LRC & MRC
Thread Starter
Join Date: Apr 2006
Location: Straight outta Compton
Posts: 96
Likes: 0
Received 0 Likes
on
0 Posts
LRC & MRC
Hi there, this may be a very simple question but how can you determine an aircrafts MRC & LRC power setting? Is it simply an FMS-calculated setting based on Cost Index, do Boeing & Airbus have charts/graphs for determining this under various conditions, or is it even simpler than that?
Join Date: Jun 2004
Location: Australia
Posts: 1,843
Likes: 0
Received 0 Likes
on
0 Posts
Most modern manuals don't have MRC as yet, because the prospect of fuel prices sky-rocketing as they have wasn't envisaged at the time of aircraft entry into service. Most paper manuals DO have LRC.
On the FMC/FMS units that I'm familiar with, LRC is one of the standard VNAV options. MRC does not usually have it's own specific "MRC select" key, but is easily obtained by entering Cost Index (CI) = 0. This is certainly so for Boeing aircraft, and I believe it to be so for Airbus. Standing by for correction from Airbii operators.
Regards,
Old Smokey
On the FMC/FMS units that I'm familiar with, LRC is one of the standard VNAV options. MRC does not usually have it's own specific "MRC select" key, but is easily obtained by entering Cost Index (CI) = 0. This is certainly so for Boeing aircraft, and I believe it to be so for Airbus. Standing by for correction from Airbii operators.
Regards,
Old Smokey
Remember that MCR/LRC won't be found at a constant power setting.
MRC occurs at 1.32 Vimd, or a constant AoA. At heavy weight, this occurs at a higher speed and as you burn fuel and lose weight, to maintain that AoA, the speed decreases and therefore power. You can however keep climbing and convert that excess power into flying at a higher altitude.
MRC occurs at 1.32 Vimd, or a constant AoA. At heavy weight, this occurs at a higher speed and as you burn fuel and lose weight, to maintain that AoA, the speed decreases and therefore power. You can however keep climbing and convert that excess power into flying at a higher altitude.
Join Date: Aug 2002
Location: Under the sea
Posts: 493
Likes: 0
Received 0 Likes
on
0 Posts
From the gouge...
What is the difference between Max Range Cruise [MRC] and Long Range Cruise [LRC]?
MRC:
The speed at which, for a given weight and altitude, the maximum fuel mileage is obtained.
It is difficult to establish and maintain stable cruise conditions at max range speeds.
1.32 Vimd constant speed with variable AoA [dependent on weight].
LRC:
Speed slightly faster than MRC at a constant AoA [slightly faster than Vimd]
As weight decreases, speed needs to decrease to maintain AoA
Reducing speed necessitates reducing thrust, though because best SFC for a given engine occurs at a particular design RPM, you must climb
What is the difference between Max Range Cruise [MRC] and Long Range Cruise [LRC]?
MRC:
The speed at which, for a given weight and altitude, the maximum fuel mileage is obtained.
It is difficult to establish and maintain stable cruise conditions at max range speeds.
1.32 Vimd constant speed with variable AoA [dependent on weight].
LRC:
Speed slightly faster than MRC at a constant AoA [slightly faster than Vimd]
As weight decreases, speed needs to decrease to maintain AoA
Reducing speed necessitates reducing thrust, though because best SFC for a given engine occurs at a particular design RPM, you must climb
Originally Posted by extreme P
From the gouge...
What is the difference between Max Range Cruise [MRC] and Long Range Cruise [LRC]?
MRC:
The speed at which, for a given weight and altitude, the maximum fuel mileage is obtained.
It is difficult to establish and maintain stable cruise conditions at max range speeds.
1.32 Vimd constant speed with variable AoA [dependent on weight].
LRC:
Speed slightly faster than MRC at a constant AoA [slightly faster than Vimd]
As weight decreases, speed needs to decrease to maintain AoA
Reducing speed necessitates reducing thrust, though because best SFC for a given engine occurs at a particular design RPM, you must climb
What is the difference between Max Range Cruise [MRC] and Long Range Cruise [LRC]?
MRC:
The speed at which, for a given weight and altitude, the maximum fuel mileage is obtained.
It is difficult to establish and maintain stable cruise conditions at max range speeds.
1.32 Vimd constant speed with variable AoA [dependent on weight].
LRC:
Speed slightly faster than MRC at a constant AoA [slightly faster than Vimd]
As weight decreases, speed needs to decrease to maintain AoA
Reducing speed necessitates reducing thrust, though because best SFC for a given engine occurs at a particular design RPM, you must climb
You can't have a constant 1.32 Vimd "SPEED" and variable AoA. If you were to slow down and increase your AoA to Vimd (best endurance) for example, you take longer to get there but burn more fuel. If you were to firewall the throttes you'd save heaps of time, but the parasite drag is so high and the power required is so high, you'd dry your tanks before arriving!
