2 Questions. Vmca & Vmcg
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I'll be inquiring of our friendly local regulators, who must have approved the 170 since Air Canada have them, when they decided to allow this, because it would seem to diminish VMCA as a design requirement. At least one of our designs would stand to benefit if treated the same way. Level playing field, and so on ...
If I look at the first line, for example, and make some really simple assumptions about takeoff accel, the distance from brakes release to minVR varies as follows (if you could set Vr at VMCA):
WT 47 50 55 60 65 70 75 80
VR 97 95 92 89 86 83 80 76
D 1 1.02 1.05 1.07 1.09 1.09 1.09 1.04
So the distance run to VR is a maximum at mid weights (but only 9% above the light weight case) and drops back down to only a 4% increase at max weight.
Conversely, if you were using a single fixed VMCA of 97, the distance increase at MTOW would be more like 70%. I know I'd like a paperwork change that reduced my takeoff run by that kind of magnitude! (Recognizing that there may be other limiting factors)
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At least one of our designs would stand to benefit if treated the same way. Level playing field, and so on ...
I think it's better to look at the min VR data rather than the vmca data for a true indication of the benifits as vmca is not always the limiting factor.
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I agree, but the point for a new design would be that you could size the fin to have a VMC that wasn't limiting for mid weights, knowing that any light weight cases shouldn't be a problem because the higher accel will roughly balance the higher VR. Right now we effectively oversize fins and rudders because VMC can affect all your performance.
I agree that for this case the VRs dont show any benefit at higher weights.
I agree that for this case the VRs dont show any benefit at higher weights.
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Vmca/Vmcg.
Mad (Flt) Scientist
There is NO REASON why Vmca or Vmcg should be a higher speed. Either is possible; we have aircraft built and certified to the exact same regulations which have one higher sometimes, or the other.
Could you please give an example of aircraft types where Vmca or Vmcg are higher for future reference.
There is NO REASON why Vmca or Vmcg should be a higher speed. Either is possible; we have aircraft built and certified to the exact same regulations which have one higher sometimes, or the other.
Could you please give an example of aircraft types where Vmca or Vmcg are higher for future reference.
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To quote the specifically relevant parts:
Originally Posted by me
Bombardier's Global Express has a slightly higher Vmca than Vmcg. The CRJ-700 has a lower Vmca than Vmcg. (In both cases, the differences in the speeds is small).
Originally Posted by Alex Whittingham
Here's another. The L1011-1 has a lower Vmca than Vmcg, usually by 3 or 4 kt.
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A bit of asking around indicates that people ARE now being allowed to schedule VMCA with weight; you learn something every day. Sounds like it's come into play in the last few years.
Thanks to FE Hoppy for correcting my dated interpretation.
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MFS-
you are most certainly welcome. I probably owe you 9999 more before we are even.
Over the last 4-5 years I've been in an environment where I've had to learn new regs and new interpretations of old regs almost every day as I have dealt with airlines from all over the world. I try not to know anything any more I just read and read again and then offer an opinion.
It's a complicated business this aviation lark!!
you are most certainly welcome. I probably owe you 9999 more before we are even.
Over the last 4-5 years I've been in an environment where I've had to learn new regs and new interpretations of old regs almost every day as I have dealt with airlines from all over the world. I try not to know anything any more I just read and read again and then offer an opinion.
It's a complicated business this aviation lark!!
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Mad (Flt) Scientist, hi there.
Had a discussion the other day with a guy regarding the subject of VMCG in conjunction with derated thrust. I mean derated thrust allows you to reduce the VMCG consequently increasing the MTOW and so on. Here is the thing though SOP says that if using flaps 2 or 3 on A346 for derated take off don't use TOGA unless above F speed. Any idea whats that all about?
Cheers.
Had a discussion the other day with a guy regarding the subject of VMCG in conjunction with derated thrust. I mean derated thrust allows you to reduce the VMCG consequently increasing the MTOW and so on. Here is the thing though SOP says that if using flaps 2 or 3 on A346 for derated take off don't use TOGA unless above F speed. Any idea whats that all about?
Cheers.
I have read every Vmca/Vmcg discussion on PPRuNe since it started back in the late 90’s. I am a current, fairly technically minded, airline pilot, - and I still get confused.
