Hard Landings

[gengis]

This may be flogging a dead horse somewhat, but I'd like to hear some diverse views on this subject. I am restricting this discussion to the 747-400 in specific.

The Maximum Takeoff Weight (MTOW) is 396 metric tonnes. Maximum Landing Weight (MLWT) - 302 tonnes (-400 Freighter) & 285 tonnes (-400 passengeer). Limiting load factors are +2.5g/-1.0g flaps UP or +2.0/-0g flaps DOWN.

For this discussion, let us define a hard landing as one in the region of 1.5g.

At the maximum landing weights permitted, simple arithmetic will show that this puts 427.5 tonnes on the main wheels (pax airplanes) & 453 tonnes (freighter). In both cases this is well above the maximum takeoff weight of the airplane. Clearly FAR 25 covers this in the required design Factor of Safety for the landing gear so this should still be a landing that the crew "walk away from".

Now let us consider "hard" landings at weights well below maximum landing weight ~ say 240 tonnes (pax) or 260 tonnes (freighter). Once again - at 1.5g on the concrete - this works out to 360 tonnes (pax) & 390 tonnes (freighter). This time however, the force on the main wheels is below the maximum takeoff weight of the airplane. In this event though, neither the limiting load on the landing gear nor the limiting load factor of the airplane would have been exceeded. Is it still incumbent on the crew to write this up in the book for a hard landing check or not? My personal take on this is..... yes. Better to err on the conservative side, for the sake of all, not least the next set of crew.

The permutations of possible weight vs vertical accelerations on landing can reveal a whole host of varying numbers on the wheels & airframe - such as very very light landing weights (such as during aircraft ferry)...... So would you say that 1.7g at a landing weight of 190 tonnes (amounting to 323 tonnes at touchdown) be less stressful to the airplane than a 1.5g landing at 240 tonnes (giving 360 tonnes) ....? Most guys i know think of hard landings in terms of number of "g"s on landing, but as the above suggests, it has surely got to be correlated with the weight as well.