V1 vs Vmcg
Join Date: Feb 2006
Location: Italy
Posts: 60
Likes: 0
Received 0 Likes
on
0 Posts
What about this. You select a V1 lesser than VMCG.Than during your take off roll exactly between V1 (that means that now you are comitted to go) and VMCG you loose one engine. Guess what is the next thing that's gonna happen
Do you think you would be still able to keep directional control and continue the take off. I don't think so.
If this is clear to you then you should be able to understand why V1 must be equal or greater than VMCG
Do you think you would be still able to keep directional control and continue the take off. I don't think so.
If this is clear to you then you should be able to understand why V1 must be equal or greater than VMCG
because
Stating the obvious I'm sure but Vmcg = Minimum Control speed on ground, put simply, below this speed, in the event of an engine failure, you will be unable to control the aircrafts direction of travel.
So ., putting a couple of numbers on this, for the sake of example say Vmcg is 150 knots but V1 is 130. If you have an engine failure at 140 knots you can't stop ( 'cos your above V1) but you cannot control the aircraft either.....
edited to add that lowandfast beat me to it
So ., putting a couple of numbers on this, for the sake of example say Vmcg is 150 knots but V1 is 130. If you have an engine failure at 140 knots you can't stop ( 'cos your above V1) but you cannot control the aircraft either.....
edited to add that lowandfast beat me to it
Join Date: Jun 2004
Location: Surrounding the localizer
Posts: 2,200
Likes: 0
Received 2 Likes
on
1 Post
Ok clear something up for me here......VMCG is based on aerodynamic controls only?..or other factors as well..such as nose wheel steering..differential braking etc...
I don't have a problem with the concept as I implement it every day I fly....just the parameters/definition
I don't have a problem with the concept as I implement it every day I fly....just the parameters/definition
Join Date: Sep 2002
Location: La Belle Province
Posts: 2,179
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by bubbers44
V1 is below VMCG only at very light weights when normal V1 is below 120K or so.
Join Date: Feb 2006
Location: Italy
Posts: 60
Likes: 0
Received 0 Likes
on
0 Posts
yep, only aerodynamic controls. otherwise if we start considering also nose steering we have to consider also the friction coefficient of the nose wheel with the runway. But this is not easy to do because of the difficulty of assessing the friction coefficient of the runway. Just consider the same runway in wet or dry conditions. The airplane companies in order to certify their new airplanes would have to produce a huge amount of graphs ecc ecc in order to calculate the VMCG in function of the friction coefficient.
Realy too much work. So just let's keep it simple and consider one aerodynamic VMCG.
Realy too much work. So just let's keep it simple and consider one aerodynamic VMCG.
Join Date: Sep 2002
Location: La Belle Province
Posts: 2,179
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by haughtney1
Ok clear something up for me here......VMCG is based on aerodynamic controls only?..or other factors as well..such as nose wheel steering..differential braking etc...
Join Date: Aug 2005
Location: fl
Posts: 2,525
Likes: 0
Received 0 Likes
on
0 Posts
(e) VMCG, the minimum control speed on the ground is the calibrated airspeed during the takeoff run at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane using the rudder control alone (without the use of nosewheel steering), as limited by 150 pounds of force, and the lateral control to the extent of keeping the wings level to enable the takeoff to be safely continued using normal piloting skill. In the determination of VMCG, assuming that the path of the airplane accelerating with all engines operating is along the centerline of the runway, its path from the point at which the critical engine is made inoperative to the point at which recovery to a direction parallel to the centerline is completed may not deviate more than 30 feet laterally from the centerline at any point. VMCG must be established with -
(1) The airplane in each takeoff configuration or, at the option of the applicant, in the most critical takeoff configuration;
(2) Maximum available takeoff power or thrust on the operating engines;
(3) The most unfavorable center of gravity;
(4) The airplane trimmed for takeoff; and
(5) The most unfavorable weight in the range of takeoff weights.
(f) VMCL
We did VMCG adjustments in the B757 performance class, but most twins are about the same for light weight take offs.
(1) The airplane in each takeoff configuration or, at the option of the applicant, in the most critical takeoff configuration;
(2) Maximum available takeoff power or thrust on the operating engines;
(3) The most unfavorable center of gravity;
(4) The airplane trimmed for takeoff; and
(5) The most unfavorable weight in the range of takeoff weights.
(f) VMCL
We did VMCG adjustments in the B757 performance class, but most twins are about the same for light weight take offs.
