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250 kts below 10000?

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250 kts below 10000?

Old 3rd Sep 2012, 08:26
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250 kts below 10000?

Hi all,

Was wondering if anyone had a good idea about the magnitude of fuel savings when an aircraft is allowed 'no speed restrictions' on departure and is allowed to immediately accelerate to the FMC recommended climb speed.

My question isn't aircraft specific but basically applies to anything equal to or faster than an A320.

I'd like to know if it's worth it to request ATC for a high speed climb or would it be better to just sit silent till 10000ft.

Thanks
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Old 3rd Sep 2012, 10:50
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Always worth asking, I don't know the specific savings - it would depend on when you got the restriction lifted etc but every little helps.
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Old 3rd Sep 2012, 11:45
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I had a few Korean CAs claim it was 3% for a short flight. I dunno where they came up with that number. Of course they thought I was crazy when I suggested they could save even more by doing visual approaches.
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Old 3rd Sep 2012, 11:52
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You are asking is it worth speeding up? But you focused only on the savings! Remember the actual reasons behind 250kts below 100!!

Bird strikes result in less damage at the slower speed.
If everyone is doing 300 kts ATC are more likely to miss their mistakes and ours.

Was the rule not originally brought in following a mid-air collision or a few near miss!?

I am interested to see a figure for the fuel savings though, i suspect they must be very small as the slower speed penalty may be countered by the higher ROC to higher levels.
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Old 3rd Sep 2012, 12:30
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The Climb is far and away the most fuel expensive phase of the flight and every effort should be made to optimize fuel use per mile in this phase.

Power increases linearly with speed. Ram recovery; the degree of compression is exponential to speed. At low speed numbers this compression is not very significant, but does partially compensate for 'Thrust Equation' losses. They are about equal at approximately M0.5 (250 Kt in ball-park figures). With further speed increase, the exponentially increasing ram effect exceeds the linearly decreasing 'thrust equation' effect, and at about M0.75, thrust is back to static thrust. Much more of the engine's internal power is now used to propel the exhaust gases, much less is used to drive the compressor. Above this mach number, net thrust can and does exceed static thrust. Ram recovery is an important factor and directly affects thrust. At a fixed engine setting, due to the effects of the 'thrust equation', thrust declines with increasing speed up to about M0.5, where the decline bottoms out and then, due to ram recovery, begins to rise again reaching parity with static thrust at about M0.75 and may exceed static thrust thereafter. If there were no ram recovery, jet aircraft would do no better than propeller aircraft.

The best rate of climb occurs at a speed where there is maximum excess power. As jet engines directly produce thrust, not power, it is necessary to consider thrust multiplied by speed (Power = Force X Velocity). Thus, for a given thrust setting, power increases as TAS increases. Thrust actually ‘dips’ as speed increases, but then there is significant ram recovery at higher mach numbers, thus further increasing Power at higher speeds.

I frequently accelerate 'slingshot' above the low altitude speed limit of 250 KIAS from 5000 feet instead of the ‘standard’ 10000 feet. For the A320, this repeatedly saves 100 Kg of fuel per SECTOR and that’s only over 5000 feet. The risk is nominal due to bird strike versus performance rewards.

Last edited by jimmyg; 5th Sep 2012 at 04:42.
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Old 3rd Sep 2012, 12:40
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irishpilot1990:

Was the rule not originally brought in following a mid-air collision or a few near miss!?
1. December, 1960: UAL DC-8 plows into TWA Connie over Brooklyn at 5 or 6,000 while blowing a holding fix (one VOR inop) doing barbar pole or there abouts.

This resulted in 250 below 10,000 within 30 miles of the destination airport. No speed restriction on departure once clear of the ATA (now Class D). Also resulted in regulation to report to ATC any en route loss of navigation capability.

2. March, 1967: TWA DC-9-10 going on short range flight (KPIT to either KCMH or KDAY) plows into a Beech Baron at 8,000 while going barbar pole or there abouts.

This resulted in 250 below 10,000 at all times.
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Old 3rd Sep 2012, 12:53
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Opting for the lower speed, let's say 230 KIAS climb to allegedly reach higher levels sooner. At your CAS/Mach changeover height for 280/M0.6 at FL200 feet, TAS at ISA is 370 Kt. At 230 KIAS at the same level, the TAS is 306 Kt, some 64 Kt slower at the same fuel flow. My low CI friends will need to find 64 Kt of Tailwind that you don't have just to remain equal. In the reverse direction, flying into a headwind, the faster climb wins again, as a given headwind component has a lesser percentage penalty upon KNM per unit of fuel at high speed than at low speed.

Last edited by jimmyg; 3rd Sep 2012 at 12:55.
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Old 3rd Sep 2012, 13:15
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Is it law?

I understand that 250 kts below 10,000 ft is a law and ATC cannot approve it except in an emergency so if there is an incident you are held accountable if you were exceeding 250 below 10,000. When ATC approves high speed climb or descent below 10,000 ft all he is telling you is that there is no conflict with traffic.
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Old 3rd Sep 2012, 13:33
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frequently accelerate 'slingshot' above the nominal low altitude speed limit of 250 KIAS from 5000 feet instead of the ‘standard’ 10000 feet. For the A320, this repeatedly saves 100 Kg of fuel per sector and that’s only over 5000 feet.
Operating in Kuwait,the land of cheap gaz,and going non standard for 5000 feet of supposely fuel saving?
How did you calculate this 100 kgs saving over 5000 feet between 250kts and your 'slingshot' higher speed?

Speed range of 260-280kts save fuel on 320 but not on heavier types such as 330s...higher speed then save fuel..

My low CI friends will need to find 64 Kt of Tailwind that you don't have just to remain equal.
Your cost index friends may just following speeds that their relevant performance officers calculated to be the most 'appropriate' for that flight,using factors that you may not have necessary data or knowledge about.

