Is VMO/MMO a "brick wall" that you must never exceed for fear that the aircraft will come apart? No, not at all. It is a limitation you must obey, but a momentary nibble into the world beyond the "maximum operating limit speed" is not a reason to panic.
— James Albright
Why ask the question? It appears some pilots will treat an excursion into the barber pole as a reason to immediately react without thinking and in some cases that has injured people. (See: Virgin Australia VH-VUE.)
It may help to understand where VMO/MMO comes from and how fast the aircraft was actually test flown. If you find yourself at or above VMO/MMO, react as your manuals tell you. More than likely: pull the throttles back, extend the speed brakes, reduce pitch gently. This is no time to test the g-limits of your aircraft.
Design cruising speed (VC/MC)
VC means design cruising speed.
Source: 14 CFR 1
When we think of "cruising" we think of normal operations, but design cruising speed is actually greater than or equal to VMO/MMO:
The maximum operating limit speed (VMO/MMO) airspeed or Mach Number, whichever is critical at a particular altitude) is a speed that may not be deliberately exceeded in any regime of flight (climb, cruise, or descent), unless a higher speed is authorized for flight test or pilot training operations. VMO/MMO must be established so that it is not greater than the design cruising speed VC and so that it is sufficiently below VD/MD or VDF/MDF, to make it highly improbable that the latter speeds will be inadvertently exceeded in operations. The speed margin between VMO/MMO and VD/MD or VDF/MDF may not be less than that determined under §25.335(b) or found necessary during the flight tests conducted under §25.253.
Source: 14 CFR 25, §25.1505
So VMO/MMO is equal to or less than VC and "sufficiently below" VD/MD or VDF/MDF.
Design diving speed (VD/MD)
VD means design diving speed.
Source: 14 CFR 1
Design dive speed, VD. VD must be selected so that VC/MC is not greater than 0.8 VD/MD, or so that the minimum speed margin between VC/MC and VD/MD is the greater of the following values:
(1) From an initial condition of stabilized flight at VC/MC, the airplane is upset, flown for 20 seconds along a flight path 7.5° below the initial path, and then pulled up at a load factor of 1.5g (0.5g acceleration increment). The speed increase occurring in this maneuver may be calculated if reliable or conservative aerodynamic data is used. Power as specified in §25.175(b)(1)(iv) is assumed until the pullup is initiated, at which time power reduction and the use of pilot controlled drag devices may be assumed;
(2) The minimum speed margin must be enough to provide for atmospheric variations (such as horizontal gusts, and penetration of jet streams and cold fronts) and for instrument errors and airframe production variations. These factors may be considered on a probability basis. The margin at altitude where MC is limited by compressibility effects must not less than 0.07M unless a lower margin is determined using a rational analysis that includes the effects of any automatic systems. In any case, the margin may not be reduced to less than 0.05M.
Source: 14 CFR 25, §25.335(b)
Speed achieved in flight test (VDF/MDF)
VDF/MDF means demonstrated flight diving speed.
Source: 14 CFR 1
How fast must the aircraft be flown in flight test? There are several things to consider.
Flight flutter testing. Full scale flight flutter tests at speeds up to VDF/MDF must be conducted for new type designs and for modifications to a type design unless the modifications have been shown to have an insignificant effect on the aeroelastic stability. These tests must demonstrate that the airplane has a proper margin of damping at all speeds up to VDF/MDF, and that there is no large and rapid reduction in damping as VDF/MDF, is approached. If a failure, malfunction, or adverse condition is simulated during flight test in showing compliance with paragraph (d) of this section, the maximum speed investigated need not exceed VFC/MFC if it is shown, by correlation of the flight test data with other test data or analyses, that the airplane is free from any aeroelastic instability at all speeds within the altitude-airspeed envelope described in paragraph (b)(2) of this section.
Source: 14 CFR 25, §25.629(e)
With the airplane trimmed at any speed up to VMO/MMO, there must be no reversal of the response to control input about any axis at any speed up to VDF/MDF. Any tendency to pitch, roll, or yaw must be mild and readily controllable, using normal piloting techniques. When the airplane is trimmed at VMO/MMO, the slope of the elevator control force versus speed curve need not be stable at speeds greater than VFC/MFC, but there must be a push force at all speeds up to VDF/MDF and there must be no sudden or excessive reduction of elevator control force as VDF/MDF is reached.
