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SunDog
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« on: April 25, 2014, 07:15:14 PM »

Anyone have an opinion on the variations on emergency descents (looking at C-172)  Do you like full flaps, roll it over to 60 degrees bank, and lock it on Vfe? Or, no flaps, higher speed (yellow arc), same bank?

It's looking like Vfe, with flaps, etc. is best, but thoughts on variations? Steeper bank, heavy slips, etc.?  Looking for the best rate of descent configuration that won't bend the airplane.  Not considering rule-book limits, just airframe limits - assume single pilot, other seats empty.
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AirDX
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« Reply #1 on: April 25, 2014, 10:08:27 PM »

Anyone have an opinion on the variations on emergency descents (looking at C-172)  Do you like full flaps, roll it over to 60 degrees bank, and lock it on Vfe? Or, no flaps, higher speed (yellow arc), same bank?

It's looking like Vfe, with flaps, etc. is best, but thoughts on variations? Steeper bank, heavy slips, etc.?  Looking for the best rate of descent configuration that won't bend the airplane.  Not considering rule-book limits, just airframe limits - assume single pilot, other seats empty.

What are you trying to accomplish, maximum descent per unit of forward progress (i.e. best angle of descent) or just maximum descent per unit of time (i.e. best rate of descent)?
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Cliff_Chambliss
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« Reply #2 on: April 26, 2014, 01:05:22 AM »

Power to idle
Full flaps
Full rudder deflection - counter with aileron
Push over to the top of the white arc.

OK, there are those out there that will say the Cessna 172 should never be slipped with full flaps.  The Flight Manuals say that under certain conditions slipping with full flaps may result in oscillation of the elevators and may cause a nose down pitching movement.  The POH for various models of the 172 say full flap slips are not recommended, others say they should be avoided, some don't even address them.

Full flaps on the C-172 thru the 172N T-41A,B,& C are 40 degrees. So slips with less than full flaps should be OK.  Now,  Full flaps for the C-172P,R,&S are 30 degrees so....
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JeffDG
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« Reply #3 on: April 26, 2014, 01:27:54 AM »

OK, there are those out there that will say the Cessna 172 should never be slipped with full flaps.  The Flight Manuals say that under certain conditions slipping with full flaps may result in oscillation of the elevators and may cause a nose down pitching movement.  The POH for various models of the 172 say full flap slips are not recommended, others say they should be avoided, some don't even address them.
Even those with the notation, I've not seen one where slips-with-full-flaps are a prohibited maneuver (like on PA-28s, where intentional spins are prohibited)

I look at that recommendation as being a "Really, don't try this for the first time on final approach when you don't really have the room to recover, because it's a bit different than other planes."
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a2capt
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« Reply #4 on: April 26, 2014, 01:59:04 AM »

..and that full 40 degrees is all I've ever known with a 172N, and it's fun to drop like a brick from abeam the numbers at pattern altitude and swoop the threshold markings just as your straightening out and drop it on the numbers. Fly it like a tail dragger ;)
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SunDog
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« Reply #5 on: April 26, 2014, 03:48:10 AM »

Anyone have an opinion on the variations on emergency descents (looking at C-172)  Do you like full flaps, roll it over to 60 degrees bank, and lock it on Vfe? Or, no flaps, higher speed (yellow arc), same bank?

It's looking like Vfe, with flaps, etc. is best, but thoughts on variations? Steeper bank, heavy slips, etc.?  Looking for the best rate of descent configuration that won't bend the airplane.  Not considering rule-book limits, just airframe limits - assume single pilot, other seats empty.

What are you trying to accomplish, maximum descent per unit of forward progress (i.e. best angle of descent) or just maximum descent per unit of time (i.e. best rate of descent)?

Short version - max rate of descent; don't give a fig about horizontal displacement from the starting point. Just come down fast, don't break the airplane. Not that it's relevant to my purposes, but emergency descent is part of the PTS, and another guy and I experimentd with full flaps, Vfe, and a steep bank. That results in a very stable, rapid descent. Not as quick as an intentional spin, but safer to continue to a much lower altitude.

I'd like to get it stable in descent, record the numbers, use them to tune some simulator code.
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SunDog
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« Reply #6 on: April 26, 2014, 04:03:29 AM »

..and that full 40 degrees is all I've ever known with a 172N, and it's fun to drop like a brick from abeam the numbers at pattern altitude and swoop the threshold markings just as your straightening out and drop it on the numbers. Fly it like a tail dragger ;)

I can get a T-41 to come down pretty quick, flaps 40, rudder to the floor, opposite aileron. It rumbles and shakes some, for sure. But it doesn't come down as quick as the coordinated steep bank, Vfe, full flaps. I can tell the sim 172 fidelity isn't so hot (rate of descent, rudder effects) compared to the real airplane, using bank, flaps, Vfe method.  I haven't tried it yet, but some sources say steep bank, and airspeed up in the yellow arc is better. . .I'll try it and see.

It makes sense to get the lift vector as far from vertical as possible, without rolling past 90 degrees, and you'd bust Vfe or Vne if you did that, anyway.

I 've heard you can hold some Pipers in a pwer -off stall, use the rudder to keep the wings level, and just let it sink like a brick - but I'm lookind strictly at 172s now.
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a2capt
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« Reply #7 on: April 26, 2014, 04:45:27 AM »

I will say that yes, it is better in the yellow arc. :)
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SunDog
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« Reply #8 on: June 22, 2014, 06:54:00 PM »

Follow up, after some experimenting - the FAA may be correct (unusual, I know!); full flaps, 60 degree bank, Vfe.  That gets a 172P down from altitude as fast or faster than other techniques, except an intentional spin, of course.  It works out to be more stable than heavily cross-controlled configs, and about as fast - just watch the airspeed, and if it starts to creep up, take out a little bank BEFORE adding back pressure.

