Kinda long winded eh? Do they let you talk on the radio?
If the main concern is flying overweight, bear this in mind (It's important):
IT'S YOUR FAULT!!!!
The decision to fly an overweight aircraft, or one that you're unsure of comes down to the flight crew. Yes, it's the captains decision, but you as an FO can refuse to fly also. Let him go single pilot. If you don't have enough backbone to refuse a flight that is THAT much over gross weight, you need to reevaluate your career choice. I asked the chief pilot how many pilots he's fired for refusing to fly due to being over gross weight. Can you guess his answer?
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I'm sick of hearing pilots whine about things their company is "making" them do. Just stop doing it. If they fire you for not flying illegally (they won't), go straight to the POI.