Defined Minimum Maneuvering Speed (DMMS)

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Brad W

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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
 
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?”
 
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
 
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
 
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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. :)
 
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brcase said:
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.
Click to expand...

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.
 
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.
 
steingar said:
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.
Click to expand...

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.
 
I just saw the video, glad I went searching for a thread before making a new one. I like it.
 
glad this thread was revived. I'd missed this post.
brcase said:
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,
Click to expand...
Brings to mind an interesting side note for me, a very long time ago...I think not long after my PPL checkride, I had a partial loss of power on takeoff in a C172N. It was a very long runway...actually I looked it up, assuming it hasn't changed much in all these years....35 at KILM is 7,754 ft today. I had done all my training based on out of an airport with a 3,200 Ft runway so 7,754 is long!
Anyway, departing maybe 200-400 ft up and I have no idea now at what airspeed...prob Vy or a little more. So the engine just sorta sputtered and died to idle. Immediately pushed and chopped power, and landed straight ahead. No problem.
So it hit me that had I been at my home field I'd likely have been in the marsh, the James River, or the woods.
What if it had been a 4,000 ft runway....5,000ft. Would I have made that?
So after that I started a habit of doing most of my initial climbs at Vx, trying to get as much altitude as possible while still above a landing surface.
I have often wondered if I was missing something in doing that.....

Oh, the other side thing I learned in that incident..... lean for taxi.....apparently the only potential cause the mechanic found was fouled plugs.... so the best guess was that my very long taxi at full rich ...or a long hold short at full rich for landing traffic after the run-up had fouled the plugs. Seems odd to me that the engine was apparently producing good power for the takeoff run, but maybe I missed something.
 
Brad W said:
glad this thread was revived. I'd missed this post.

Brings to mind an interesting side note for me, a very long time ago...I think not long after my PPL checkride, I had a partial loss of power on takeoff in a C172N. It was a very long runway...actually I looked it up, assuming it hasn't changed much in all these years....35 at KILM is 7,754 ft today. I had done all my training based on out of an airport with a 3,200 Ft runway so 7,754 is long!
Anyway, departing maybe 200-400 ft up and I have no idea now at what airspeed...prob Vy or a little more. So the engine just sorta sputtered and died to idle. Immediately pushed and chopped power, and landed straight ahead. No problem.
So it hit me that had I been at my home field I'd likely have been in the marsh, the James River, or the woods.
What if it had been a 4,000 ft runway....5,000ft. Would I have made that?
So after that I started a habit of doing most of my initial climbs at Vx, trying to get as much altitude as possible while still above a landing surface.
I have often wondered if I was missing something in doing that.....

Oh, the other side thing I learned in that incident..... lean for taxi.....apparently the only potential cause the mechanic found was fouled plugs.... so the best guess was that my very long taxi at full rich ...or a long hold short at full rich for landing traffic after the run-up had fouled the plugs. Seems odd to me that the engine was apparently producing good power for the takeoff run, but maybe I missed something.
Click to expand...
Thank you for sharing your experience.
After a long hold, could carb ice have been an issue? At what point did you pull the throttle back to idle?

Brian
 
brcase said:
After a long hold, could carb ice have been an issue? At what point did you pull the throttle back to idle?
Click to expand...
I suppose carb ice could have been a factor I suppose.... but I still can't wrap my head around it being clear for takeoff and initial climb

it was a long time ago...but the way I remember it the engine just sorta stumbled and died, then it kicked back up a bit but not fully, as I was going through the whole surprise and pitch for speed thing. I remember it being a quick and easy decision to land, so it was probably sometime just after pitching over and I didn't want it kicking back up while landing.

Ravioli said:
A renter could get one of these and set it each time: https://www.aircraftspruce.com/cata...Mq4GWPbKmeoXF7OiepWdzZ94X93kfZWhoCr9gQAvD_BwE
Click to expand...
that was one of the first things I thought of.... and back in the day I would have added one to my flight bag for sure.... At some point I really went on a downsizing of my flight bag. Dragging all the stuff in and out....fastening clamps to the yoke, getting everything set up....and then having to remember to repack it all and not leave it in the plane...got really old. One reason renting stinks.....
 
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