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Classic Wings Magazine Luftwaffe Resource Center WWII Naval Aviation Research Pacific
Final Cut-The Post War B-17 Flying Fortress and Survivors - 5th Edition


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PostPosted: Sat Dec 09, 2017 11:31 pm 
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Location: Salt Lake City
How does keeping the gear down when they drop fire retardant keep the aircraft from stalling? Do they do this to have extra drag while having the engines spooled up just in case they need the extra thrust from the engines?


http://fireaviation.com/tag/md-87/


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PostPosted: Sun Dec 10, 2017 7:57 am 
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It is to do with the EPGWS I am pretty sure and the FAA.


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PostPosted: Sun Dec 10, 2017 9:32 am 
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From my understanding from chatter (Erickson has never officially talked about it), is that there are two things in play here -

1) The MD-80 has a mode on the Gear Horn where it activated continually when the flaps are deployed beyond a certain point without the gear being down, regardless of throttle position. This has to do with the design of the airplane. With the T-Tail, if you stall the airplane with the flaps deployed it can easily develop into a "deep stall" whereby the tail is in the "shadow" of the wing and cannot get sufficient smooth airflow to be able to push the nose over. This results in an unrecoverable stall. The MD-80 has a "stick pusher" that helps to push the nose over to prevent this, however it can occur rapidly during approach, so there are additional warning systems in place to keep this from happening. While this system is on an independent circuit as with most systems on transport-category aircraft, the FAA denied Erickson from doing that during the certification process. While this wasn't initially an issue (they did their initial certification and early operational drops with the gear up), it appears when they went to re-certify the system, the FAA added that the horn cannot be allowed to continuously operate. From my perspective, I would suspect this has to do with contamination of the CVR recording as the alarm going off like that would make it hard to hear anything else.

2) When the MD-87 was originally certified for drops, they were having issues with retardant going over the top of the wing and being ingested by the engines. While this wasn't a huge issue for Erickson (they thought they could fix it with some changes after the season), the FAA saw a problem and forced them to go back immediately and change things as they saw it as too big of a risk. Part of this change was making changes to the drop profile that included using more flaps than prior. I don't know if this change is what activated the flap/gear disagree alarm or if was going off in the original profile, but the fact is that they're now using more flaps than before. The gear also changes the airflow around the wing which may also assist in keeping the retardant from going over top of the wing and into the engines, so if that's what was found, then they keep them down to ensure no retardant goes where it shouldn't.


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PostPosted: Tue Dec 19, 2017 12:23 pm 
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I was discussing this subject with a member of our EAA chapter who was a McD test pilot on the MD-80 series aircraft. He saw no reason to have the requirement of the gear down for the drops as far as how the plane flies.


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