FPA questions

[MED1473 18]

Hi all!

I know you can display the FPA of choice on the HGS via its control panel and that you can select the required FPA in the DEP/ARR page of the MCP if no specific approach is selected for a rwy but I was just wondering, is there a way to select it via the MCP as an alternative to VS?

 

Also, can somebody more knowledgeable than me explain the difference in between a slope expressed in º and as a % and how to convert it?

Thankyou!

[cva1077 68]

First part, no idea! Second part:

 

If we take a typical glide path for an aircraft:

 

 

This slope can be expressed in three main ways:

1) 1:20 (1 in 20) this tells us for every unit up(or down) we will travel 20 units left (or right). Generally expressed as y:x, where y is the vertical distance and x is the horizontal distance.

 

2)As a percentage:

1/20=0.05

0.05x100=5%

 

3)As an angle:

So, we want to know angle a, the angle from the horizontal at which the aircraft will fly. Through the alternate angle rule, a=b. To work out angle b, we can use some right angle trig:

 

tan(b)=opposite/hypotenuse

hence: inverse tan(opposite/hypotenuse)=b

inverse tan(1/20)=2.86 degrees

because b=a, a=2.86 degrees

 

As for converting between, draw a small triangle and fill in the values you know(eg. the angle) then substitute 1 into the horizontal component and resolve for the vertical component. Then divide the vertical by the horizontal and multiply by 100 to get the percentage.

 

I hope this helps!

[MED1473 18]

It does thanks, very well explained. That's all clear now thank you!

Anybody else got an idea about the first question?

[Spin737 102]

You can fly a runway extension in LNAV/VNAV. No need for any special modes.

[MED1473 18]

Yup but what about if it's not a straight in landing but I'm still required a prescribed fpa (eg am offset approach?

[rsvit 16]

Yup but what about if it's not a straight in landing but I'm still required a prescribed fpa (eg am offset approach?

 

You can still fly VNAV down to MDA or use V/S from tables in approach charts.

[MED1473 18]

Ok, does this mean there is no way to imput the fpa in the mcp?

[KriVa 307]

That's exactly what it means... It's not an Airbus, you know ;-)

[rsvit 16]

It's not an Airbus, you know ;-)

 

You can do that on triple seven, E-jets and maybe some other also.

[KriVa 307]

Did not know that, although it is pretty logical in retrospect.

Thanks!