Vmcg- calling John Tullamarine.
Thread Starter
Vmcg- calling John Tullamarine.
There is a since locked thread in which John Tullamarine offered guidance on Vmcg when operating at light weights. I understand the relationships between the forces at work...to a degree.
Had a light weight aircraft, when requesting perf, it wouldn’t return FLEX thrust, only returning normal NORMAL thrust numbers. Our performance provider was queried and they determined it was a Vmcg limitation.
Trying to wrap my head around this. With FLEX and an engine failure, the aerodynamic control surface (rudder) forces required to maintain directional control are less than what would be required with Normal thrust. Why then (if our perf provider is accurate) would FLEX number be inhibited as related to Vmcg?
Had a light weight aircraft, when requesting perf, it wouldn’t return FLEX thrust, only returning normal NORMAL thrust numbers. Our performance provider was queried and they determined it was a Vmcg limitation.
Trying to wrap my head around this. With FLEX and an engine failure, the aerodynamic control surface (rudder) forces required to maintain directional control are less than what would be required with Normal thrust. Why then (if our perf provider is accurate) would FLEX number be inhibited as related to Vmcg?
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lnteresting. I can't immediately come up with a very plausible story. However, we do have specialists in the area on PPRuNe who, no doubt, will do better than I.
Were you looking at a ferry from a very short runway, perchance ? in which case you may have been distance-limited for your actual weight, co-incidentally, by min V1 on the day ?
Were you looking at a ferry from a very short runway, perchance ? in which case you may have been distance-limited for your actual weight, co-incidentally, by min V1 on the day ?
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What plane?
if it’s an Airbus, the perf software could have give you a fixed de-rate/ no FLEX, which may sometimes be better for a light plane due to Vmc considerations.
On the Bus you can’t FLEX and de-rate at the same time (on Boeing you can) - so perhaps you got something like D-TO1/no FLEX?
if it’s an Airbus, the perf software could have give you a fixed de-rate/ no FLEX, which may sometimes be better for a light plane due to Vmc considerations.
On the Bus you can’t FLEX and de-rate at the same time (on Boeing you can) - so perhaps you got something like D-TO1/no FLEX?
If you're talking A320 family...
On our CFM engines TOGA gives a Vmcg limit at light weights on a long runway. Any Flex at all gives a Vmca limit.
Doesn't do it for our less powerful IAEs.
On our CFM engines TOGA gives a Vmcg limit at light weights on a long runway. Any Flex at all gives a Vmca limit.
Doesn't do it for our less powerful IAEs.
Thread Starter
lnteresting. I can't immediately come up with a very plausible story. However, we do have specialists in the area on PPRuNe who, no doubt, will do better than I.
Were you looking at a ferry from a very short runway, perchance ? in which case you may have been distance-limited for your actual weight, co-incidentally, by min V1 on the day ?
Were you looking at a ferry from a very short runway, perchance ? in which case you may have been distance-limited for your actual weight, co-incidentally, by min V1 on the day ?
Imagining temp could play a factor in driving Vmcg in one direction or another. I have access to SCAP data. A good night sleep and I’ll see if it yields and information.
Only half a speed-brake
The way FLEX (the assumed temperature method) is calculated allows full thrust at any moment. Maybe something got lost in the translation, but the answer from the performance provider as stated in post #1 makes no sense.
Somebody remind us, please, what happens in RTOW flex calculation once you hit the Vmu / minimum V2 limit from the FCOM. Do you just use the higher speed or are the FLEX calculation results disregarded altogether?
Somebody remind us, please, what happens in RTOW flex calculation once you hit the Vmu / minimum V2 limit from the FCOM. Do you just use the higher speed or are the FLEX calculation results disregarded altogether?
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How about this for a possibility? At full thrust the aircraft was not limited by FLL OEI and therefore a theoretical range of decision speeds was available at the actual TOM. Using made-up numbers assume VEF = 138KT V1 = 140KT and the range of V1s exists from 130KT to 150KT. Now assume a VMCG at 143KT, VEF thus calculated is not greater than or equal to VMCG but a procedure would allow VEF and V1 to be raised within the range of speeds available so that VEF = 143KT, V1 say 145KT. Now you have a legal take-off at full thrust.
A FLEX take-off has the effect of making the take-off limiting at the lower thrust. If there were no other constraints such as climb limit, 2nd sector etc, this would mean the take-off becomes FLL at 140KT-ish, but it doesn't meet the VMCG criteria with a VMCG (calculated at full thrust as observed above) of 143KT.
A FLEX take-off has the effect of making the take-off limiting at the lower thrust. If there were no other constraints such as climb limit, 2nd sector etc, this would mean the take-off becomes FLL at 140KT-ish, but it doesn't meet the VMCG criteria with a VMCG (calculated at full thrust as observed above) of 143KT.