Join Date: Jun 2004
Location: Surrounding the localizer
Posts: 2,200
Likes: 0
Received 2 Likes
on
1 Post
Cheers guys
Yep I do it everytime I perform a t/o calc..a nice little table sits in the corner of the checklist
We did VMCG adjustments in the B757 performance class, but most twins are about the same for light weight take offs.
Join Date: Sep 2002
Location: La Belle Province
Posts: 2,179
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by bubbers44
V1 is below VMCG only at very light weights when normal V1 is below 120K or so.
Originally Posted by bubbers44
We did VMCG adjustments in the B757 performance class, but most twins are about the same for light weight take offs.
Join Date: Aug 2005
Location: fl
Posts: 2,525
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Mad (Flt) Scientist
Just so I don't misunderstand this: are you saying that the scheduled V1 speeds for a Boeing 757 at light weights are below Vmcg?
Join Date: Sep 2002
Location: La Belle Province
Posts: 2,179
Likes: 0
Received 0 Likes
on
0 Posts
Ah, good.
Then V1 is always above Vmcg, as required by regulation.
The "V1" value you are talking about is in effect an intermediate, calculation step.
Maybe I will agree to board 757s after all.
Then V1 is always above Vmcg, as required by regulation.
The "V1" value you are talking about is in effect an intermediate, calculation step.
Maybe I will agree to board 757s after all.
Join Date: Feb 2005
Location: flyover country USA
Age: 82
Posts: 4,579
Likes: 0
Received 0 Likes
on
0 Posts
I'm not sure how this is taken into account -
Surely Vmcg depends on the nosewheel load. If the control column is held fully fwd it adds load to the nosewheel. On what condition is Vmcg based - stick fwd, or neutral?
Surely Vmcg depends on the nosewheel load. If the control column is held fully fwd it adds load to the nosewheel. On what condition is Vmcg based - stick fwd, or neutral?
Join Date: Aug 2005
Location: fl
Posts: 2,525
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by barit1
I'm not sure how this is taken into account -
Surely Vmcg depends on the nosewheel load. If the control column is held fully fwd it adds load to the nosewheel. On what condition is Vmcg based - stick fwd, or neutral?
Surely Vmcg depends on the nosewheel load. If the control column is held fully fwd it adds load to the nosewheel. On what condition is Vmcg based - stick fwd, or neutral?
VMCG, the minimum control speed on the ground is the calibrated airspeed during the takeoff run at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane using the rudder control alone (without the use of nosewheel steering), as limited by 150 pounds of force, and the lateral control to the extent of keeping the wings level to enable the takeoff to be safely continued using normal piloting skill. In the determination of VMCG, assuming that the path of the airplane accelerating with all engines operating is along the centerline of the runway, its path from the point at which the critical engine is made inoperative to the point at which recovery to a direction parallel to the centerline is completed may not deviate more than 30 feet laterally from the centerline at any point.
Nose gear stearing is not allowed in calculating vmcg. I am sure it would help to use some forward yoke to help the situation however.
Join Date: Sep 2002
Location: La Belle Province
Posts: 2,179
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by barit1
I'm not sure how this is taken into account -
Surely Vmcg depends on the nosewheel load. If the control column is held fully fwd it adds load to the nosewheel. On what condition is Vmcg based - stick fwd, or neutral?
Surely Vmcg depends on the nosewheel load. If the control column is held fully fwd it adds load to the nosewheel. On what condition is Vmcg based - stick fwd, or neutral?
You are going to obviously slightly change the downforce on the mainwheels as a result of the stick position, but it's not likely to be big percentage change, and given the fact that you aren't testing on a representatively wet runway, so the main wheel friction/cornering powers are all wrong anyway....
Our procedure is usually to keep the stick somewhat neutral, but I suspect that the variability of the test in terms of, for example, pilot reaction time will swamp any stick position effects.
Join Date: Aug 2005
Location: fl
Posts: 2,525
Likes: 0
Received 0 Likes
on
0 Posts
The B757 nose wheel steering has a limited input from rudders so it gives you a free hand on taxi to drink your coffee with the left hand on the taxiway. On takeoff once rolling down the runway you transition to rudders for nose wheel steering and maintain it until nose wheel strut is airborne and centers. We can not shut it off. I think it would help to delay rotation. I used it for V1 cuts in the sim and it worked.
Moderator
.. a few things to note
(a) Vmcg is a line in the sand certification animal for nominally critical conditions. Like everything else to do with certification, it is not a guarantee, only a benchmark considered to provide an adequate risk level.