1.32 Vimd (best TAS/DRAG ratio) occurs at a CONSTANT AoA and the speed at which that AoA occurs is dependent on weight. SPEED is the variable, proportional to weight, which decreases during the flight, as noted on your LRC notes.
LRC is referred to as 99% MRC. Same principles, but you fly about 5% faster with only a 1% decrease of the best SAR. ie: a good compromise.
Join Date: Jun 2004
Location: Australia
Posts: 1,843
Likes: 0
Received 0 Likes
on
0 Posts
I think that if you want to stick to the old MRC = 1.32 Vmd, you’d better restrict your operations to low level. There may be some validity in making it a discussion point for low level operations, e.g. depressurised operations at 10,000 feet, but at normal optimum cruise levels for Boeing and Airbus aircraft, MRC and LRC are defined by Mach Number, i.e. all operations are above Mcrit and the low speed drag polars attributable to EAS are only a portion of the total drag.
A very heavy 4 engined jet aircraft might have a MRC speed defined by EAS alone very early in the flight, but not for long.
The original post did refer to Boeing and Airbus aircraft, so here goes for a B777-200ER at 240,000 Kg (529,100 Lb) at F/L 350. Published Holding Speed is 279 KIAS. Assuming that Boeing have given me a 5% buffer above Vmd for speed stability, that makes VMD equal to 265.7 Kt. Multiplying by 1.32 makes it 350.7 KIAS, hey wait!, I can’t do that, it’s way above Vmo, and what’s more, is supersonic!
OK then, let’s assume that Boeing gave me 10% above Vmd for extreme speed stability (unheard of). That makes Vmd equal to 253.6 KIAS, and multiplied by 1.32 equals 335 KIAS, still above Vmo but we’re sub-sonic now at a mere M 0.962, way way above Mmo as well.
What of very high flying aircraft like the Learjet 45? Mcrit is about 0.72, and at higher levels (up to F/L 510 for this aircraft), Vmd no longer exists, minimum drag is defined by Mach Number. Mmd is always greater than Mcrit, except at the cross-over level where Vmd = Mcrit. So 1.32 X 0.72 = M 0.95 – Any Learjet pilots out there willing to take a run up to M0.95 to prove or disprove the theory?
For aircraft operating way below optimum altitude, the 1.32 Vmd story is a nice little bit of theory. For everyday operations at or close to optimum level, it has no relevance whatsoever. It’s all about Mach Number.
Regards,
Old Smokey
A very heavy 4 engined jet aircraft might have a MRC speed defined by EAS alone very early in the flight, but not for long.
The original post did refer to Boeing and Airbus aircraft, so here goes for a B777-200ER at 240,000 Kg (529,100 Lb) at F/L 350. Published Holding Speed is 279 KIAS. Assuming that Boeing have given me a 5% buffer above Vmd for speed stability, that makes VMD equal to 265.7 Kt. Multiplying by 1.32 makes it 350.7 KIAS, hey wait!, I can’t do that, it’s way above Vmo, and what’s more, is supersonic!
OK then, let’s assume that Boeing gave me 10% above Vmd for extreme speed stability (unheard of). That makes Vmd equal to 253.6 KIAS, and multiplied by 1.32 equals 335 KIAS, still above Vmo but we’re sub-sonic now at a mere M 0.962, way way above Mmo as well.
What of very high flying aircraft like the Learjet 45? Mcrit is about 0.72, and at higher levels (up to F/L 510 for this aircraft), Vmd no longer exists, minimum drag is defined by Mach Number. Mmd is always greater than Mcrit, except at the cross-over level where Vmd = Mcrit. So 1.32 X 0.72 = M 0.95 – Any Learjet pilots out there willing to take a run up to M0.95 to prove or disprove the theory?
For aircraft operating way below optimum altitude, the 1.32 Vmd story is a nice little bit of theory. For everyday operations at or close to optimum level, it has no relevance whatsoever. It’s all about Mach Number.
Regards,
Old Smokey
Me thinks, this 1,32 Vimd comes from HTBJ and is just one of many other approximations made in that book and other publications. Nice to test during ATPL examination and during job interviews. I reckon, it´s always best to fly your airplane by the book.
Join Date: Sep 1998
Location: wherever
Age: 55
Posts: 1,616
Likes: 0
Received 0 Likes
on
0 Posts
Old Smokey has it right again.
Almost.
1.32vmd isn't totally theoretical. Certain maritime patrol aircraft can be found trundeling around at exactly that or vmd +5 for edurance and shutting down engines to keep the others in the best RPM band for TSFC.
Ahhh..... those days of old when men were men and we all took our turn on watch.
Almost.
1.32vmd isn't totally theoretical. Certain maritime patrol aircraft can be found trundeling around at exactly that or vmd +5 for edurance and shutting down engines to keep the others in the best RPM band for TSFC.
Ahhh..... those days of old when men were men and we all took our turn on watch.
