Climb and descent

[dwfdwf]

I am working with some people who are developing flight planning software, and an issue came up today that made me want to check against real life experience.

One of them commented that because of wind etc., the optimal route (according to the software) might involve climbing and descending several times during the flight -- so much so, that the calculated flight path might need to be "smoothed" so that the the aircraft wasn't constantly changing altitude during the flight.

However, from my minimal knowledge of aviation, I had the impression that an aircraft would pretty much only climb during a normal flight (as it burned fuel and became light enough for higher flight levels), then descend only at the end of the flight.

In practice, do aircraft tend to both ascend and descend during a flight, or just ascend?

(If there is a more appropriate forum to ask this question, let me know).

[wiggy]

I had the impression that an aircraft would pretty much only climb during a normal flight (as it burned fuel and became light enough for higher flight levels), then descend only at the end of the flight.

That is generally the case in the real world but there are indeed times it might be advantage to consider decending to get out of strong headwinds/significant turbulence and I've seen flight plans that ask you to do exactly that. However, again in the real world, especially Longhaul and most especially on some of the more congested routes if you descend en-route you may be unable to get clearance to climb back up again to a higher level because another aircraft has joined your route of flight above you and is blocking you.

Most folks game plan is to climb the higher levels when it's economical to go there..and stay there.

[Hartington]

Surely, in an ideal world, ATC would no longer exist (at least not in the form we know today). Each aircraft would perform a "perfect" flight plan without having to fly from beacon to beacon at a fixed height. They would fly the great circle at optimum altitude. To achieve this you'd need both air and ground based systems (I suspect) with frequent position reports generated automatically from the aircraft and the ground systems calculating where to route the aircraft for avoidance purposes and aircraft based systems along the lines of TCAS to deal with unexpected problems.

I love science fiction!

[KLOS]

On the topic of requested flight plans, friend on flight to TPA which was delayed from departure from LGW said that the Capt.told them that to make up time he was given permission to go faster ( I know there is fuel cost involved) but is it normal for companies to specify speeds ( do ATC have to be consulted in advance) ? Excuse my ignorance ( why I post on here:8

[OhNoCB]

This is my understanding of it, keep in mind that I fly for a small operator and don't utilise these procedures myself so someone else may be better placed to asnwer.

Operators will not assign speeds explicitly, but will tend to specify a Cost Index. This is a value derived from operating (time) cost / fuel cost. The higher the cost index, the higher the speed and vice versa.

With regards to informing ATC, flight plans have to have the planned speed on them, and if the speed you want to do/require has a difference of more then 5% TAS or M0.01 of the flight planned speed, then you must inform ATC, who will grant or deny the speed change unless it's for safety reasons when it is ultimately down to the Captain.

[KLOS]

Thanks for your helpful response and for taking the trouble -appreciated:ok:

[foxmoth]

Surely, in an ideal world, ATC would no longer exist (at least not in the form we know today). Each aircraft would perform a "perfect" flight plan without having to fly from beacon to beacon at a fixed height. They would fly the great circle at optimum altitude. To achieve this you'd need both air and ground based systems (I suspect) with frequent position reports generated automatically from the aircraft and the ground systems calculating where to route the aircraft for avoidance purposes and aircraft based systems along the lines of TCAS to deal with unexpected problems.

I love science fiction!

We are actually moving in that direction, not sure how far along it is, but there are moves to do away with airways in Europe, and you will basically be cleared great circle track from dep to destination, though level will for some time be assigned, a/c will need to be CPDLC and, I think, have sat nav.

[Groundloop]

They would fly the great circle at optimum altitude.

The great circle is more often than not not the ideal routing because of headwinds/tailwinds.

[Trackdiamond]

Is there a quick planning rule of thumb for determining optimum altitude for jet transport (discounting advantages of Jetstream rides, tailwinds,weather avoidance, and temperature considerations)..just a simle rule that can factor weight and average climb wind components.

I aslo invite critique for the following simplistic quick and dirty approach:
Given the sector distance you would wanna cover 1/3 in climb, a third in cruise and step climbs, and a third in descents (assuming Constant Angle/Speed profile descents or similar) .Divide distace by 3 and multiply by 3 for the cruise Altitude and check if weight limits...any further tips will be appreciated.

[Denti]

Just get as high as you can and glide back down for the remainder of the flight, who needs a cruise phase on short hops? However, there are some real life concerns, ATC might not be able to fit in a profile like that, nor might airspace structure allow it.

[phiggsbroadband]

The third climb, cruise, descend would most likely only work for one given mileage. (say 240nm.) This is because the climb usually takes about 80 nm and so does the descent. On long haul of 3000nm it is unreasonable to climb or descend for 1000nm.


As for the speed question, there is something called 'Coffin Corner' which, at high altitudes limits how fast you can fly because of engine power, and how slow you can fly because of aerodynamic stall. At very high Flight Levels these two numbers get closer together.

[HEATHROW DIRECTOR]

Scrub ATC and put everyone on GC tracks? Er.... how about Cb avoidance?

