Pete,
I just converted from -300 to -800 4 month's ago. Here are some of the main differences:
Flightdeck, apart form the display's is identical to that of the -300 series, sytems that hide behind the switches are a lot different though. For instance 2 batteries i.s.o. one, completely different, and much more reliable electrical system. Better and more powerful APU i.e you can use two aircopacks on the ground.
MCP and AFS are exactly the same, except for some speed limiting and reversion differences.
These are just a few examples. The thing that struck me the most was the noise in the cockpit. Despite higher cruising speeds. ( M 0,8 against M 0,74 ) it's much quieter in the NG office. Seems to be because of newly designed rivets and better insulation.
To my knowledge the flight controls are identical. ( One more spoiler panel and leading edge slat because of longer wingspan ).Displays are the best, LCD 777-style and very easy to read even during sunny conditions.
Apart from all of this. In the beginning, it's a bastard to land. Especially with flaps 40. Approach speeds are also much higher. For a 66 ton -800 with flaps 30 around 150 knots VREF. With max wind correction that means 170 knots FAS!!
Hope this gives you an idea.
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