Only half a speed-brake
In practical terms, Vmcg for A320 is around 107 kt, minimum V2 around 112 kt, IIRC?
A lightly loaded one is 55 tonnes, best guess. Lifting 66 tonnes out of 1800 m runway is possible even with tiny flex using Conf 3 (large flap, a certified landing setting), on a narrow runway (30 m vs. standard 45 m).
Need more info from the OP
----
Oops, I just assumed a narrow-body Airbus.
A lightly loaded one is 55 tonnes, best guess. Lifting 66 tonnes out of 1800 m runway is possible even with tiny flex using Conf 3 (large flap, a certified landing setting), on a narrow runway (30 m vs. standard 45 m).
Need more info from the OP
----
Oops, I just assumed a narrow-body Airbus.
Only half a speed-brake
As V1 is not allowed to be lower than Vmcg...
Full power allows for a decision point with a higher speed because in the stop scenario with the higher thrust it will be reached sooner during the roll, leaving more distance remaining to kill the increased energy (Ek = 1/2*m*v*v).
Full power delays the decision point to a higher speed because in the go scenario the higher thrust can still accelerate the aircraft to flying speed in the shorter remaining runway length.
Last edited by FlightDetent; 30th Apr 2020 at 09:20.
Airbuses 
why can't they just say derate and ATM? other than V1 and Vmcg I can't understand anything you guys are really talking about Lol
And also I'm just trying to be funny and probably failing at it
why can't they just say derate and ATM? other than V1 and Vmcg I can't understand anything you guys are really talking about LolAnd also I'm just trying to be funny and probably failing at it
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Somebody remind us, please, what happens in RTOW flex calculation once you hit the Vmu / minimum V2 limit from the FCOM. Do you just use the higher speed or are the FLEX calculation results disregarded altogether?
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Your post V1 less than VMCG a big no no is not exactly a Eureka moment of aviation world.
Only half a speed-brake
Macsboss The OP came here looking for arguments to challenge that Vmcg answer. Some folks above share the concern, namely people who had seen the inside of performance software and/or earn their living by flying aircraft on the wrong side of V-speeds.
And thanks for that.
I was looking for a similarity:
V2 is a chosen speed. When using paper charts on my type the printed figures observe all the limits. Once case you start applying corrections for non-standard influences (QNH, bleeds, MEL) it is possible the manually adjusted V2 becomes too low invalid. In such a case, the instructions say to disregard the whole FLEX(=ATM) thing and use the TOGA paper charts. Which is funny for us the nerdy types.
I think it is not, but you get a certified way out like that. Constrained by the already printed numbers, there is no other option.
The manually adjusted values from the paper FLEX table provided an invalid V2. The TOGA tables will give a good one, so off you go. If there were a chance to print the paper FLEX charts again, forcing a calculation with all the restrictions so that no manual adjustments to the speeds are required, there would be a valid V2 making FLEX possible.
Speculation towards the OP's dilemma:
V1 is also a chosen speed. If the calculation for FLEX perhaps hit the limit of Vmcg in an iterative manner, could it be the SW defaulted into TOGA results (similar to the fixed printed scenario above) instead of re-doing the FLEX values properly? It sounds like a far shot...
Gut feeling what happened: The TOGA displayed was for a De-rated result.
The goal of crunching TKOF performance numbers is enabling a departure with the lowest permissible thrust setting. On a certain number of occasions - when Vmca/g limited - a De-TOGA
will give you better (lower N1) value, because it is calculated using reduced thrust for Vmca/g (and all other) purposes. Whereas FLEX needs to absorb the full adverse kick from the live engines. My argument here is that FLEX was possible, but the SW gave a de-rated TOGA showing a more desirable result.
Question (general): Can Vmcg sometimes be actually higher than Vmca?
My understanding is that both "G" and "A" are calculated using aerodynamical means only (no pavement gear friction for "G"), the difference being
G: gear down, level wings
A: gear up, 5° helpful bank permitted.
How close / apart are the values in real life? What about L/G down ferry, or aircraft versions with a short fuselage and strong engines?
And thanks for that.
I was looking for a similarity:
V2 is a chosen speed. When using paper charts on my type the printed figures observe all the limits. Once case you start applying corrections for non-standard influences (QNH, bleeds, MEL) it is possible the manually adjusted V2 becomes too low invalid. In such a case, the instructions say to disregard the whole FLEX(=ATM) thing and use the TOGA paper charts. Which is funny for us the nerdy types.
- V2 is limited on the low side by Vmca(controllability) or Vmu(airframe geometry),
- for FLEX(ATM), it is a regulatory requirement to account for full TOGA when calculating the limiting parameters
I think it is not, but you get a certified way out like that. Constrained by the already printed numbers, there is no other option.
