Ah! That 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

Would the ATC always prefer that the aircraft slows first rather than continuing the descent?

 

Yes, they do.

 

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?

 

Yes and no, when we refer to being "on profile", we are mainly referring to the altitude, but energy does come into the equation, just not that particular phrase. For example, if we were high, but slow, we'd say we were high on the profile, but we'd not be worried about it, if you catch my drift....

 

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?

 

As above.

 

Hope this helped,

Rónán.

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?

 

In principle you will gain speed by having to descend at a higher rate of descent. But slow and above the profile is not necessarily a problem. Imagine two planes. Plane A has a ground speed of 60 knots (so 1 nm / minute) and plane B has a ground speed of 120 knots (so 2 nm / minute). Both planes descend at 1000 ft/min. It will take both planes 1 minute to descend 1000 ft. However in that time plane B will have covered 2 miles, and plane A will only have covered 1 mile. So although the rate of descent is the same, plane A is descending much more steeply (i.e. at a higher flight path angle).

These differences are probably a bit more extreme than is possible in practice (if both planes are of the same type) and the numbers are made up for ease of calculation, but the example does show that if you slow down and end up above the profile, you can descend more steeply to intercept it again, even if your vertical speed might be a bit lower.