As a service to us all, could somebody (MFS ?) please post up a quick pilots guide on the subject along with a few FAQs or myths. I am sure it would be much appreciated.
S&L
As a service to us all, could somebody (MFS ?) please post up a quick pilots guide on the subject along with a few FAQs or myths. I am sure it would be much appreciated.
S&L
CaptainSandl.
While on the general discussion on VMC and I will the first to admit it is probably a stupid and far fetched question (age shall not weary us and all that sort of jazz) - but, if by chance one inadvertently enters an incipient or full spin in a 737 such as may be caused by slow reaction to a flame-out on one engine at high altitude, will full rudder be available for recovery action? - keeping in mind relatively low IAS at the time and the fact that the rudder pressure reducer would have been activated automatically at 700ft (?) after take off?
While on the general discussion on VMC and I will the first to admit it is probably a stupid and far fetched question (age shall not weary us and all that sort of jazz) - but, if by chance one inadvertently enters an incipient or full spin in a 737 such as may be caused by slow reaction to a flame-out on one engine at high altitude, will full rudder be available for recovery action? - keeping in mind relatively low IAS at the time and the fact that the rudder pressure reducer would have been activated automatically at 700ft (?) after take off?
There are at least three different stages of 737 rudder modification in service. It is important that you know which you have installed as they work differently.
It is my understanding that if you have the RPR and/or RPL installed rudder authority is reduced by 1/3 when above 1000ra on take-off or 700ra on approach. So full rudder authority is not available at high altitude.
If you are in an NG you have load limiters instead of the RPR/RPL, these reduce rudder authority by 25% at blowdown speed. The LL’s become active above approx 135kts with no altitude constraints. So full rudder authority is available at any altitude if below 135kts. This may catch your spinning scenario.
Bringing this back to Vmcg/Vmca, does this mean that the 737 has two different Vmca’s depending upon whether or not the rudder limiter is active or not? I suppose it must.
It is my understanding that if you have the RPR and/or RPL installed rudder authority is reduced by 1/3 when above 1000ra on take-off or 700ra on approach. So full rudder authority is not available at high altitude.
If you are in an NG you have load limiters instead of the RPR/RPL, these reduce rudder authority by 25% at blowdown speed. The LL’s become active above approx 135kts with no altitude constraints. So full rudder authority is available at any altitude if below 135kts. This may catch your spinning scenario.
Bringing this back to Vmcg/Vmca, does this mean that the 737 has two different Vmca’s depending upon whether or not the rudder limiter is active or not? I suppose it must.
Mach 3
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Centaurus,
My understanding would be that in the situation you describe, if you fly an a/c without the RSEP modifications but with a RPR you would retain full rudder authority in this case because there is another condition which allows full system pressure back to the actuators, namely, when the difference between left and right N1 exceeds 45%.
The situation with a RSEP modified aircraft, wouldn't appear to be any different?
Am I missing something?
My understanding would be that in the situation you describe, if you fly an a/c without the RSEP modifications but with a RPR you would retain full rudder authority in this case because there is another condition which allows full system pressure back to the actuators, namely, when the difference between left and right N1 exceeds 45%.
The situation with a RSEP modified aircraft, wouldn't appear to be any different?
Am I missing something?
SR71,
Non-RSEP a/c you are correct; full rudder pressure will return with a 45% N1 difference at any altitude.
Quote from Vol 2:
“A rudder pressure reducer is connected to the A system hydraulic line upstream of the main rudder PCU. Hydraulic pressure to the rudder is reduced when the airplane climbs above 1000 feet AGL. Hydraulic pressure returns to normal when the airplane descends through 700 feet AGL, or if B hydraulic system depressurizes, or whenever the N1 difference between the left and right engines exceeds 45%.”
However for RSEP a/c the only factor is airspeed:
“At speeds above approximately 135 kts, both hydraulic system A and B pressure are each reduced within the main PCU by approximately 25% each. This function limits full rudder authority in flight after takeoff and before landing.”
Pulling this back to the thread again does Vmca change depending upon your a/c mod status???
It is all very confusing.
Non-RSEP a/c you are correct; full rudder pressure will return with a 45% N1 difference at any altitude.
