Same A/c Type-Different Max Structural Weights?

[BMM389EC]

Take 2 different 737-200s. One with -15 engines and the other with -17 engines. Why would the -17 engined aircraft have a higher max structural take-off weight of about 2 tons? Surely the -17 engines would only come into play in allowing higher RTOWs at higher temps and altitudes than the -15s? Any ideas?

[Genghis the Engineer]

(1) Single engine performance minima in JAR-25.

(2) Airlines sometimes like an aircraft certified to a lower weight, it permits them to use smaller airports.

G

[BMM389EC]

Perhaps the latter, these aircraft operating in South Africa. However, if it was the first then surely that would be under the RTOW and not the structural weight.

[Checkboard]

3) Airlines also like aircraft certified to lower weights as Airnav charges and landing fees are usually predicated on the certified Max Structural Take Off Weight.

As it is the Maz Zero Fuel Weight that usually determins the load the aircraft can carry, the MTOW generally only determines the range. If you are operating a full 737, but only on short hops, then you never get close to the MTOW, so you ask Boeing for a new flight manual page limiting the weight to attract lower landing and Airnav fees.

[Jamesel]

The airline I fly for has over 20 737-200s, and the MTOWs vary between 114,500#, 115,500#, 117,000#, 117,500#, and 119,500#, with -9A powered machines (Basic & Advanced) certified at all the above weights. We used to have a -17A powered machine with a MTOW of 110,200#! It has been re-certified at 119,500#. I was told by our maintenance dept. that some of the limits are paper differences and some are due to differences in the Landing Gear and Brake assemblies.

[quid]

Another reason may be that "structural" doesn't necessarily mean the structural strength of the components.

It may be the certification for noise that is driving the lower weights. The max weight to meet noise standards is not called "max noise weight". If the weight is reduced to meet noise requirements, the "structural" weight is reduced.

Check to see if some of your CDL procedures require an adjustment to "structural" weights. If so, it's a dead giveaway.

[Siddique]

A afdditional consideration is the initial cost of the aircraft. eg aircraft 'a' is sold at say 580,000lbs MTOW. To get it at say 600,000 costs more even if there is no other change except the performance curves in the AFM? Why, its called a derived benefit. You operate at higher weight for more profit and therefore the manufacturer gets some of that benefit...can be a very large sum per 1000lb of weight. The engine manufacturers are also in on this. An engine certified at say 45,000lbs thrust will be less than the same engine at 50k thrust? Only the program plug on the ECU are different and the AFM...so basically the manufacturers putting the icing on the cake eh? Even if by operating at higher weight / thrust you wear it out quicker and pay more in spares!! Wish I was in the game selling this!!!

[mustafagander]

I know that the B767 and B747 are able to have increases and decreases to "structural" weights by writing a note in the AFM and paying a fee to Boeing.
My lot "lightened" their B762s a few years ago to lower the airways/landing charges in the domestic ops. Our newer B744s have MLW approx 10 tonnes lighter than the rest of the fleet.
Over the last 20 years or so, the "structural" weights of the B742 and B743 fleets were upped with minimal mods - new flap load relief computer etc.