View Full Version : FPA on Airbus

Muzungu

3rd Jun 2002, 05:10

I am just converting to the A330 from the 757 and am madly working my brain around a few new techniques whilst trying to dump Boeing stuff and become "assimilated".

I am wondering if there is a formula for figuring out the FPA for a non precision approach based on FAF altitude and distance from threshold. I have seen a chart, but our instructor can't give it to us as it is non SOP and as a contract instructor he doesn't want to get in the brown stuff.

Thanks

CaptA320

3rd Jun 2002, 06:03

Check on the chart what the gradient is, so if you have gradient of 5.6% for example multiply that by 0.6 and you get 3.36 FPA.

Now for selected VOR approaches we generally use 3.0FPA unless it is a real steep approach in which case you check the above gradient and start calculating from the threshold upwords.

For example you want to cross the threshold at 50' so 1 nm before the threshold you want to be 50'+ 336' (converted fpa) = 386', 2 nm before threshold 386'+336=722' and so on until you go back to FAF. However if you do it this way make sure your altitudes meet any restrictions on the chart.

All the best

manuel ortiz

4th Jun 2002, 04:10

Muzu,

Alt. divided by distance in feet , the result "shift" SINE and that's the angle .

Cheers,

Manuel

DC-3 >>>> A-320

Muzungu

4th Jun 2002, 06:09

Thanks guys,

We have FPA on our plates but some in the Carribean and Europe , the Jepps don't.

This should help.

Cheers

thermostat

8th Jun 2002, 04:37

Yes there is a formula , actually two.

First, if you don't have a chart with the FPA, % gradient and gradient , then you will have to find the gradient (grad) of the required FPA. To do that you find the SIN of the angle then multiply that # by 6076 (the number of feet in one N.M.) This will give you the "gradient". e.g Enter 3 degrees and press the SIN button. You will get 0.052335956. (This numner by the way IS the % Grad - 5.2%)

Next multiply that # by 6076 - result = 317.99 or 318 ft down for every one N.M. forward. That is the Grad.

Now for the second formula : The Grad. (318) can now be multiplied by the distance in N.M. to get height to lose i.e. aircraft height above the airport. Now you can divide the height to lose in feet by either a). the Grad to find the distance in N.M. or b). the distance in N.M. to get the Grad.

EXAMPLE : Say you would like to start down from the 10 nm fix from the threshold on an angle of 3.3 degrees. Runway threshold elevation 700 ft. asl. Where do you start the descent?

First formula:

Press 3.3, press SIN = 0.05756 (% Grad = 5.7%) X 6076 = 349.7 or 350. This is the Gradient.

Second formula:

350 X 10 = 3500 feet - height to loose.

To find the aircraft altitude, add 3500+700+50 = 4250 ft.

(50 is the threshold crossing height)

So at 4250 ft at 10.3 n.m. from the threshold (thats .3 of a nm before 10) you would pull the FPA knob and set -3.3.

You can work another way. Say you want to maintain a 3000 ft altitude for the same airport above so now you want to find out at what DISTANCE to start down for 3.3 degrees.

Start with 3000 ft minus 700, minus 50 to get height to lose = 2250 ft. Divide 2250 by 350 Grad = 6.4 n.m.

So at 6.7nm pull and set -3.3 degrees on the FPA knob and slide down the "glide slope".

Here are the numbers for the FPA's recommended for JET aircraft.

FPA 3.0 %Grad 5.2 Grad 318

" 3.1 " 5.4 " 329

" 3.2 " 5.6 " 340

" 3.3 " 5.8 " 350

" 3.4 " 5.9 " 361

" 3.5 " 6.1 " 372

" 3.6 " 6.3 " 382

" 3.7 " 6.5 " 393

Check them out for yourself on a sientific calc using the formulae above.

i.e. Grad=SIN of angle X 6076

Height to lose = Grad X Dist

Dist = Height divided by Grad

Grad = Height divided by Dist

Angle = Grad divided by 6076 Inv SIN

Hope that will answer your question.

thermostat

8th Jun 2002, 04:46

Yes there is a formula , actually two.

First, if you don't have a chart with the FPA, % gradient and gradient , then you will have to find the gradient (grad) of the required FPA. To do that you find the SIN of the angle then multiply that # by 6076 (the number of feet in one N.M.) This will give you the "gradient". e.g Enter 3 degrees and press the SIN button. You will get 0.052335956. (This numner by the way IS the % Grad - 5.2%)

Next multiply that # by 6076 - result = 317.99 or 318 ft down for every one N.M. forward. That is the Grad.

Now for the second formula : The Grad. (318) can now be multiplied by the distance in N.M. to get height to lose i.e. aircraft height above the airport. Now you can divide the height to lose in feet by either a). the Grad to find the distance in N.M. or b). the distance in N.M. to get the Grad.

EXAMPLE : Say you would like to start down from the 10 nm fix from the threshold on an angle of 3.3 degrees. Runway threshold elevation 700 ft. asl. Where do you start the descent?