Last edited by de facto; 3rd Sep 2012 at 13:36.
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Old 3rd Sep 2012, 14:00
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I understand that 250 kts below 10,000 ft is a law and ATC cannot approve it except in an emergency so if there is an incident you are held accountable if you were exceeding 250 below 10,000. When ATC approves high speed climb or descent below 10,000 ft all he is telling you is that there is no conflict with traffic.
Depends in which States airspace you are flying. It is legal in many places.
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Old 3rd Sep 2012, 15:25
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Flying out of Dallas one is told to "go fast" as often as not, I think they want you out of their space ASAP
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Old 3rd Sep 2012, 18:36
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There are getting to be more and more restrictions, or planned restrictions on departure speeds.

With an increased effort on throughput, departures are happening at reduced time separation, but then the ac are piling up and violating distance separation.

The solution is looking at more speed restrictions for the departures, if they cannot have a diverse departure path...
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Old 3rd Sep 2012, 21:47
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Boeing FCTM says ECON with no restrictions saves gas.

Couldn't this be tested on the ground by checking the fuel req'd in both conditions?
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Old 3rd Sep 2012, 23:36
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According to Boeing performance manual based on 737-800 CFM56-7B26:

Trip fuel and time charts are based on an initial climb speed of 280 KIAS. Local ATC may require that 250 KIAS not be exceeded below 10000ft. Approximately 25kg of additional fuel is burned when this restriction is imposed.
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Old 4th Sep 2012, 01:07
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At any time that ANY aircraft conducts a climb at less than, or more than, optimum range climb speed you will consume more fuel. This speed is typically slightly higher than best Rate of climb speed, and significantly higher than the best gradient speed.

The best Rate of climb can be refined further to optimize time and fuel. Remembering that up to 20 to 30 Knots off optimum will have only minimal effect upon excess power. An econ climb speed a little above best rate will have a negligible effect upon rate of climb, but put the aircraft 30 miles or so further down track at top of climb. The very slight increase in climb time and fuel is more than off-set by the extra distance covered in the ‘fuel expensive’ climb, and elimination of the incremental cruise.

Operation cost equations vary from organizations but are most always benefited by a time reductions. Low cost indices in many cases are opted for based purely on face value of misinformed accountancy rather than laws of physics and cost factors.

Last edited by jimmyg; 5th Sep 2012 at 04:52.
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Old 4th Sep 2012, 02:49
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Low cost indices in many cases are a function of misinformed accountancy rather than laws of physics.
What are you smoking?
Tell that to Airbus....

An Airbus 320(that s what you fly?) will burn 45kgs more flying at 320 kts than 280 kts for a constant climb to FL350.

Last edited by de facto; 4th Sep 2012 at 02:53.
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Old 4th Sep 2012, 04:20
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If you care to have a informed intelligent conversation rather than a childlike self depreciating " what are you smoking" Then I do not care to partake.

In all engineering endeavors, nothing proves the laws of physics and mathematics more succinctly than does the jet airplane.

I find it intriguing though, for most of us who don't have too high an opinion of our innate knowledge start off knowing nothing. A year or two later and with a few hundred hours, know that we know everything and then spend the rest of our careers coming to realize just how little we really do know.

Learning commences with humility.

Last edited by jimmyg; 5th Sep 2012 at 04:38.
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Old 4th Sep 2012, 06:03
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Learning commences with humility.
True and by reading..

Here is a good start pointer, google airbus 'getting to grips with fuel economy' and airbus 'getting to grips with cost index'


The smoking thing was light hearted comment not meant to be pejorative.

Last edited by de facto; 4th Sep 2012 at 06:21.
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Old 4th Sep 2012, 06:41
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Irishpilot talked about a higher rate of climb to cruise altitude at 250kts. Thats not accurate. Best rate of climb will be above 250kts in most airliners. If you want to save gas in a 737 you accelerate low (ie 1000ft) and you accelerate to best rate (about 280kts). You'll save about 10kg by accelerating NADP2 instead of NADP 1 and you'll save about 25kg by going straight to best rate. (Boeing info as best as I can recall).
SR 22 talked about cost index flying resulting in low climb speeds anyway..... Thats not accurate. Cost index zero will give you the least fuel burn, which is the best rate, which is well above 250 kits .
If you really want to be efficient you'll depart with the least flap possible and accelerate to best rate of climb as low as you can. There will obviously be slight adjustments for head and tail winds but that's the guts of it.
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Old 4th Sep 2012, 06:53
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Irishpilot talked about a higher rate of climb to cruise altitude at 250kts. Thats not accurate. Best rate of climb will be above 250kts in most airliners. If you want to save gas in a 737 you accelerate low (ie 1000ft) and you accelerate to best rate (about 280kts). You'll save about 10kg by accelerating NADP2 instead of NADP 1 and you'll save about 25kg by going straight to best rate. (Boeing info as best as I can recall).
SR 22 talked about cost index flying resulting in low climb speeds anyway..... Thats not accurate. Cost index zero will give you the least fuel burn, which is the best rate, which is well above 250 kits .
If you really want to be efficient you'll depart with the least flap possible and accelerate to best rate of climb as low as you can. There will obviously be slight adjustments for head and tail winds but that's the guts of it.
Agree,just that CI does not account for winds during climb and descent.The ECON speed is a speed where from the start of the climb segment to a point into the cruise segment,the cost of fuel and time cost is minimized.
To save a tad more,max rate speed can be used,a speed dependent on weight.
Accelerating early (800ft) NDP2 will obviously reduce fuel burn as drag is reduced earlier and speed increased.

Last edited by de facto; 7th Sep 2012 at 05:08.
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