TacomaSailor wrote on Yesterday at 4:16pm:
2) AC & AP are working perfectly - AC is stable in NAV-LOC with ALT HOLD on.  FL 20 at Mach .71  - suddenly the AC starts falling from sky at 5,000 fpm while maintining Mach.71 and N1 at ~85%.   IAS starts to drop rapidly, I push over, increase the throttle to 99% but the IAS continues to drop and the Vertical Airspeed does not change.  If I push over into a near vertical dive the flight ends with a an Overspeed/Overstress and IAS of less than 100 kts just before breakup.

:D :D :D

NOW I UNDERSTAND!! - It is a DEEP STALL and Captain Sim modeled it perfectly - as far as I can tell from my limited experience.

In test flying at 12,000' I found that the 727 will go into a deep stall at somewhere around 138 - 140 knots at a AoA of 7 degrees. Once in the deep stall the vertical airspeed quickly reaches 6,000 fpm downward. If recovery is not accomplished within 10 seconds of stick shaker it is almost impossible to recover. However, if at stick shaker onset I push the yoke and increase the power (AFTER the nose drops!) I can recover with the loss of about 4,000 feet. The AC begins to have a positive climb rate at an IAS of around 200 knots.

I'm sure many of you know about deep stalls but since it is a phenomena pretty much restricted to T-Tails it is something I had never experienced or read about.

At a high AoA the horizontal portion of the T-Tail becomes blanketed by the disturbed air from the wing. When the wing stalls - the elevator has little authority to force the nose down. As the stall deepens the elevator looses all authority and becomes useless. The AoA (documented in real life) continues to increase -OR- the AC settles into a wings level nose level attitude with a very high vertical air speed. The AC then pancakes in a near level vertical drop. Well documented on the WEB (Wiki is a good start) and fun to read about.

Now that I understand it I see Captain Sim did a great job of building the proper flight dynamics.

This happens when I fly the 727 on AP and for whatever reason the AoA increases with a drop in IAS. If I am not paying attention and don't hear the stick shaker - then it quickly becomes unrecoverable.

SO - the question becomes - WHY when the AC has been under control of the AP in level stable flight for many minutes, or even hours, does the AoA suddenly increase and the IAS drop? It seems to occur in AltHold and for some reason the throttles slowly retard - thus forcing the AP to trim for pitch up and allows IAS to decrease.

I KNOW - the AP does not control the throttles but at Mach .71 changing the AoA by 5 degrees should not get the AC to Vs.  I need to find a flight envelop diagram for the 727-100 to determine the critical AoA at Mach .71/FL20 - MAYBE it is that sensitive.  Although I thought Vno was around Mach .85.

Good Job on the Deep Stall - Captain Sim!!

this is a copy (with more AP details) of a reply I added to the AVSIM 727 Opinion Thread