The manually adjusted values from the paper FLEX table provided an invalid V2. The TOGA tables will give a good one, so off you go. If there were a chance to print the paper FLEX charts again, forcing a calculation with all the restrictions so that no manual adjustments to the speeds are required, there would be a valid V2 making FLEX possible.
Speculation towards the OP's dilemma:
V1 is also a chosen speed. If the calculation for FLEX perhaps hit the limit of Vmcg in an iterative manner, could it be the SW defaulted into TOGA results (similar to the fixed printed scenario above) instead of re-doing the FLEX values properly? It sounds like a far shot...
Gut feeling what happened: The TOGA displayed was for a De-rated result.
The goal of crunching TKOF performance numbers is enabling a departure with the lowest permissible thrust setting. On a certain number of occasions - when Vmca/g limited - a De-TOGA
will give you better (lower N1) value, because it is calculated using reduced thrust for Vmca/g (and all other) purposes. Whereas FLEX needs to absorb the full adverse kick from the live engines. My argument here is that FLEX was possible, but the SW gave a de-rated TOGA showing a more desirable result.Question (general): Can Vmcg sometimes be actually higher than Vmca?
My understanding is that both "G" and "A" are calculated using aerodynamical means only (no pavement gear friction for "G"), the difference being
G: gear down, level wings
A: gear up, 5° helpful bank permitted.
How close / apart are the values in real life? What about L/G down ferry, or aircraft versions with a short fuselage and strong engines?
Last edited by FlightDetent; 30th Apr 2020 at 15:12. Reason: grammar, grammar
Only half a speed-brake
We ain't going any further until the OP comes with few more breadcrumbs. The last thing may have gotten lost in the translation, about people who are paid to go and figure out what the V speeds are; invariably stepping on both sides of the line before it gets drawn based on their findings.
You are correct about the challenge v.s. understand disparity. Certainly, I personally did fail to distinguish.
You are correct about the challenge v.s. understand disparity. Certainly, I personally did fail to distinguish.
Thread Starter
Sorry, meant to share the answer I received explaining the prohibition on FLEX. Despite the answer, I'm not sure I've wrapped my head around it. It was written by an engineer a heck of a lot smarter than I, so hopefully I have it right.
When calculating FLEX data, Vmcg is calculated at the OAT, while Vr is calculated at the assumed temp. As weight decreases, given the differing temps used in the calculations there's a risk that Vr will get closer and closer to Vmcg. Eventually the Vr at the assumed temperature will cross over and go below Vmcg. This meaning (my assumprion) that you could find yourself counter to FAR part 25 by having a rotaation speed lower than Vmcg.
Clear as mud?
When calculating FLEX data, Vmcg is calculated at the OAT, while Vr is calculated at the assumed temp. As weight decreases, given the differing temps used in the calculations there's a risk that Vr will get closer and closer to Vmcg. Eventually the Vr at the assumed temperature will cross over and go below Vmcg. This meaning (my assumprion) that you could find yourself counter to FAR part 25 by having a rotaation speed lower than Vmcg.
Clear as mud?
Only half a speed-brake
Not buying it. Probably he means Vmcg is calculated at OAT TOGA thrust, whereas Vr at AST TOGA thrust. That sounds alright and not beyond what was mentioned several times above.
Does not explain why not limit the Vr to stay above Vmcg and allow a FLEX departure.
If you can still use TOGA, but not FLEX: Does that mean that TOGA Vr is greater compared against FLEX Vr? Dubious.
West Coast The breadcrumbs I was looking for is airfield data and airframe/engine combination. Any chance you did not purge those from the memory banks?
Does not explain why not limit the Vr to stay above Vmcg and allow a FLEX departure.
If you can still use TOGA, but not FLEX: Does that mean that TOGA Vr is greater compared against FLEX Vr? Dubious.
West Coast The breadcrumbs I was looking for is airfield data and airframe/engine combination. Any chance you did not purge those from the memory banks?
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@Flight Detent.
Could it be a case of a simplified set of performance calcs (or charts more likely) which don't have provision for any "special" flex calcs?
So the option of your second sentence might not exist - my only choices are the FULL set of OAT performance or the FULL set of FLEX temp performance - no mix and match.In that case, since I have to account for the full thrust scenario, I have to pick the more conservative set, and use that?
Could it be a case of a simplified set of performance calcs (or charts more likely) which don't have provision for any "special" flex calcs?
So the option of your second sentence might not exist - my only choices are the FULL set of OAT performance or the FULL set of FLEX temp performance - no mix and match.In that case, since I have to account for the full thrust scenario, I have to pick the more conservative set, and use that?