Quote from Vol 2:
“A rudder pressure reducer is connected to the A system hydraulic line upstream of the main rudder PCU. Hydraulic pressure to the rudder is reduced when the airplane climbs above 1000 feet AGL. Hydraulic pressure returns to normal when the airplane descends through 700 feet AGL, or if B hydraulic system depressurizes, or whenever the N1 difference between the left and right engines exceeds 45%.”
However for RSEP a/c the only factor is airspeed:
“At speeds above approximately 135 kts, both hydraulic system A and B pressure are each reduced within the main PCU by approximately 25% each. This function limits full rudder authority in flight after takeoff and before landing.”
Pulling this back to the thread again does Vmca change depending upon your a/c mod status???
It is all very confusing.
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The limitation of the rudder displacement comes from the hydraulical force available to fight the aerodinamic force.With low pressure the rudder displacement will be limited by the amount required to fight the blowdown on the rudder at that actual IAS.
In case of speed near stall the blowdown will be reduced,so my guess ,the rudder will go all the way, even with the RSEP.
It's not a mechanical limitation,it's a pressure force one.
In case of speed near stall the blowdown will be reduced,so my guess ,the rudder will go all the way, even with the RSEP.
It's not a mechanical limitation,it's a pressure force one.
Is your interview as a CFI -ME?
in that case the real signifigance of the lowered VMC [many training twins far23] have only one Vmc and many times it's very near-or below- the stall speed so as the engine power is reduced with the temperature ratio at a certain point Vmc will be lower than the stall speed-on the ground this is fine- but for the required Vmc(a) demonstration it cannot be done and as it is required for the test standard--you have to use your feet to block the student's pedals and create a fake vmc.
in this application only does the question makes sense to me,
otherwise a very silly interview question IMHO
in that case the real signifigance of the lowered VMC [many training twins far23] have only one Vmc and many times it's very near-or below- the stall speed so as the engine power is reduced with the temperature ratio at a certain point Vmc will be lower than the stall speed-on the ground this is fine- but for the required Vmc(a) demonstration it cannot be done and as it is required for the test standard--you have to use your feet to block the student's pedals and create a fake vmc.
in this application only does the question makes sense to me,
otherwise a very silly interview question IMHO
Moderator
MFS has a very sound background in this stuff so I would counsel heeding his commentary. For my thinking, Vmc/Vmcg are potentially hazardous certification animals and, as line pilots, we are better off giving them a moderate berth where feasible. Better to leave the risky stuff to the QTPs of the world (who are trained in managing those risks).
Does anyone know how to get the book “Jet Airplane Performance” of Lufthansa Consulting
I don't think I have read that particular book but there are others of a similar vein on the net, eg Performance of the Jet Transport Airplane etc by Prof Trevor Young (Wiley), and others. Also the Boeing Performance Engineers Manual and the Airbus Getting to Grips manuals are a good read. These can be found on the net.
you have to use your feet to block the student's pedals and create a fake vmc.
In a similar vein, a colleague had need to pick up a GA twin endorsement prior to renewing an instrument rating years ago. His experience didn't warrant much of the stuff commonly done on light aircraft. So, when the mentally programmed instructor set out to have him demonstrate Vmc (which you can't really do, anyway) he held some rudder in reserve. This perplexed the instructor who then proceeded to demonstrate the exercise. My colleague held the rudder blocked to keep things sensible and the instructor went home quite confused by what he had experienced ......
Does anyone know how to get the book “Jet Airplane Performance” of Lufthansa Consulting
I don't think I have read that particular book but there are others of a similar vein on the net, eg Performance of the Jet Transport Airplane etc by Prof Trevor Young (Wiley), and others. Also the Boeing Performance Engineers Manual and the Airbus Getting to Grips manuals are a good read. These can be found on the net.
you have to use your feet to block the student's pedals and create a fake vmc.
In a similar vein, a colleague had need to pick up a GA twin endorsement prior to renewing an instrument rating years ago. His experience didn't warrant much of the stuff commonly done on light aircraft. So, when the mentally programmed instructor set out to have him demonstrate Vmc (which you can't really do, anyway) he held some rudder in reserve. This perplexed the instructor who then proceeded to demonstrate the exercise. My colleague held the rudder blocked to keep things sensible and the instructor went home quite confused by what he had experienced ......