slow VNAV descent speeds

[sho69607 29]

On my most recent flight in the 744, I flew from JFK-ORD at FL380 with a cost index of 90. I don't know the typical cost index used for this aircraft type, so 90 was a guess for me. I have noticed though, that VNAV has me descending at 260kts down from FL300. Isn't this a really slow speed for an aircraft of this size? With no speed restrictions, I would expect more along the lines of 280-300kts. Is there something I may be doing wrong or is this normal?

[scandinavian13 5,192]

10 hours ago, sho69607 said:

On my most recent flight in the 744, I flew from JFK-ORD at FL380 with a cost index of 90. I don't know the typical cost index used for this aircraft type, so 90 was a guess for me. I have noticed though, that VNAV has me descending at 260kts down from FL300. Isn't this a really slow speed for an aircraft of this size? With no speed restrictions, I would expect more along the lines of 280-300kts. Is there something I may be doing wrong or is this normal?

If you're using a CI, then it's dependent on the CI. Keep in mind that, when you're up that high, the more important number is your mach number. The aircraft's reference crossover (to IAS) is lower than FL300, if I'm remembering correctly.

For the 747-400, the CI range is 0-9999, so 90 is quite low in the range (though not uncommon).

[sho69607 29]

1 hour ago, scandinavian13 said:

If you're using a CI, then it's dependent on the CI. Keep in mind that, when you're up that high, the more important number is your mach number. The aircraft's reference crossover (to IAS) is lower than FL300, if I'm remembering correctly.

For the 747-400, the CI range is 0-9999, so 90 is quite low in the range (though not uncommon).

Ok good to know, thanks!

[Captain Kevin 586]

On 10/6/2019 at 4:48 AM, sho69607 said:

I don't know the typical cost index used for this aircraft type, so 90 was a guess for me.

That would be dependent on the airline and the operational requirements for a particular flight.

[jon b 2,041]

Yes that 260kts econ speed looks about right for the cost index.

Cost indexing is a bit of a black art, in years gone by in my real world ops we'd just use a standard CI250, nowadays its normally well below 100 unless things have been sped up by flight planning for some reason.

The descent speeds given by econ are as you have noticed a bit slow and the aeroplane doesn't really like it, invariably this will be overwritten by the crew. Personally I'll use .85/300kts as the aircraft stays higher for longer then descends purposefully at idle, but 280-300 works well. London ATC set up a trial a few years ago of having everyone descending into the London TMA to do so at 270kts, but this was quickly abandoned as they found everyone was arriving over the holding fix at the same time, rather than allowing a bit of variation in speeds between types and therefore a bit of natural selection. The 280-300kt bracket works well as you've got the option to open the speed window and increase even further if you need to increase descent rate to meet a late issued crossing restriction. Slow speed descents result in too  much waffling for my liking, Some people worry about going over 300kts in the descent but the 744 econ climb speed is often well over 320kts so it's really not a problem. 

The other advantage of the higher speed descents is that if you can plan it right and level off at the holding fix altitude well ahead of the fix you can often get 10-15 miles of free flying were the plane will be flying level at idle thrust while it bleeds speed back from 320 to 230 kts for the hold, much better than waffling down to arrive at level flight just before the hold then need speedbrake to slow it up at the last minute. Not really applicable in the US though where the <250<10,000ft is mandated.