Simviation Forum - Rate of descent for landing in a Piper Dakota?

Mar 3^rd, 2010 at 12:12am

Mr._Ryan Offline
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Hey,

I just recently got the DreamFleet Dakota (PA-28-236), which comes with a manual, but one of the things the manual doesn't say is what the proper rate of descent is for landing. Speed and all that jazz is in there, but not the rate of descent. I've tried it around 400-500 fpm and generally find myself too high (above the glide slope). I've googled everything I can think of (I'm probably not using the proper terminology, who knows), but can anyone answer this question for me?

Thanks!

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Reply #1 - Mar 3^rd, 2010 at 6:54am

Brett_Henderson Offline
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You don't fly an approach for a target vertical speed. In fact (especially in light GA), you might as well cover the vertical-speed indicator.

Your goal, is an approach speed (Vref)... then you use pitch to maintain that speed... and then manage your vertical position with the throttle.

Referencing your example: If you started the final approach at the proper altitude; properly configured (gear/flaps/airspeed).. maintaining that airspeed by pitch, and noticed you were a little high (never even glancing at the vertical-speed indicator).. you'd simply reduce power and continue pitching for Vref. If too low.. increase power and continue pitching for Vref.. During this process (especially if there are changing winds).. your vertical speed will fluctuate quite a bit.

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Reply #2 - Mar 3^rd, 2010 at 9:58am

Mr._Ryan Offline
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Ah, so Vref in the Dakota is 72 KIAS, so if you nail that and start the final approach at the proper altitude, you're good. I suppose that's pretty good information to have Wink

Thank you for answering and not rolling your eyes, I appreciate it.

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Reply #3 - Mar 3^rd, 2010 at 10:06am

Brett_Henderson Offline
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Mr._Ryan wrote on Mar 3^rd, 2010 at 9:58am:

Ah, so Vref in the Dakota is 72 KIAS, so if you nail that and start the final approach at the proper altitude, you're good. I suppose that's pretty good information to have Wink

Thank you for answering and not rolling your eyes, I appreciate it.

My pleasure

Now.. during all of this, your airspeed will fluctuate too.. and there's a big difference in how the approach will be flown comparing zero-wind to say something like a 20knot headwind.. You just keep using an ever-fluid combination of pitch (for airspeed) and power (for vertical position) until you're over the pavement.. then it's just a matter of bleeding off the airpseed (power reduced as needed.. near idle at touchdown) until the airplane just stops flying, and you have no choice but to "let" the wheels touch .. Cool

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Reply #4 - Mar 3^rd, 2010 at 10:19am

Mr._Ryan Offline
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Brett_Henderson wrote on Mar 3^rd, 2010 at 10:06am:

Mr._Ryan wrote on Mar 3^rd, 2010 at 9:58am:

Ah, so Vref in the Dakota is 72 KIAS, so if you nail that and start the final approach at the proper altitude, you're good. I suppose that's pretty good information to have Wink

Thank you for answering and not rolling your eyes, I appreciate it.

My pleasure

Now.. during all of this, your airspeed will fluctuate too.. and there's a big difference in how the approach will be flown comparing zero-wind to say something like a 20knot headwind.. You just keep using an ever-fluid combination of pitch (for airspeed) and power (for vertical position) until you're over the pavement.. then it's just a matter of bleeding off the airpseed (power reduced as needed.. near idle at touchdown) until the airplane just stops flying, and you have no choice but to "let" the wheels touch .. Cool

Yeah, I do just fine with the flare, most of the time anyhow. So I understand you correctly: the airspeed will fluctuate by a bit above and below 73 KIAS, but my target is to always maintain that, using adjustments in pitch and power, correct? Presumably you would pitch the nose down a bit flying into a 20 mph headwind vs. little or no headwind and would probably add a bit of power. Do I have the concept right?

It's interesting, because I did all the lessons in FSX for private pilot, and I don't remember them being specific about this. For some reason I remember the target being a specific vertical speed. Either I remember wrong or you're giving me better information than Rod

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Reply #5 - Mar 3^rd, 2010 at 11:13am

Brett_Henderson Offline
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The vertical speed is like an afterthought.. or fore-thought as it pertains to a glide-path. All things being standard.. a certain ground-speed, from a certain altitude and distance out, WILL have an "average" vertical speed. That's geometry..

It's just not something you, as a pilot try to "fly". You really can't fly a vertical-speed to a point on the ground... there are too many variables.

Quote:

Presumably you would pitch the nose down a bit flying into a 20 mph headwind vs. little or no headwind and would probably add a bit of power. Do I have the concept right?

Ponder that.. A pitch down will increase airspeed.. adding power will increase it even more ..so much for that target airspeed Cheesy

A headwind will reduce your ground-speed, so you need to reduce the "steepness" of the approach. With your normal approach configuration as a starting point.. you'll quickly notice the you're getting a bit low. We control vertical position by throttle, so add a bit of power... then as airspeed increases, we pitch up a bit to keep it at Vref.. See how that would work ?

It's counter-intuitive, but you gotta keep it in mind. Your elevators control your airspeed (not the throttle).. and your throttle controls you're veritcal position (not the elevators).

