Vmcg: Understanding the minimum control speed on ground

Back at flight school, we all learned about the “30 ft-Vmcg-rule”. While that might get you through the ATPL exam, it does not provide the necessary understanding of the implications that certification requirements have on the daily operation of aircraft. This article aims to do exactly that: Provide a summary of Vmcg certification aspects and demonstrate how they affect daily operation.

Take-off speeds overview

The take-off process involves several critical speeds and the background of each is beyond the scope of this article. In order to have a good idea of where Vmcg lies in all of this, we shall look at the following picture:

Figure 1: Vmcg in the take-off process

The picture above makes it clear that Vmcg is a critical speed during take-off as it sets the lower limit for Vef and V1 respectively. If we go back to the certification specifications, we find the following:

EASA CS 25.149(e) states [1]:

“VMCG, the minimum control speed on the ground, is the calibrated airspeed during the take-off run at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the aeroplane using the rudder control alone (without the use of nose-wheel steering), as limited by 667 N of force (150 lbf), and the lateral control to the extent of keeping the wings level to enable the take-off to be safely continued using normal piloting skill.”

The associated conditions are set out below:

Figure 2: Vmcg determination (Base picture credit: [2])

How is it tested?

Testing of Vmcg is considered high-risk [3]. Several pre-requisites must be completed before commencing Vmcg testing. One of the key points being a good estimation of Vmcg. Once the configuration is established, the crew will rehearse the process in the simulator. The pattern needs to be clear and repeatable, since Vmcg heavily depends on pilot reaction time. Generally, there are two options: Continued take-off or rejected take-off testing. For aircraft with three or more engines, the take-off is usually continued, on twin-engine aircraft both techniques are used [3]. As these tests involve an actual fuel-cut at high thrust, this will degrade the engine which is shut down, and the number of “attempts” is therefore limited. This is also the main reason for the subject engine to be heavily instrumented [3]. Needless to say, that a wide runway and calm winds are required. The tower wind is not “good enough” and precise wind sensors are needed. Crosswind will either be restricted to a negligible amount or, if unavoidable, compensated for by performing test-runs in opposite directions.

Figure 3: Vmcg testing considerations, based on [3]

Typical Vmcg physics

As described above, the determination of Vmcg is a delicate undertaking. Physics are highly non-linear and it is not surprising that the Vmcg changes dramatically with even minor changes of the environment as indicated below. As Vmcg is tested on a dry runway with nosewheel-steering (NWS) OFF, the other curves in the graph below are estimations, based on a NASA model [4]. The exact effects will vary for each aircraft/environment combination.

Figure 4: Estimated NWS and RWY condition effects on Vmcg based on [4]

The events that follow an engine failure on ground are highly dynamic and a complex interaction between aerodynamic and ground forces. For a better understanding, the Figures below show the situation after an engine failure, once before any correcting controls are applied, and once after correcting controls are in place. In the first Figure, the thrust asymmetry and the associated yawing moment are clearly visible. Also note the cornering force (Fc) of the tires. Further, the aerodynamic sideslip produces a side force.

Figure 5: Situation immediately after engine failure without correcting controls, derived from [4]

Once the necessary controls are applied and the corrections start having an effect, something interesting happens: As the yaw rate is reversed, so is cornering force on the main gears. This is different for the nose gear, as NWS is applied. The aerodynamic side force also reverses.

Figure 6: Situation after correcting controls have been applied, derived from [4]

Tire cornering force

It will be intuitive to understand that a contaminated runway provides less friction. This fact reduces the cornering force generated by aircraft tires [5]. The implications of this are that despite the NWS being castering, there is still a significant effect on the aircraft main gear tires on the trajectory when comparing dry and contaminated runways.

Figure 7: Aircraft tire skidding, derived from [5]

The cornering force as indicated above diminishes when the runway friction is reduced. This will inevitably affect the centerline deviation of the aircraft when experiencing an engine failure close to Vmcg. Crucially for the flight crew, EASA AMC 25.1591 [1] contains an important note:

“7.4.1 Minimum V1

For the purpose of take-off distance determination, it has been accepted that the minimum V1 speed may be established using the VMCG value established in accordance with CS 25.149(…). As implied in paragraph 8.1.3, this may not ensure that the lateral deviation after engine failure will not exceed 30 ft on a contaminated runway.”

And later:

“8.1.3 The provision of performance information for contaminated runways should not be taken as implying that ground handling characteristics on these surfaces will be as good as can be achieved on dry or wet runways, in particular following engine failure, in crosswinds or when using reverse thrust.”

V1 selection policy: Do you have one?

