VNAV/Vertical profiles

[tumtiddle]

I saw the B737 descent planning thread just below, but I feel my question is different enough to warrant a new thread.

This is the topic I can't seem to get my head fully around. It may sound simple but I'm finding descents and approaches a little annoying in that I don't seem to fully appreciate their complexity. I've just been watching Baixado e Travado's latest video:





I'm curious...At the start of the video, the aircraft is in a managed THR IDLE descent (and is descending within a speed 'window), the speed is then set to a selected 250kts. The aircraft then pitches up slightly, reducing the rate of descent but slowing the aircraft down (speed controlled by pitch). Then 230kts is requested, and the same happens again. Would the ATC always prefer that the aircraft slows first rather than continuing the descent?


If the aircraft reduces vertical speed, surely then the aircraft is above the vertical profile? And you would gain speed by having to descend faster to get back on profile?

I have a vague understanding that being 'on profile' doesn't just include the altitude, but rather a combination of factors such as altitude and speed, and is therefore more about energy management than just height? Is that correct? And if so, does that mean by slowing down as requested, even though you are above the profile 'height' you are actually still on profile because you are at a lower speed, and therefore have less energy to expend?





I hope my questions make sense and don't look like some random babble!

Cheers

[Check Airman]

Regarding ATC, it has been my experience that if they need speed or altitude first, they will let you know.

[bucket_and_spade]

Exactly. It's about energy management not speed or height per se.

In my type (B757 - and I'm sure it's similar for most jet transports) in a normal descent (circa .79/290 knots) three times my current FL (minus the last zero) + 10 miles to slow down (to clean speed) gives all I need for descent planning (based on no speedbrake/early gear etc.). Beware overcomplicating. The FMC vertical profile (if programmed with what you expect/want to do correctly) is a good backup/cross-check and esp. useful in a managed approach.

Assuming an airfield close to sea level...

If I'm at 250 knots and at FL120 I'll look for (12 x 3) + 5 miles (if I was 300 knots, it would be + 10 miles, if I was 240 knots it would be + 4 miles, etc.) to slow down i.e. 41 miles. If the FMC says 60 miles until touchdown (and the FMC has been updated during descent to reflect the likely reality) I'm low (on energy) and will shallow off the descent/speed up etc. If it says 25 miles until touchdown I'm high (on energy i.e. speed/height) and will probably use some speedbrake to improve things.

It's all about doing a quick calculation (based on the above) at several points during the descent and reacting accordingly...in my humble, and still limited, experience!

Seems to work well. In the latterstages of the approach you'll have glideslope/DME/the picture out of the window to tweak it a bit.

You'll fluff it some of the time though due to ATC doing unforeseen things or you just miscalculating!

Shizzle happens...

Hope this helps?

B&S

[gatbusdriver]

Minor thread drift as 757.

New box now believes that if you select speed then you have done that because that is the priority so it will fly the speed in preference to the profile even with VNAV selected.

[172_driver]

If the aircraft reduces vertical speed, surely then the aircraft is above the vertical profile? And you would gain speed by having to descend faster to get back on profile?

The VNAV profile takes into account the deceleration required, normally first at FL100 and then a second time approx. 20 nm from the runway. So if you let the aircraft fly in VNAV PTH (B737 term) you will see the aircraft decelerate without getting high on profile.

I have a vague understanding that being 'on profile' doesn't just include the altitude, but rather a combination of factors such as altitude and speed, and is therefore more about energy management than just height? Is that correct?

Sounds like your understanding is correct. If you are being held high by ATC you slow down towards Up-speed (B737 term again..) When you are cleared for further descent you can go down with a good rate of descent to while trading speed for altitude.

And if so, does that mean by slowing down as requested, even though you are above the profile 'height' you are actually still on profile because you are at a lower speed, and therefore have less energy to expend?

Slowing down usually means you will glide flatter, and that in itself will put you high. Increasing the speed will bring you down steeper and often required when provided shortcuts. Sometimes ATC (usually at smaller airports, especially those without radar) can tell you to slow down very early for separation purposes. So there you can be at FL300 at 220 kts with speedbrakes out to maintain profile (or energey management, if you will..) Not so pretty… but you do what you need to do.