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Why Isn't The 77L Certified For 775K Lb Mtow?  
User currently offlineEA772LR From United States of America, joined Mar 2007, 2836 posts, RR: 10
Posted (5 years 11 months 4 weeks 1 day 3 hours ago) and read 3495 times:

I was wondering why Boeing didn't maximize the range of the 77L even a tad more and take the 77L's MTOW of 766,000lb up to the 77W's 775,000lb MTOW?? They share the same structural improvements and engines, so why not certify them for the same weight?? Would the 9,000 extra pounds make the difference in the 77L being able to do LHR-SYD-LHR nonstop year round??
http://www.boeing.com/commercial/777family/pf/pf_lrproduct.html


We often judge others by their actions, but ourselves by our intentions.
5 replies: All unread, jump to last
 
User currently offlineBrendows From Norway, joined Apr 2006, 1020 posts, RR: 4
Reply 1, posted (5 years 11 months 4 weeks 1 day 1 hour ago) and read 3436 times:



Quoting EA772LR (Thread starter):
I was wondering why Boeing didn't maximize the range of the 77L even a tad more and take the 77L's MTOW of 766,000lb up to the 77W's 775,000lb MTOW?? They share the same structural improvements and engines, so why not certify them for the same weight?? Would the 9,000 extra pounds make the difference in the 77L being able to do LHR-SYD-LHR nonstop year round??

I've been wondering about the same thing a few times, and here's some possible reasons.

The 77L is fuel volume limited, there are no more room to store fuel, and if they installed more auxiliary fuel tanks in the cargo holds, they would end up having c/g issues (and eat up revenue space.) The best way to make LHR-SYD-LHR possible year round would be to lower fuel burn/OEW.

Another issue is that the 77L has the same rudder as the 77W but is quite a bit shorter, which makes handling with an engine out at high TOW challenging, or "interesting" as Suzanna Darcy-Hennemann called it during the 77L flight testing.


User currently offlineSunriseValley From Canada, joined Jul 2004, 4952 posts, RR: 5
Reply 2, posted (5 years 11 months 3 weeks 2 days 13 hours ago) and read 3209 times:



Quoting Brendows (Reply 1):
Another issue is that the 77L has the same rudder as the 77W but is quite a bit shorter, which makes handling with an engine out at high TOW challenging, or "interesting" as Suzanna Darcy-Hennemann called it during the 77L flight testing.

I believe this is the answer. There is a concurrent thread going in civil aviation that gave this reason.


User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3502 posts, RR: 66
Reply 3, posted (5 years 11 months 3 weeks 2 days 8 hours ago) and read 3170 times:



Quoting Brendows (Reply 1):
Another issue is that the 77L has the same rudder as the 77W but is quite a bit shorter

I believe that the tail arm is the reason, but it's not the yaw axis as suggested above. Instead, it's the pitch axis. It's a certification requirement that the wing must be able to produce enough lift for 3.75 g's ultimate load. At that load, the h. tail must be able to trim the wing lift at the forward CG position. This means the h. tail will have a download for this condition.

Since the 772LR has a shorter tail arm than the 773ER, the 772LR wing will be producing more lift than the 773ER wing at the same weight because the h. tail download will be higher. Therefore a 772LR wing at 775K MTOW would need to be stronger than a 773ER wing at the same weight.

I suspect that for simplicity of construction, the 772LR and 773ER wings are the same structurally. This means the 772LR MTOW will be less than the 773ER, hence we see the 772LR MTOW limited to 766K.

Just a guess, but it seems to fit the geometric dimensions of the two models.



Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offlineThegeek From Australia, joined Nov 2007, 2638 posts, RR: 0
Reply 4, posted (5 years 11 months 1 week 3 days 16 hours ago) and read 2865 times:



Quoting EA772LR (Thread starter):
Would the 9,000 extra pounds make the difference in the 77L being able to do LHR-SYD-LHR nonstop year round??

No. I remember reading that the 77L needed to lose 5t in OEW to make it. A 4.5t MTOW increase wouldn't do.


User currently offlineEA772LR From United States of America, joined Mar 2007, 2836 posts, RR: 10
Reply 5, posted (5 years 11 months 1 week 3 days 7 hours ago) and read 2816 times:

Very informative answers guys. There is a lot about aircraft design that I'm just incredibly ignorant to...Anyway, would a possible fix to solve the problem be to place a larger stabilizer on the 77L? It could act as a one-two punch. First the taller stabilizer could handle engine out characteristics, and second, could handle additional fuel.


We often judge others by their actions, but ourselves by our intentions.
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