If the actual take-off weight is less than the performance-limited weight, a range of V1 speeds will be available. Some operators have a V1 selection policy, others leave the decision to the crew. A third option is to use the “admin settings” in the performance software to simply provide the crew with one value for V1 in all cases. Either way, it will be clear from the discussion above that an increased “gap” between Vmcg and V1 will improve lateral controllability, while using “min. V1” will maximize the stop-margin [4].

When “less is more”

And then, there is the situation when reducing thrust can increase the permissible take-off weight. This happens, typically on contaminated runways when the take-off weight is limited by the accelerate-stop-distance available (ASDA). But careful, there are strict rules:

Reduced-thrust take-off: Assumed temperature vs. derate

In brief terms, there are two ways to perform a reduced-thrust take-off. Manufacturers can allow both individually, both combined, only one, or none of them. The key points are below:

Assumed temperature (FLEX) take-off

This employs one set of performance data, and the thrust is reduced to the equivalent of TOGA at the assumed temperature. All minimum control speeds are based on TOGA. The flight crew may apply TOGA thrust at any point during the take-off.

This procedure is prohibited on contaminated runways [1].

Derate take-off

This contains sub-sets of performance data with individual limitations. Thrust reduction is usually specified in proprietary terms. All minimum control speeds are based on derated thrust. Application of TOGA is prohibited, as it can lead to a loss of control. Some manufacturers specify a speed, above which the crew may re-consider TOGA after having accelerated. This procedure is permitted on contaminated runways [1].

It is now clear how a derated take-off can increase the allowable take-off weight under the aforementioned conditions: The derated thrust leads to a lower Vmcg, which allows a lower V1, which in turn reduces the accelerate-stop-distance.

Advanced flight control systems

While the certification specifications stipulate pilot inputs during Vmcg determination [1], there is no prohibition for the flight control engineers to get creative: The use of some form of automatic thrust asymmetry compensation using the rudder is nothing new and is present on many aircraft [6] [7]. But what about more advanced ideas, such as deflecting spoilers asymmetrically or splitting ailerons in order to reduce Vmcg?

Asymmetric spoiler deployment and aileron split

Higher Order Flight Control (HOFC) aircraft can take credit from a massive increase in possibilities regarding control surface deflections. One such example is the A380, where the value of Vmcg has been reduced by about 10 kt using such a facility [8] [9]. The idea is simple and genius: When the aircraft is on the ground, in the take-off run and encounters an engine failure that requires a significant rudder deflection to compensate, the aircraft will also deflect spoilers and split the aileron on the side where the rudder is deflected to [8]. The control surface deflections are then removed, as the aircraft transitions to flight [8]. Similar concepts are used on other HOFC aircraft [10].

This is just one of many applications that demonstrate in an exemplary way what advantages HOFC aircraft can provide compared to conventional aircraft.

References:

[1]	EASA, «Easy Access Rules for Large Aeroplanes (CS-25) amdt 27,» 2023.
[2]	Vecteezy, «Vecteezy,» [Online]. Available: https://de.vecteezy.com/vektorkunst/2127665-draufsicht-auf-die-landebahn-des-flughafens-mit-flugzeugvektorillustration. [Zugriff am 22 03 2026].
[3]	P. J. Bolds-Moorehead, V. G. Chaney, T. L. Lutz und S. Vaux, «Ground Minimum Control Speed Testing of Transport Aircraft,» in AIAA Flight Testing Conference, Dallas, TX, 2015.
[4]	E.-J. A. M. Huijbrechts, H. J. Koolstra und J. A. Mulder, «Using Vmcg-Limited V1, Controllability Issues on Contaminated Runways and in Crosswind,» JOURNAL OF AIRCRAFT, Bd. 56, Nr. 4, pp. 1342-1352, 2019.
[5]	Y. Nakajima, Advanced Tire Mechanics, Springer, 2019.
[6]	R. S. Breuhaus, K. R. Fowler und J. J. Zanatta, «Innovative aspects of the BOEING 777 development program,» Seattle, Washington, 1996.
[7]	C. Lelaie, «The Evolution of Airbus Fly-by-Wire,» Translated by Alex Scerri.
[8]	C. LELAIE, «Minimum control speed tests on A380,» The Airbus Safety Magazine, pp. 11-14, 2011.
[9]	T. H. Orra, L. R. De Souza Cruz und M. Faustino Oliveira, «Handling qualities evaluation of a VMCG reduction functionality,» in AIAA Atmospheric Flight Mechanics (AFM) Conference, Boston, 2013.
[10]	C. FAVRE, «Fly-by-wire for commercial aircraft: the Airbus experience,» International Journal of Control, Bd. 59, Nr. 1, pp. 139-157, 1994.