Defined Minimum Maneuvering Speed (DMMS)

Discussion in 'Hangar Talk' started by Brad W, Feb 9, 2020.

  1. Brad W

    Brad W Pre-takeoff checklist

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    I'm curious your thoughts about this.
    I was watching the Flight Chops recent video on AQP AFR, which pointed back to a vid of his I remembered watching a few months back with Dan Gryder that in part covered DMMS

    starts about 9:40 into the video,but I appreciated the whole thing and think it's worth watching all the way through for sure.

    anyway
    So using the multiplier they presented (1.404 VS1) for the cessna 172N I'm currently renting for my rusty clean up/BFR I get
    50Kt x 1.404 = 70.2 KTS CAS

    Taken from the narrative under the flightchops vid....from Dan, in part "Vs1 x 1.404 becomes a)best glide b) safe climb speed) c) safe DMMS in the pattern."

    It works for my normal landing/pattern speeds...70 no flaps....

    The part I'm trying to wrap my head around handling is the best glide and climb.
    70.2Kt is of course higher than Cessna's book best glide speed which is 65Kt, and is a good bit higher than the 59 Kt Vx
    I get that it's a maneuvering idea....so
    Vx only in straight flight....initial climbout without turns
    Vbg on in straight flight....if a turn is needed in the glide, increase speed to 70

    Anyway, just thought it was worth discussion..... thanks
     
  2. denverpilot

    denverpilot Tied Down

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    I thought it was an interesting concept. I think we had a thread already too but maybe not.

    The idea that there should be monitored speeds upon which one will not go below isn’t truly new, and I remember older instructors essentially teaching the same thing long ago without marking it.

    It was more of a “If you just got THAT slow, something is totally screwed up with this pattern/approach/whatever... how about we execute an immediate go around and go back out and set this all up again, and do it right this time?”

    Dan setting it as a must act trigger that’s marked on the ASI is more formal than that, but the concept is similar. Why are we pushing on into an approach or maneuver that’s already hit a major airspeed error? It’s only going to get worse unless you get aggressive about fixing it, right now. And it’s a big hint your head isn’t in the game.

    We all talk about hearing old instructor’s voices in our heads saving our bacon later down the road. I can think of a few times I heard “Too slow, how about we just go back out and start this over?”
     
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  3. Brad W

    Brad W Pre-takeoff checklist

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    I would have thought there would be. Did a very quick search though before posting and came up empty
     
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  4. brcase

    brcase Pattern Altitude

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    So the idea is if you are maintaining altitude you can bank up to 60 degrees (in the video he said 30 deg, but I believe the math is actually 60 degrees) without getting into an accelerated stall using the 1.404 number.

    I agree with the general idea. Vx is not a “safe climbspeed”. So the 1.404 number matches up better with a Vy climb. Get an instructor and try doing some engine failure scenarios at Vx. There are some portions of a Vx take-off from which a power failure at Vx will result in a damaged airplane at minimum, in quite a few airplanes. We use Vx to provide more margin for error in some takeoffs, but if a Vx climb is required for a safe take off, you probably shouldn’t be taking off. This is a risk management thing, where we decide the chances of a power failure are less than the odds of us the airplane or pilot not performing up to expectations and decide the extra obstacle clearance is less risky than a possible power failure.

    Also when you look at performance charts for most things you find the penalty for being 5kts fast is about 1/2 the penalty for being 5kts slow. Hypothetically if you needed a Vx climb to exactly clear an obstacle and you flew it 5kts to fast you would not clear by perhaps 10 feet. If you flew it 5kts to slow you would not clear by 20 feet and have a much great risk of a stall.

    Brian
    CFIIG/ASEL
     
  5. 455 Bravo Uniform

    455 Bravo Uniform En-Route

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    I watched this a few nights ago. I liked the concepts. Saved the PDF.

    Vs1 is 40 kts for the Peterson STC, so DMMS comes out to 56. That’s also near my plane’s recommended short-field landing speed (55). My calc turned out to be just under Vx (57).

    I put this mark on my ASI. I’m not sure it will help, but it can’t hurt I don’t think.

    25B4A90B-C2AB-4757-BCD5-B2F7B9500C8E.jpeg
     
    Last edited: Feb 9, 2020
  6. dtuuri

    dtuuri En-Route

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    Your, as he calls it, DMMS would depend on stalling speed in the configuration and for your bank angle, I would think. In type rating airplanes you usually figure the speed in reference to Vref, i.e., 1.3 Vso. You start with a basic Vref for zero bank and full flaps at the current weight which varies with passenger load and fuel burn. Then you adjust that upward for any partial flap setting so as to maintain the same safety margin. Since jet jocks don't ever bank more than 30°, a nominal additional increment of speed, usually 10 kts, is added to Vref. So, it might look like this:

    Full flaps........= Vref+10
    Approach flaps = Vref +20
    Clean............= Vref +30​

    Basic Vref is with zero fuel, so you'd add one knot, say, per 200 lbs of jet fuel and another knot for every passenger, then set the adjusted Vref on the ASI bug. Easy peasy. :)
     
    Last edited: Feb 9, 2020
  7. MauleSkinner

    MauleSkinner Final Approach

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    As @dtuuri indicated, this isn't just to "not stall", it's to maintain an angle of attack at or below that which provides a 30% stall margin...so it's the math for a 30 degree bank on top of 1.3 Vs.
     
  8. steingar

    steingar Taxi to Parking

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    I think the biggest thing to take away is if you loose an engine on takeoff better be ready to push hard and fast. Unpowered, speed decays quickly in most of our airplanes.
     
  9. denverpilot

    denverpilot Tied Down

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    Dan covers that concept in other videos and articles. He uses the acronym LOTOT or Loss of Thrust on Takeoff and has written about it fairly extensively and how deadly it is. He also trains the “big push” as an instant and immediate response to it.

    This video was more about the concept of low level yanking and banking and setting some real objective limits on speeds for it with simple math. Then showing an ASI can simply be marked for it. No guesswork. If you’re above the line you can accept a certain bank angle.

    Not as helpful for renters as owners but still an interesting point that’s not often made by marking it in small aircraft. Makes it instantly easy to see.
     
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