(b) usually, the figure is not particularly relevant to real world operations due to the configuration's being different (especially CG and actual thrust) .. this has the result of lowering the real Vmcg on the day .. however, the certification figure still determines the minV1 boundary
(c) based on the aircraft for which I have had a Vmcg test involvement, the handling changes in the Vmcg region are very rapid and quite critical. That is to say, a continued failure just above the Vef for Vmcg will be OK and the aircraft will deviate and then return to the centreline. In the space of a few knots reduction through Vmcg, the deviation will rapidly become uncontrollable and a close encounter of the grassy kind becomes the predictable outcome if the throttles are not closed very promptly
(d) generally the continued takeoff is the critical case, the accel-stop being comparatively straightforward .. subject to an appropriate pilot response
(e) when the NWS physically can be disconnected for the tests, that is a usual test configuration
(f) with max aft CG, the nosewheel load is going to be very light and, if the event occurs for critical conditions, by the time the average startle-affected pilot gets the pole forward .. it is probably a case of too little, far too late .. For instance, I had a period training in a 732 sim which appeared to be reasonably realistic for minV1 failures. The particular operator appeared never to address this case as they routinely used significant overspeed takeoff schedules .. it was interesting to watch the open eyes on the crews when I started exposing some of them to the problems ...
(g) the centreline deviation varies with the certification basis but the philosophy is the same regardless of the maximum deviation permitted
(h) generally Vmcg is determined for nil wind. The effect of crosswind in real world considerations is critical and may increase the real world Vmcg above the certification animal in the order of 0.5 kt/kt (twins) to in excess of 1.0 kt/kt for four motor aeroplanes ... an important consideration for the selection of speed schedules (or considering delaying departure) if such options are available
(i) for some aircraft Vmcg is so low that it is not limiting (eg Citation)
(a) Vmcg is a line in the sand certification animal for nominally critical conditions. Like everything else to do with certification, it is not a guarantee, only a benchmark considered to provide an adequate risk level.
(b) usually, the figure is not particularly relevant to real world operations due to the configuration's being different (especially CG and actual thrust) .. this has the result of lowering the real Vmcg on the day .. however, the certification figure still determines the minV1 boundary
(c) based on the aircraft for which I have had a Vmcg test involvement, the handling changes in the Vmcg region are very rapid and quite critical. That is to say, a continued failure just above the Vef for Vmcg will be OK and the aircraft will deviate and then return to the centreline. In the space of a few knots reduction through Vmcg, the deviation will rapidly become uncontrollable and a close encounter of the grassy kind becomes the predictable outcome if the throttles are not closed very promptly
(d) generally the continued takeoff is the critical case, the accel-stop being comparatively straightforward .. subject to an appropriate pilot response
(e) when the NWS physically can be disconnected for the tests, that is a usual test configuration
(f) with max aft CG, the nosewheel load is going to be very light and, if the event occurs for critical conditions, by the time the average startle-affected pilot gets the pole forward .. it is probably a case of too little, far too late .. For instance, I had a period training in a 732 sim which appeared to be reasonably realistic for minV1 failures. The particular operator appeared never to address this case as they routinely used significant overspeed takeoff schedules .. it was interesting to watch the open eyes on the crews when I started exposing some of them to the problems ...
(g) the centreline deviation varies with the certification basis but the philosophy is the same regardless of the maximum deviation permitted
(h) generally Vmcg is determined for nil wind. The effect of crosswind in real world considerations is critical and may increase the real world Vmcg above the certification animal in the order of 0.5 kt/kt (twins) to in excess of 1.0 kt/kt for four motor aeroplanes ... an important consideration for the selection of speed schedules (or considering delaying departure) if such options are available
(i) for some aircraft Vmcg is so low that it is not limiting (eg Citation)
Join Date: Aug 2005
Location: fl
Posts: 2,525
Likes: 0
Received 0 Likes
on
0 Posts
A friend of mine took off in a Citation 500 with a bad starter/generator with one engine in Mexico to get it back empty so since the failure was at the ramp his vmcg was zero. Of coarse max power had to be delayed until he wasn't using full rudder to keep it straight. With a very long runway most any procedure will work One guy forgot the flaps on a 737 but still had an uneventful takeoff except for a slight stick shaker at rotation.
I was given a stall recovery lesson in a Falcon 20 and the instructor failed an engine when I went max power so I reduced power on the good engine to maintain wings level and it upset him I didn't crash. We didn't have to repeat it to prove his point though. Most of the time a little common sense and long runways can get you out of most anything whether it is legal or not.
I was given a stall recovery lesson in a Falcon 20 and the instructor failed an engine when I went max power so I reduced power on the good engine to maintain wings level and it upset him I didn't crash. We didn't have to repeat it to prove his point though. Most of the time a little common sense and long runways can get you out of most anything whether it is legal or not.