Originally Posted by
Auxtank
Now, why did they do that? Genuinely interested to know.
Surely it wouldn't be a weight consideration. Was it part of certification - keeping it simpler, etc.
I wrote the material below in a thread some time back to explain the regulation that contains the requirements that determine whether a jettison system must be installed. Hopefully this responds to your question. The earliest 767s with the dry center wing box were not required to have a jettison system. Whether subsequent variants were required to have jettison depended on their thrust rating and their max takeoff weight because, as discussed below, it's a quick return to land climb gradient requirement that determines if jettison is required. Whether jettison was actually installed on those variants could have been driven by it being required or by the customer electing it as an option. Here's my previous post:
The regulation that determines whether a jettison system is required is 14 CFR 25.1001(a). It has nothing to do with the relationship between max takeoff weight and max landing weight. The requirement for a jettison system is instead based on climb capability at a weight equal to max takeoff weight minus the weight of fuel necessary for a 15 minute flight consisting of a takeoff and return to land. If the climb gradient requirements of sections 25.119 (all engine climb in the landing configuration flaps down gear down) and 25.121(d) (engine out climb in the approach configuration flaps down gear up) cannot be met at this weight, then a jettison system is required by 25.1001(a).
The flow performance of the jettison system, if required by 25.1001(a), is required by 25.1001(b) to be able to get the airplane within 15 minutes to a weight that allows the airplane to meet the climb gradient requirements of 25.119 and 25.121(d).
The braking system regulation, section 25.735, sets the energy absorption capability requirements for the brakes. Landing at max landing weight at anticipated speeds must be withstood repeatedly as normal operation with no damage other than normal wear. Landing at max takeoff weight must be withstood without hazard, but parts can be destroyed or require inspection/maintenance. The structural regulations (25.473) set the landing loads that must be met by setting descent rates that must be accommodated as limit loads at max takeoff weight (6 feet per second descent rate at touchdown) and at max landing weight (10 feet per second descent rate at touchdown). A design can be limited by the braking and structural regulations to a maximum landing weight that is significantly less than the maximum takeoff weight, and whether or not a jettison system is required would have nothing to do with this. It's all design choice - how strong do you want to build your gear and brakes, and what climb performance do you want to provide.
Of course, performance information (climb gradient and runway distance) for landings in excess of the maximum landing weight up to the max takeoff weight is required to be provided in the AFM by 25.1587(b)(3).
So yes, as most of you pilots already know, you can theoretically land safely above the maximum landing weight, but you are going to at least have some inspections required afterward, and worst case could blow the tires and damage the brakes and wheels. I am not qualified to speak about the judgment of whether you should do this in a real world situation.