First formula:

Press 3.3, press SIN = 0.05756 (% Grad = 5.7%) X 6076 = 349.7 or 350. This is the Gradient.

Second formula:

350 X 10 = 3500 feet - height to loose.

To find the aircraft altitude, add 3500+700+50 = 4250 ft.

(50 is the threshold crossing height)

So at 4250 ft at 10.3 n.m. from the threshold (thats .3 of a nm before 10) you would pull the FPA knob and set -3.3.

You can work another way. Say you want to maintain a 3000 ft altitude for the same airport above so now you want to find out at what DISTANCE to start down for 3.3 degrees.

Start with 3000 ft minus 700, minus 50 to get height to lose = 2250 ft. Divide 2250 by 350 Grad = 6.4 n.m.

So at 6.7nm pull and set -3.3 degrees on the FPA knob and slide down the "glide slope".

Here are the numbers for the FPA's recommended for JET aircraft.

FPA 3.0 %Grad 5.2 Grad 318

" 3.1 " 5.4 " 329

" 3.2 " 5.6 " 340

" 3.3 " 5.8 " 350

" 3.4 " 5.9 " 361

" 3.5 " 6.1 " 372

" 3.6 " 6.3 " 382

" 3.7 " 6.5 " 393

Check them out for yourself on a sientific calc using the formulae above.

i.e. Grad=SIN of angle X 6076

Height to lose = Grad X Dist

Dist = Height divided by Grad

Grad = Height divided by Dist

Angle = Grad divided by 6076 Inv SIN

Hope that will answer your question.

A340Driver

8th Jun 2002, 23:17

Thermostat he'll be calling "go around flaps" by the time he does all that:confused:

thermostat

9th Jun 2002, 04:52

I agree with you on the surface. However the time to do that is in cruise flight when you have the time to examine the approach plate.

I have with me a small booklet with all the info already published, so there is no need for long calculations. I simply turn to the appropriate FPA page and read off the distance and height required. Takes about 20 seconds.

I fail to understand why airlines are so reluctant to capitalize on new equipment that can make flying simpler and safer. I am sure that the development of the FPA feature on the A320 family aircraft must have cost a wopping lot of money yet I find most pilots unwilling to use it.

The manual published by Airbus (two volumes) on CFIT accidents points to step-down non-precision approaches as being the biggest cause of fatal accidents and goes on to indicate that a single constant FPA would in almost all cases have prevented the ground contact.

I have always increased the FPA to 3 degrees (5.2% Grad) on any approach that is published at less than 3. As mentioned above the recommended safe FPA for JET aircraft is from 3 degrees to 3.7 degrees yet there are many approaches published at less than 3 degrees. (The chart makers are only concerned with obstacle clearance).

It is necessary to have the FPA charts on hand for the 3 to 3.7 degree slopes and to use them (with raw data backup and dme vs altuitude crosscheck) until one becomes comfortable with the procedure. It's a lot easier than you might think, just needs pratice like anything else. A lot safer too.

We should use EVERY bit of information in the cockpit to our advantage and the FPA feature is no exception. Imho it's the best invention pilots have had for many years, so lets use it. I used the Lotus 123 spreadsheet on my computer to do the charts and if I can do it, anyone can.

Thermostat.

aluminum ovcst

9th Jun 2002, 18:46

Muzungu,

You will notice that many non-prec approaches in Europe have been redesigned so as to fly a constant 3.0º path from FAF to rwy threshold. At least those which are ILS overlays anyway. Of course, normally you will not end up flying these approaches. In other corners of the world, however, be careful and make sure your calculated path takes you above the minimum altitudes. One more thing - Always have your PNF call out the progress of the approach clearly: e.g. "8 DME, 50 ft high", "6 DME, on path", etc.. Good luck with your transition. You will end up loving th "bird".

hptaccv

9th Jun 2002, 19:11

..a feeble question from a wannabe; What exactly does FPA stand for? I'm currently working on my CPL/IFR, and overstressing my brain working out constant descent approaches, thus my interest...

Thanks!

Scallywag

9th Jun 2002, 21:57

hptaccv, FPA=Flight Path Angle

You can have flight director on in one of 2 modes.

1. Heading & Vertical Speed

2. Track & Flight Path Vector

Or you can switch FD off and have the "bird" (FPA) displayed, as well as still having your black dot representing aircraft attitude. The beauty of FPA is that you see your angle of dangle rather than attitude which as you know changes with power, configuration etc. This way you can fly a constant angle descent without reliance on external aids.

This site will give you in depth explanation and pretty pics if you want to see what I mean.

http://www.meriweather.com/fd/fd.html

777AV8R

10th Jun 2002, 00:18

And then there is..VNAV on the 777, which figures it all out for you, if you have the coded approach...or..put your own numbers in.....sit back...and have another coffee.

hptaccv

10th Jun 2002, 00:24

thanks Scallywag..

jetboy

10th Jun 2002, 06:22

777AV8R, the 'bus can do it too, if the approach is part of the database. "If it ain't Boeing, I'll take the 'bus."