off-runway excursions.
Off-runway excursions occur either on hard, even surfaces that do not create higher-than-normal loads or on uneven surfaces with depressions and obstructions that also may include soft and muddy conditions. The latter situation can create high vertical, high drag, and side loads when the gear goes over rough terrain or when the airplane stops suddenly in soft terrain.
Travel onto surfaces with depressions or obstructions will generally require close inspection of all fuse pins during the two-phased inspection process outlined in the AMM. The gear then may be removed for closer inspection depending on flight crew judgment, FDR/QAR data review, consultation between the operator and technical experts, or the discovery of any structural anomalies. In addition to fuse pin deformations, axle and truck deformations may be discovered during close inspection of the gear. When an airplane goes into soft or wet turf or the gear picks up debris (fig. 5), in addition to high-drag-load conditional inspections, the wheel and tire assemblies should be replaced because water or dirt may have contaminated the wheel bearings. Also, the wheel speed transducers should be removed and inspected; brakes should be washed, examined for obvious damage, and operationally checked; and the entire gear should be cleaned of debris, especially under the axle sleeves, and relubricated.
In one instance, an axle fracture was attributed to moisture and mud under the axle sleeve following an off-runway excursion. According to the maintenance records, no inspection or cleaning was done, and the contamination resulted in corrosion and crack initiation.