How is VNAV used on a typical real flight?

[Sweet19blue]

Hi,

I did my come back flight a few days ago, all went well.

I kept noting though that my TOD point was way off from the beginning when I inserted the flight plan. I did enter the whole flightplan, then I edited some constraints of altitude and speed on the arrival (all this was done on ground before takeoff). I could see that the TOD did not change after entering the constraints. 

Well, I know the VNAV is currently being worked on, I guess because of such behavior. Nevertheless, I read from this forum that the real airplane VNAV system was not so reliable. So, as the title says, I am wondering how the VNAV is really used during a typical flight on the real 737 Classic?

Bedt regards,

Pierre

[Litjan]

Hello Pierre,

often the TOD does not change when you enter or change constraints on the descent path - because it is not affected by a new constraint, or lifting an old one doesn´t affect it.

Imagine a "idle descent" path that passes well under a restriction of "be at FL250 or below". Now clearing this restriction won´t affect the calculated path, because it never affected it in the first place!

The calculation of the VNAV PTH descent in the real 737 worked fine - but it was complex and had some potential pitfalls. Especially when winds were unknown (as they mostly are) or when ATC changed your lateral flightpath (as they almost always do) the calculation of the optimum path can get you in trouble. This is due to the fact that the FMS will plan the descent in an optimal way - without any conservative reserves. So if there is less headwind or the routing gets shortened you will immediately be "too high" to start your approach. This does not work well with the conservative and safety-oriented attitude that airline pilots have. Also the complexity and missing transparency of VNAV calculation make real pilots shy away from using it - don´t use what you can´t understand and doublecheck.

So speaking for my airline, the typical use of VNAV was seen in climb and cruise. In those portions of the flight VNAV works exactly like FL CHG, with the difference being that the target speed is calculated by the FMS.

Usage in descent was close to 0. We used the TOD and vertical deviation bar to "sanity check" our own descent calculation, but the VNAV PTH descent mode was barely ever used.

Cheers, Jan

 

Edited March 29, 2020 by Litjan

[Sweet19blue]

Thanks for the answer. 

Actually, I introduced an “at” altitude at the waypoint before the initial T/D at FL220. Cruise level was FL260, so I was expecting the T/D to move before that waypoint so I am able to descent from 260 to 220. But I did not move. I will have to try again.

Regarding real flight, we’re you using V/S or FLCH for the descent or maybe a combination of the 2 modes?

Best regards,

Pierre

[Litjan]

Hi Pierre,

It is very well possible that your change confused our FMS. Especially with the descent calculations it is very unreliable. It works "ok" when you have no restrictions. With restrictions - not so much

In the real aircraft it was dependent on pilot´s preference what modes were used. In general, for smaller descents (<3000 feet) V/S was used. Also when ATC gives you a descent rate restriction (descend with more than 2000 feet per minute...)

The most economical way to descend is using an idle power descend (= FL CHG). But especially when your target speed is still a Mach number, this can result in very high rates/low pitch, so some pilots didn´t like to use that for passenger comfort.

Cheers, Jan