Neat thing is, get it trimmed and it's very stable - gives you time to muck about with whatever issue precipatated the need to get down quickly.  The "brand-X" simulator doesn't track the real-world airplane behaviour with great fidelity, so expect the real thing to differ from the sim to a meaningful degree.

X-control at the edge of the yellow works, too, but it's awkward and takes more attention, at least for me. The steep bank approach keeps the terrain below you in good view, adds a little margin for choosing (or changing your mind) about where to park it. That's subjective, of course. 

Anyway, after a few hours of fooling around with it, my personal preference, when faced with a need for an emergency descent, will be to chop power, drop flaps, peg Vfe, and roll to near knife edge; trim for the condition, and then try to sort out my problem.
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PHall
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« Reply #9 on: June 23, 2014, 01:24:01 AM »

Follow up, after some experimenting - the FAA may be correct (unusual, I know!); full flaps, 60 degree bank, Vfe.  That gets a 172P down from altitude as fast or faster than other techniques, except an intentional spin, of course.  It works out to be more stable than heavily cross-controlled configs, and about as fast - just watch the airspeed, and if it starts to creep up, take out a little bank BEFORE adding back pressure.

Neat thing is, get it trimmed and it's very stable - gives you time to muck about with whatever issue precipatated the need to get down quickly.  The "brand-X" simulator doesn't track the real-world airplane behaviour with great fidelity, so expect the real thing to differ from the sim to a meaningful degree.

X-control at the edge of the yellow works, too, but it's awkward and takes more attention, at least for me. The steep bank approach keeps the terrain below you in good view, adds a little margin for choosing (or changing your mind) about where to park it. That's subjective, of course. 

Anyway, after a few hours of fooling around with it, my personal preference, when faced with a need for an emergency descent, will be to chop power, drop flaps, peg Vfe, and roll to near knife edge; trim for the condition, and then try to sort out my problem.

Pulling out at the bottom of the descent would be nice too. >:D  i.e. don't get so busy fixing the problem that you forget to fly the airplane.

It has happened, way more then once. :o
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SunDog
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« Reply #10 on: June 23, 2014, 03:52:31 AM »

Funny you should say that - I did take it lower than I intended on one try, getting a little wrapped up in making notes and fine-tuning trim. Not danger-low, but enough to scold myself a bit.  No danger of that when spinning, since the 172 spins so steep - mother earth is in plain view in the whole time. . .so, about done with Cessna flying now; been there, done that, got the T-shirt. On to something different, hopefullly with some more performance and character.
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ReCAP
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« Reply #11 on: June 23, 2014, 02:14:14 PM »

In a true emergency long term engine reliability is not the priority, but is there an issue with sustained flight at 100kts with the engine at idle, thermal-stress wise? 
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SunDog
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« Reply #12 on: June 23, 2014, 06:30:03 PM »

It's closer to 85 knots, I think, with full flaps.  As to thermal stress, I didn't see anything in the POH.  This particular day was pretty warm, so it's not like I was wiping power off on a cold winter's day, then suddenly laying the coals to it to recover after a long idle..  I entered from low-cruise power, bottom of the green arc, just above carb heat required RPM, already leaned, about 5K' AGL.

But it was idle power all the way down to about 1K' AGL, then roll level, bring it back up to 2100 RPM or so, make some notes, then climb back up.  Not being Chuck Yeager, I didn't descend nearly so long (or low) out of a spin, of course.  After three or four turns, I'm happy enough to stop watching the world rotate. Was just looking to measure altitude loss vs time, after established.  I guess a Sierra Hotel, Ace of the Base type might try to get down real fast that way, but it doesn't seem practical to me (subjective opinion, of course).  I just don't see myself doing it - "Wow, got a engine fire! I know, I'll yank power, smoothly increase AOA while trying to hold altitude, then stomp rudder at the stall break - that'll get me down quick!"  if you were at cruise speed, it's take a bit to slow down, and if you got a little excited, it'd be easy to pull too soon, too hard, and end up with a view out the windscreen you wouldn't much like much, or a find yourself in a steadily expanding cloud of airplane parts. . .

And it'd take a cool head to start running a checklist and diagnosing a problem while auto-rotating. . .and since this was 172 specific, in my experience, I'm usually not that high in a 172 - most often under 5K' AGL, and even less than 3K' AGL.  I think the spin entry and spin recovery time would offset any gains from spinning down.
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ReCAP
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« Reply #13 on: June 23, 2014, 07:20:01 PM »

Oops, yes, 85 kts is correct.  I was thinking 100 mph.  Shows you how long its been since I've flown. 

Regarding spins, I do remember reading a fictional account of an early transatlantic flight where they intentionally spun down through a cloud deck rather than attempting to continue visual flight into IMC. 
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SunDog
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« Reply #14 on: June 24, 2014, 05:48:00 PM »

I think I've read something similiar, though can't recall the source.  Pretty rad, but why not?

Ernest Gann describes a very, very tight spiralling descent through a small hole (WWII, somewhere way up in the NE, or the Canadian Maritimes?) in a burning airplane (multi-engine), and needing to get down NOW, and not enough time to fly the published approach. . .the book is Fate is the Hunter
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