[John Hill]

I thought there was a thing called 'zoom cruise' where the aircraft is flown at optimum power for the route and would naturally rise as fuel is consumed, then descend at reduced power?

[Lord Spandex Masher]

Scrub ATC and put everyone on GC tracks? Er.... how about Cb avoidance?

Windows or weather radar?

[AirborneAgain]

As for the speed question, there is something called 'Coffin Corner' which, at high altitudes limits how fast you can fly because of engine power, and how slow you can fly because of aerodynamic stall. At very high Flight Levels these two numbers get closer together.Surely the upper speed limit in the "coffin corner" is determined by mach number, not engine power?

[phiggsbroadband]

Err.. Yes you are correct. The upper speed is limited to Mcrit or the FAA imposed Mmo (Mach max ops.) So maybe not related to engine power, as that speed could also be achieved in a dive.


Quote from Wiki...
As an airplane moves through the air faster, the airflow over parts of the wing will reach speeds that approach Mach 1.0. At such speeds, shock waves (http://en.wikipedia.org/wiki/Shock_waves) form in the air passing over the wings, drastically increasing the drag due to drag divergence (http://en.wikipedia.org/wiki/Drag_divergence_Mach_number), causing Mach buffet, or drastically changing the center of pressure (http://en.wikipedia.org/wiki/Center_of_pressure_(fluid_mechanics)), resulting in a nose-down moment (http://en.wikipedia.org/wiki/Moment_(physics)) called "mach tuck (http://en.wikipedia.org/wiki/Mach_tuck)". The aircraft Mach number at which these effects appear is known as its critical Mach number (http://en.wikipedia.org/wiki/Critical_Mach_number), or Mach CRIT. The true airspeed corresponding to the critical Mach number generally decreases with altitude.


Datei:CoffinCorner.png ? Wikipedia (http://de.wikipedia.org/wiki/Datei:CoffinCorner.png)

[phiggsbroadband]

Just a reply to John Hall.... quote...
I thought there was a thing called 'zoom cruise' where the aircraft is flown at optimum power for the route and would naturally rise as fuel is consumed, then descend at reduced power?


Any idea of allowing an aircraft to settle at its own level must be balanced with the fact that ATC will only give you a set Flight Level which you must maintain.


I haven't heard of Zoom Cruise only Zoom Climb... From Wiki...


Zoom climb - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Zoom_climb)

[wiggy]

You might be thinking of a "cruise climb", where you try and keep the aircraft at it's optimum level as the fuel burns off. In reality it's usually a series of 100 foot steps rather than a smooth continuous climb and in any event due traffic it's rarely if ever available over most parts of the world.

[AirborneAgain]

You might be thinking of a "cruise climb", where you try and keep the aircraft at it's optimum level as the fuel burns off. In reality it's usually a series of 100 foot steps rather than a smooth continuous climb and in any event due traffic it's rarely if ever available over most parts of the world.Standard procedure for the Concorde in supersonic flight, I believe. Of course the Concordes were pretty much alone on their supersonic cruise flight levels...

[phiggsbroadband]

Actually from a practical point of view, and looking at what is happening on Flightradar24.com, you will see that if a plane is over an ocean going west it will be given an even FL, either 360, 380 or 400. And if it is going east then the usual FLs are 370, 390 or 410.
The exact track most likely takes into account the jet-stream winds aloft, so the east goers are separated from the west goers, by quite a distance.

[wiggy]

Being really really picky I'd point out that on the the Organised Atlantic Tracks the "odds east, evens west rule" doesn't apply, everyone's going the same way with 1000 foot separation. The level change to the cleared oceanic level may happen just before Oceanic entry, in Shannon's case well offshore and may not always show on the likes of Flightradar. As a consequence it is sometimes possible to step up in 1000 foot increments but often as not traffic doesn't allow it.
As you rightly say the jet stream tends to means one set of traffic wants to be in the jet, the other lot don't, so usually don't get much opposing traffic though it does happen occasionally.

[John Hill]

phiggsbroadband wrote..


Any idea of allowing an aircraft to settle at its own level must be balanced with the fact that ATC will only give you a set Flight Level which you must maintain.

That is true but there are bits of the world, quite big bits really, where this may not be an issue..

For example Aerolineas Argentinas nonstop Sydney-Buenos Aires, LAN Chile Auckland-Santiago and Qantas Sydney-Santiago and of course Qantas Sydney-Johannesburg can likely choose any level that would suit. I am wondering are those flight even in controlled airspace?

I expect there are routes too in the South Pacific where the only conflicting traffic is likely to be yourself on your way back!

[wiggy]

I am wondering are those flight even in controlled airspace?

I expect there are routes too in the South Pacific where the only conflicting traffic is likely to be yourself on your way back!

I'm afraid you simply can't rely on the "big sky theory", ...anywhere

C141HEAVEN - All there is to know, and lots more, about the Lockheed C141 Starlifter! (http://c141heaven.info/dotcom/65/pic_65_9405.php)

[John Hill]

Seems that collision was at least partly due to a breakdown of ground communications and/or no effective dissemination of traffic information within that FIR.