meters per second descent rate in glideslope and landing modes

[WildBillKelsoe]

any thoughts? I reckon 5 m/s sink rate but I could be wrong.

[Vedexent]

I know the kneeboard mod that someone published has suggested decent rates, but digging into Aero's actual published manual for the L-39C, I don't see any suggested rates listed in the landing procedure diagrams.

 

However, using the VFR technique of "set your throttle so that the point on the runway that you want to hit isn't moving up, or down, in your field of vision" results in about 4-6 m/s decent (2 or 3 marks).

 

That'll go up or down, depending on whether you're above or below the glide path, of course, but on the proper slope seems to be around 5 m/s or a bit less.

 

Edit: on page 2-14 of the manual found here, it does indeed state 4-5 m/s decent rate on the base leg of the pattern.

 

Edit2: on page 2-22, in the diagram for the RSBN box landing they indicate 2-3 m/s on the final past the outer marker beacon. 200m AGL, 250 Kph over the outer beacon, 2-3 m/s sink, cross the inner beacon at 60-80m AGL @ 220Kph, touchdown @ 180kph

Edited March 4, 2016 by Vedexent

[frumpy]

4-5 meters.

I think this guy knows pretty well what he is doing:

[ame]

[/ame]

[Vedexent]

4-5 meters.

I think this guy knows pretty well what he is doing:

 

He does 2-3 m/s decent between the outer and inner markers, but hits the inner marker around 90-100 meters - well about the 60-80 recommended, so has to crank up the sink rate to 5-6 m/s.

 

That seems to suggest that 2-3 m/s past the outer beacon is too low to get you on the runway just past the threshold.

 

I'm going to go try it out; I'm guessing a steady 4 past the outer marker beacon will probably do the trick.

 

Once you get all the exact particulars - try it with zero viability (hood down?) and see how it works as an ILS approach ;)

[1.JaVA_Platypus]

It is possible to land like that. But you must realize that any VSI/VVI has a few seconds delay. At least in real-life. So getting a decent sink rate is done by maintaining the correct nose-up attitude and forward speed. Only half an eye should be kept at the VSI dial. And One and half an eye on the horizon and speedometer. And the guy in the vid has a rather wobbly approach.

 

AI is doing an approach and touch down at 200 km/h. This is decent. However you can not see the runway over the gunsight that way.

[Curly]

I devised a handy method for figuring a 3 degree glide slope for aircraft configured in metrics. Be advised it has some accuracy limitations depending on airspeed. This method is most accurate between 150 kph and 300 kph. My method is accurate to within .4 degrees between those speeds.

 

Here is what you do. First, take your approach speed and multiply it by .01 then add 1. A simpler way to preform the function is to insert a decimal point after the first digit in your air speed and add 1 to that. For example lets say our approach speed is 250 KPH, to find our 3 degree glide slope we would just place a decimal point between the 2 and the 5 and add 1. So 250 becomes 3.5, or 250 * .01 + 1 = 3.5. So to fly a 3 degree glide slope at 250 kph we simple have to maintain a sink rate of 3.5 meters per second. This method works for a range of speed from ~150 to 300Kph with an accuracy of .4 degrees. This gives you a quick and easy method to fly a standard approach.

 

They way this works is that if we convert our airspeed to meters per second and divide the sink rate by the airspeed and take the inverse tangent of that number, we’ll have our flight path angle (glide slope). Or the Inverse tangent of VVI / Air speed = flight path angle (alpha). There are some examples below

 

 

Example 1

 

250 kph = 69.4444 mps

 

The VVI from the above method is 3.5, 250 *.01 +1 = 3.5

 

3.5 / 69.4444 = 0.05040003225602

 

Arctan of .0504 = 2.8 degrees which is ~ 3 degrees

 

 

Example 2

 

200 Kph = 55.5556 ms

 

 

3 mps VVI

 

3/55.55556 = 0.539

 

Arctan .053999 = 3.090

 

 

Example 3

 

280 Kph = 77.77 ms

 

3.8 Ms VVI

 

 

3.8 / 77.778 = .0488570032657

 

Arctan of .0488570032657 = 2.797 degrees

 

 

 

150 Kph @ 2.5 MPS VVI = 3.43 degree .4 degree deviation

 

300 Kph @ 4 MPS VVI = 2.74 degree .3 degree deviation.

[Ramsay]

Your figures pretty much match my rough rule of thumb derived empirically and using the relationship of similar triangles.

 

When I've followed the PRGM Glideslope/Penetration profile I've found

 

• 
  
• 400 km/h TAS = 6 m/s descent to stay on the profile
  

with +/- 4 m/s adjustments to correct if above or below the electronic glideslope profile.

 

Using "Similar Triangles" - if I half my true air speed, I need to half my decent rate i.e.

 

• 
  
• 200 km/h TAS ~ 3 m/s descent to stay on the profile
  

with +/- 2 m/s adjustments to correct if above or below the glideslope profile.

 

So when I find a speed and decent rate that follows the glide slope - I try to reduce my descent rate proportionately as I reduce my air speed.

 

As others have said, I'm mostly looking at the runway, glancing at the VVI trend, IAS, etc. rather than chasing the VVI needle - but the simple rule of thumb gives me a "feel" for what numbers to aim for, without the need to remember a table of speeds and descent rates.

 

The triangle also makes it obvious that if I land at higher speeds than usual (perhaps due to load or configuration), I'll also need a higher descent rate to stay on the glide slope.

[doodenkoff]

Imma put this in my kneepad.

[Vedexent]

Or you could just "pitch you nose for the correct speed, set you throttle so the black scuffy part of the runway isn't moving up or down with regards to your dashboard" - it woks for landing a RL Cessna 172 :)

 

Once you've set up your course correctly - HSI needle set up to runway heading, on HSI needle heading with zero deflection on the ADF as you hit the inner marker beacon - if you hold the course, and hold the rate of descent, you should be fine until touch down, but you can check peripheral vision just before flare.

 

You're absolutely right that there's some delay in RL VSI needles - and most likely in the simulator - but if you're chasing large deviations of the VSI that close to the ground, you're in trouble and should do a go-around anyway.

 

By the time you cross the inner beacon marker, you can/should pretty much be set up on course and rate of descent (calculated or eyeballed), and should be able to just 'ride her down'.

 

I worked trial and error for a few hours last night. Straight and level at 200m AGL (radar alt.), 250 Kph over the outer beacon, nose up to pull airspeed back to 230Kph, while pulling throttle back slow to target -4m/s VSI, eyeball the "scuffy patch" of the runway to trim the descent rate - this all puts me over the inner beacon at ~230Kph, and ~80m AGL.

 

You can ride that down until you see the threshold pass in peripheral vision, throttle to zero, glance at the radar altimeter to see it hit 10m AGL, gentle flare to level flight, hold until she starts to drop, pull the nose up to about 10 degrees, and wait for your rear tires to kiss the pavement :)

 

----

 

In the end, though, use whatever works for you. Any technique that gets you on the gound safely, reliably, and without damaging you or the plane (and NOT screwing up other air traffic), is a good one :)

Edited March 5, 2016 by Vedexent