"Pitch for airspeed.. power for altitude" ... all the way down, and no mater what the wind might be.. you'll arrive at the runway (at the proper airspeed)..

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Reply #6 - Mar 3^rd, 2010 at 1:38pm

Mr._Ryan Offline
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Brett_Henderson wrote on Mar 3^rd, 2010 at 11:13am:

The vertical speed is like an afterthought.. or fore-thought as it pertains to a glide-path. All things being standard.. a certain ground-speed, from a certain altitude and distance out, WILL have an "average" vertical speed. That's geometry..

It's just not something you, as a pilot try to "fly". You really can't fly a vertical-speed to a point on the ground... there are too many variables.

Quote:

Presumably you would pitch the nose down a bit flying into a 20 mph headwind vs. little or no headwind and would probably add a bit of power. Do I have the concept right?

Ponder that.. A pitch down will increase airspeed.. adding power will increase it even more ..so much for that target airspeed Cheesy

A headwind will reduce your ground-speed, so you need to reduce the "steepness" of the approach. With your normal approach configuration as a starting point.. you'll quickly notice the you're getting a bit low. We control vertical position by throttle, so add a bit of power... then as airspeed increases, we pitch up a bit to keep it at Vref.. See how that would work ?

It's counter-intuitive, but you gotta keep it in mind. Your elevators control your airspeed (not the throttle).. and your throttle controls you're veritcal position (not the elevators).

"Pitch for airspeed.. power for altitude" ... all the way down, and no mater what the wind might be.. you'll arrive at the runway (at the proper airspeed)..

Makes perfect sense! It seems as if I was half-right in terms of adding a touch of power, but being half-right in this case is the equivalent of being wrong

It's funny, because adding power when I'm too low is instinctive for me - which is a good thing, because it's the right thing to do. And now with this information you gave me it really does make sense. Essentially the headwind will slow you down, so if you were to fail to adjust to this, you would meet the ground short of the runway, because you are descending at the same rate but not traveling forward as much. So you add the power to keep from coming up short of the runway and pitch the nose up to maintain the proper speed.

I really appreciate the time you took to explain this, thank you so much.

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Reply #7 - Mar 3^rd, 2010 at 2:09pm

aeroart Offline
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Your rate of descent on approach is based on the distance between the final approach fix and the decision height, and your ground speed. Say, for example, the distance is six miles and your final approach ground speed is 90 kts. It will take you four minutes to cover that distance. If the descent is from 1500 ft at the final approach fix to a decision height of 200 feet, a descent of 1300 feet, you have to do that in four minutes, making the desired descent rate 325 fpm (1300 divided by four).

Art

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Reply #8 - Mar 3^rd, 2010 at 2:37pm

SeanTK Ex Member

Also, keeping in mind that your focus will be on the proper airspeed, which in most cases would make what I am about to say a moot point, but anyway:

It not a very good idea, at least in my experience when descending from a reasonable cruise altitude in a light aircraft, to descend at more than about 800 fpm. Anything more has the potential to be rather uncomfortable to either you or your passengers due to the rapid pressure changes. I say this keeping in mind that there may be a time when you start your descent a little late, and are focused on just getting down rather than nailing a nice speed range for the descent.

I actually learned in my training to focus a little more on the VS rate. (Of course one has to pay attention to the airspeed too...) Part of my training consisted of practicing either constant airspeed climbs/descent, or constant vertical speed climbs/descents while maintaining a reasonable and safe airspeed. Wink

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Reply #9 - Mar 3^rd, 2010 at 2:53pm

aeroart Offline
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Yes, SeanTK is right: airspeed is most important, particularly in not letting it get too slow and risking stalling at a low altitude. Brett is right in that a properly flown approach is accomplished by understanding the relationship between pitch and power.

Mr_Ryan, if you would like to practice airspeed and vertical speed control for a longer period of time than a final approach allows, take your airplane up to, say, 10,000 ft, and set a target approach speed and vertical speed, and try to get both in your descent. Then do it again with different target speeds and rates. I'm sure you will find this very useful in improving your flying.

Have fun!

Art

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Reply #10 - Mar 3^rd, 2010 at 5:31pm

olderndirt Offline
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By all means use those gauges but also pay attention to the plane's attitude as you descend or whatever. Someday you may have to fly it without some of those precious clocks so it's good to know how things should look 'over the nose'.

THIS IS NOT A PANAM CLIPPER

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Reply #11 - Mar 6^th, 2010 at 10:45pm

beaky Offline
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olderndirt wrote on Mar 3^rd, 2010 at 5:31pm:

By all means use those gauges but also pay attention to the plane's attitude as you descend or whatever. Someday you may have to fly it without some of those precious clocks so it's good to know how things should look 'over the nose'.

You beat me to it... I was going to say the same thing, sort of.
There's more to it than nailing the airspeed, distance and altitude and just "locking that in", although starting your final with everything just right is the best way to go.
However- if adjustments need to be made (and they usually do), chasing the VSI needle will just get you in trouble.

The "secret", with any aircraft, on a visual approach, anyway, is to do whatever you have to in order to keep the sight picture looking as it should. If the angle looks good throughout and your airspeed is within limits, you should not descend more rapidly than you want.

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