I belive most airlines today use 600ft/min or 2G as their hard landing limit on their aircraft.If remember correct Scandinavian use this for their entire fleet.But at what sink rates is it likely that some sort of damage will occur,like gear damage or bent stringers.In the BA038 incident, I been told that the gear would have withstand the 1300 ft/min impact if it wasn,t digging into the grass.In a A320 tailscrape landing with a sink rate of 800 ft/min the engineers fould no structural damage except for the scrape damage.There are several videos on youtube were the sink rate has to be well over 600 ft/min as for example and awful landing of a 757 at Manchester.I also asking if a Widebody like 767 or 777 will withstand higher sink rates at touchdown because of it,s longer legs than for example a 737 or MD80? Someone with more knowledge please comment.
One interesting fact is that hard landings tend to appear worse than they actually are. I back this statement up with the evidence from my company's ASRs. You see "suspected hard landing written up in tech logs, but the subsequent analysis shows that there were nowhere near the 600fpm limit. In fact most don't even exceed the 360fpm limit for the overwight case.
I've been retired for a while, but from memory, 600fpm is the design limit for landings at weights up to MLW as specified by the airworthiness requirements (JAR25 in Europe or FAR25 in USA). Any landing with a vertical speed in excess of 600fpm is therefore outside the certified limits and inspections are needed to check for damage. As far as I know, all fairly modern aircraft are designed to these limits and you can't assume that any make or model is more capable than another.
I have just checked and both FAR 25 section 25.473 and JAR 25, also section 25.473 specify that the aircraft must withstand loads due to landing at MLW at a limit descent velocity of 10 fps, i.e. 600 fpm. I have no idea whether that was the regulation limit for the 727 or whether you are correct about a test having been performed at a higher descent velocity.
"... 2G as their hard landing limit ... what sink rates is it likely that some sort of damage will occur ... bent stringers...."
The various definitions were discussed in the August 20004 issue of Flt Safety Digest:
"... Flight crews primarily use their judgment to identify and report hard landings ..."
= = = = = = = = ==
Excerpts from other cases: Crown fractures have resulted when PITCH RATE combined with firm landing:
"... severe circumferential wrinkling of the fuselage crown skin
... running from one window belt line to the other....
... N373AA subsequently bounced three times resulting in 4 touchdowns over a total period of 6 seconds ... vertical velocity at the Centre of Gravity ... of 1.5g; this was not excessive but, if coupled with high pitch rates and vertical velocities of the nose gear, could result in high stresses on the airframe. There were high pitch rates generated during the 'bounces'. However, the recorded time histories of pitch attitude and elevator angle could not be used to analyse accurately the dynamics of the aircraft attitude during this period. Firstly, the DFDR did not sample pitch attitude and elevator angle sufficiently frequently and secondly, control column position was not recorded...." [B767-323 ER, N373AA, 15JAN99 at Heathrow]
Decades earlier fight crews demonstrated that Boeing airliners fly OK even after Crown Fracture, days later tower alerted later flt crew to the earlier damage:
"... LAX84AA132 . ... CONTINENTAL ... Wednesday, December 28, 1983 in NEW YORK ... BOEING 727-224, registration: N79750 ... WINDS WERE REPORTED 170 DEG AT 26 KTS GUSTING TO 40 KTS. THE PLT ADDED 20 KTS TO HIS V-REF SPEED OF 132 KTS & SELECTED AUTO- SPOILER DEPLOYMENT. ... WINDSHEAR ALERT IN SEVERAL QUADRANTS. AFTER TOUCHDOWN THE ACFT BOUNCED & THE AUTO-SPOILERS DEPLOYED. THE ACFT TOUCHED DOWN AGAIN IN WHAT THE CREW DESCRIBED AS A FIRM LANDING.
THE FLT ENGR FOUND NO APPARENT DAMAGE SO THE CREW DID NOT WRITE IT UP AS A HARD LANDING.
ON 12/31 DAMAGE WAS FOUND ON THE UPPER CROWN OF THE FUSELAGE BETWEEN STATIONS 660 & 680 FROM STRINGER S-10 LEFT TO S-10 RIGHT. LATER INSPECTIONS REVEALED NOSE GEAR DAMAGE ALSO.
A hard landing is defined as a landing with a vertical acceleration of more than 2.6 g at the center of gravity or:
-a vertical speed (v/s) of more than 600 ft/min.
An overweight landin is defined as a landing at more tha the maximum landing weight with a vertical acceleration of more then 1.7 g at center of gravity or:
-a vertical speed (v/s) of more then 360 ft/min.
On A-320 if the aircraft has DMU/FDIMU with an enhanced load report 15, do the "inspection after hard/overweight landing for aircraft with enhanced DMU/FDIMU load repot 15".
It is responsability of the flight cr ew to make a report if THEY THINK THERE WAS A HARD/OVERWEIGHT LANDING.
Recorded g is not always a reliable method of determining if a landing was hard. The impact shock through the airframe is transmitted directly to the accelerometer and this shock itself can influence the reading.
The A320 has its own monitoring accelerometers around the CG. They're much higher rate and much less precise accelerometers than the IRUs. Hopefully somebody at the factory figured out how they correlate to hard landings before they certified the thing.
The LRs have a ~10 of them throughout the airframe for the anti-vomit system too. The 777 has something similar.
Back in the "Good Old Days" the DC8-50, and maybe other DC8 models, had a small strip of Al clamped to the bottom of each MLG strut pointing up. Part of the engineers' turnaround check was to see if they were still straight along the strut. If bent, it was a heavy landing and an inspection required, if straight no problem.
We called them "lie detectors" and it saved any arguing the toss with the pilots.
A hard landing limit of 2.6 G sounds high.After reading this,I don,t belive any more that an A 320 series has a weaker airframe than the 737 Ng and Md80 series,or maybe it has something to do with the long leg,s of the bus.A 737 driver once told me that you didn,t bother with a hard landing check unless hitting over 700 ft/min so there is some variations of the information.One more thing! Is it possible to overload the aircraft if you flare to late,with a violent pitch rate even with a decent rate within the limits? I saw a video on youtube with a Zoom 767 witch probaly had the cg well in front of the mains.When flaring late,he rotated the main gear into the runway with great force.It looked like a hard landing,but were the sink rate was well inside the limits.Since I not into computers,I am not able to give you a link.