How to calculate VS during approach? Saab 340A

[FloB]

My problem: How to perform a (smooth) descent/approach and meet the altitude restriction. I'm frequently in trouble (mostly flying too high and thus beeing too steep on final) if visibility is restricted or when flying approaches that require turns > 100° during descent.

 

How would, for example, a pilot calculate his V/S for an approach like

 

https://skyvector.com/files/tpp/1513/pdf/00545RRY13R.PDF

 

with IAF PSP and without an FMS (LES Saab340A)

 

What is the general idea and procedure for calculating V/S during approaches? How do I compensate for upwind / downwind? How for KIAS changes?

How to use the GPS's VNAV best?

 

Any hints appreciated.

 

Flo

Edited December 13, 2015 by FloB

[JGregory]

I have sent your questions off to one of our test pilots and will let you know.

 

In the meantime, maybe another user will have some insight.

Edited December 13, 2015 by JGregory

[FloB]

Great, thank you Jim.

 

F.

[ilias.tselios]

To my best of my knowledge, Saab 340A is not properly equipped for the execution of that kind of approach. That's why there are other types of approaches available like: https://skyvector.com/files/tpp/1513/pdf/00545VGB.PDF

 

In general, there are tables at Charts.pdf in the Manuals folder, on page 105 and 106 that gives you the altitude where you will start the descend, the distance to land with headwinds/tailwinds, even the fuel consumption. These tables assume that you are flying in you depart your cruising (or current) altitude in IAS HOLD mode and power levers to flight idle. That's the way that mostly this type of aircrafts meant to be flown.

[Hovercat]

If you want to calculate your required v/s for a 3° Glidepath, as seen on the above approach chart, you could use this estimation:

 

V/S=GS*5=160kt*5=800fpm as an example

 

Of course this only works for a GP=3°, if its steeper (some are 3,5° or more), you have to decent faster...

Furthermore, most approach plates have a small table with calculated V/S for a range of ground speeds. Yours doesnt have that tho...

And if you want to be on the save side, you can always decent on a higher rate, but leveling of at the "steps" specified. 

Hope it helps a little...

[mgeiss]

You can also use "half GS * 10", which is often easier to calculate in the head.

 

Another common formula for descents is "altitude-difference/1000 * 3" for calculating your TOD.

Let's say you are cruising at 20.000ft  and you want to be at 5.000ft  at a given waypoint. That's (20-5)*3 = 15*3 = 45 nautical miles. So you would start your descent when there are about 50nm to go to that waypoint  (just to be on the safe side).

 

You can also use the first formula for calculating your v/s for the descent. Just make sure to constantly adjust for the change in GS which goes along the change in altitude, even when your IAS stays the same.

Edited December 14, 2015 by mgeiss

[FloB]

Thanks guys,

 

this helps. Half GS * 10 (for 3° approaches) will have to do, anything more complicated will fail to be "constantly adjusted" in my head for now. I guess I have to be more strict about my approach speed to not mess things up. Will give it a try.

I also came up with the idea of descending faster until the next restiction is hit, but that would require a lot of thrust adjustments und thus add a new source of mistakes and stress.

 

Best

Flo

[JGregory]

Flo,

 

Here is a response from one of our Saab pilots... (sorry for the delay)

 

So descent in a Saab. 

Speed is planned at Vmo - 10 kts (240 usually). The v/s to use is 1500fpm. Calculate TOD using 3nm for every 1000 feet you need to lose. 

For example if I need to be at 2000ft AGL above the airfield on arrival, and I am at 20 000ft AGL, then I need to lose 18 000ft. So I will need to start my TOD at 54NM (3 x 18) from the airport. As far as wind goes, add 1nm for every 10kts of tailwind and subtract 1nm for every 10kts of headwind. 

The trick with this is to keep doing the calculations to see if you are high or low. So in the above example I know that passing 12 000ft I need to be 30nm from the field. So I can adjust my V/S to make sure I pass through that gate correctly. And of course if ATC gives a speed limit, then I may need to back off the V/S a bit so I can still meet my height and speed goals. Eg say in the above example I want to be at the aerodrome at180kts at 2000ft joining crosswind. So I obviously cannot keep at 1500v/s and 240kts till 3nm from the field. So I will need to start slowing down in enough time that I can meet those goals. The SAAB will happily descend at 1500ft at 240kts with minimal power set, so there has to be some planning in advance to start reducing the V/S and the speed to meet the speed requirements at the aerodrome. 

Edited January 7, 2016 by JGregory

[VHFly]

Descents, while complex, are usually fine if you plan ahead and keep on top of it by continually analysing where you are. 

There are a few things to keep thinking about such as distance to go, the wind, the altitude you need to lose, the weather, turbulence (eg if you need to come back to the turbulence penetration speed, then the VS may need to be reduced), cabin pressure, ATC requirements, minimum safe altitudes and approach steps, other traffic. The point is that everything has an impact on a successful descent. It is not just set and forget. 

 

And as to the approach there, I would probably chose a normal RNAV to try as the 340A isn't equiped in the real world to do an RNP approach. 

 

With RNAVs generally, you look at descending 320ft/nm. So work out what height you want to be at each waypoint and then depending on the speed set a vs that will make you achieve it.

 

Hope all that makes sense. 

[FloB]

Thanks everybody for your feedback! And insight from a real Saab pilot is always very much appreciated.

I think with all that I have all I need to improve my descend/approach planning and management.

Now it's on me...

 

Best

Flo