Speed control and energy conservation

[Fokjok]

A posting to raise a point for ATCOs.

In jet aircraft, in the descent, most FMS have a mode called energy conservation (or similar). In this mode, if the aircraft gets high on the profile, the aircraft will fly level and reduce speed to minimum clean. When further descent is granted, the aircraft will accelerate to a high speed (320kts perhaps) to dive off speed and get back onto the profile.

This all assumes that the aircraft is left in 'PROF' or 'VNAV' for descent. Alternatively, if being flown in 'LVLCH' (basic IAS mode) by the pilots, the same result is achieved by varying the commanded speed as each descent clearance and level off occurs. This has the added advantage of ensuring that speedbrake use is minimised (more economic and more comfortable especially in Boeing types) and that the engines are kept at idle (quiet for the passengers) throughout the descent unless sustained flight at minimum clean is necessary.

Thus, if for example an aircraft checks in at high speed, but is then levelled off and gets high on the profile, the crew may well want to reduce speed dramatically. Unless a speed control for sequencing is required, it is probably best not to apply restrictions, nor to be surprised when the hardware changes speed dramatically throughout the available speed range during a stepped descent.

I must admit that a lot of pilots don't seem to bother with this more efficient and comfortable way of operating, but some do.

More importantly, most ATCOs don't seem aware of the above - perhaps those of you who do know about it could spread this through the training system and make life eaisier for those of us doing the driving?

Your thoughts and comments will follow, I am sure.....

[This message has been edited by Fokjok (edited 08 February 2000).]

[Avernus]

Very informative, Fokjok. Although what you said was commonsense, in a great many years on the job I have never been made aware of the details before. One of the great problems between 'glorified bus drivers' and their earthbound counterparts is a lack of understanding of the other guy's job. Consider at least one of us informed.

[U R NumberOne]

Fokjok,

An interesting point made there. Certainly at my unit we try very hard to give continuous descent to a suitable level for ILS capture (subject to terrain) so this shouldnt be a big issue. The one problem we do encounter is aircraft arriving quite high with 20-30nm to touchdown, still at 240kts (props) or 300+ on some jets. It is perfectly feasible for you to be number seven or worse behind a number of significantly slower aircraft who are much closer in - and because of the need to get the height off it is difficult to come back on the speed resulting in many additional track miles in order to fit in. Is this type of profile (high & fast) down to following the FMS?

Also, going from your username are you on FK100s? If so an interesting point came up the other day about minimum clean speeds - a FK100 was asked to come back to min clean and seemed to stop. When asked what speed he was doing the pilot replied 170kts! Is this true or was there a misunderstanding on what the ATCO wanted?

[Fokjok]

No, I used to fly the FK100 - and luuuuurrrved every moment on it. It's worth mentioning that the aircraft is quite happy to take-off 'clean', and this is an indication of the quality of the wing's performance at low IAS. I recall that minimum clean in the descent was 'green dot' plus 20kts, green dot being best lift/drag speed and variable depending upon weight. So, minimum clean is practically about 200/220kts or so (memory may be a little bit out). I believe the manual quoted 'green dot' as the minimum clean speed, but we added twenty knots for mum and the kids.

Regarding the high, fast, and close problem, this is better for the pilot than being slow and close, as it is more difficult to get the height off at a low drag speed than to lose the height in a high IAS descent and then deccelerate and dirty up. If there's traffic ahead, then two solutions are available, either using the speedbrake (uncomfortable on some types - though not on the Fk100 if you take flap 8 and stow the brake as you do so!), or putting gear and flap down horribly early to get more drag and less lift. A swift 360 is another answer.

Remember, a big aircraft's best clean lift/drag speed is around 220kts, and the aircraft will have great difficulty losing height at this speed - even more impossible is reducing to this speed in the descent, and the automatics don't know about the 500fpm minimum ROD rule in CAS.

So far as turboprops are concerned, I used to arrive in my (modern, twenty and a bit tonne) aircraft doing 250kts to 5 miles without embarassment, as going to flight fine pitch gives terrific braking even on a three degree descent, so I wouldn't worry about this unless it is a FK27, Herald, or similar, that you are dealing with. These older aircraft need more time to slow down, on account of the engine and propeller combination. Happy Days!!

Hope this is helpful - best of luck to you all.


[This message has been edited by Fokjok (edited 09 February 2000).]

[Flanker]

Fokjok I have to disagree when you say'high,fast and close is better than slow and close'.Certainly in the 757/767 fast and close (say inside 20 miles) only go together when you are low on profile already and you have 10 miles or so in level flight to slow down from say 300kts. If you are high close in on these aircraft you need to either slow right down ie gear and flap or you need more track miles. Obviously the wind is a major consideration and the aircraft weight too.Gear down with speedbrake at around 230 kts works well if you're high around thirty miles out too.