Source: 14 CFR 25, §25.253(3)
Adequate roll capability to assure a prompt recovery from a lateral upset condition must be available at any speed up to VDF/MDF.
Source: 14 CFR 25, §25.253(4)
(a) From an initial condition with the airplane trimmed at cruise speeds up to VMO/MMO, the airplane must have satisfactory maneuvering stability and controllability with the degree of out-of-trim in both the airplane nose-up and nose-down directions, which results from the greater of—
(1) A three-second movement of the longitudinal trim system at its normal rate for the particular flight condition with no aerodynamic load (or an equivalent degree of trim for airplanes that do not have a power-operated trim system), except as limited by stops in the trim system, including those required by §25.655(b) for adjustable stabilizers; or (2) The maximum mistrim that can be sustained by the autopilot while maintaining level flight in the high speed cruising condition.
(b) In the out-of-trim condition specified in paragraph (a) of this section, when the normal acceleration is varied from + 1 g to the positive and negative values specified in paragraph (c) of this section—
(1) The stick force vs. g curve must have a positive slope at any speed up to and including VFC/MFC; and
(2) At speeds between VFC/MFC and VDF/MDF the direction of the primary longitudinal control force may not reverse.
(c) Except as provided in paragraphs (d) and (e) of this section, compliance with the provisions of paragraph (a) of this section must be demonstrated in flight over the acceleration range—
(1) −1 g to + 2.5 g; or
(2) 0 g to 2.0 g, and extrapolating by an acceptable method to −1 g and + 2.5 g.
(d) If the procedure set forth in paragraph (c)(2) of this section is used to demonstrate compliance and marginal conditions exist during flight test with regard to reversal of primary longitudinal control force, flight tests must be accomplished from the normal acceleration at which a marginal condition is found to exist to the applicable limit specified in paragraph (b)(1) of this section.
(e) During flight tests required by paragraph (a) of this section, the limit maneuvering load factors prescribed in §§25.333(b) and 25.337, and the maneuvering load factors associated with probable inadvertent excursions beyond the boundaries of the buffet onset envelopes determined under §25.251(e), need not be exceeded. In addition, the entry speeds for flight test demonstrations at normal acceleration values less than 1 g must be limited to the extent necessary to accomplish a recovery without exceeding VDF/MDF.
(f) In the out-of-trim condition specified in paragraph (a) of this section, it must be possible from an overspeed condition at VDF/MDF to produce at least 1.5 g for recovery by applying not more than 125 pounds of longitudinal control force using either the primary longitudinal control alone or the primary longitudinal control and the longitudinal trim system. If the longitudinal trim is used to assist in producing the required load factor, it must be shown at VDF/MDF that the longitudinal trim can be actuated in the airplane nose-up direction with the primary surface loaded to correspond to the least of the following airplane nose-up control forces:
(1) The maximum control forces expected in service as specified in §§25.301 and 25.397.
(2) The control force required to produce 1.5 g.
(3) The control force corresponding to buffeting or other phenomena of such intensity that it is a strong deterrent to further application of primary longitudinal control force.
Source: 14 CFR 25, §25.255
Maximum speed for stability characteristics (VFC/MFC)
VFC/MFC means maximum speed for stability characteristics.
Source: 14 CFR 1
VFC/MFC may not be less than a speed midway between VMO/VMO and VDF/MDF, except that, for altitudes where the Mach number is the limiting factor, MFC need not exceed the Mach number at which effective speed warning occurs.
Source: 14 CFR 25, §25.253(b)
VMO/MMO is the lower number below VFC/MFC and VDF/MDF. Your maximum operating limit speed is below the maximum speed for stability characteristics and the demonstrated diving speed.
Maximum operating limit speed (VMO/MMO
VMO/MMO means maximum operating limit speed.
Source: 14 CFR 1
Allowing for pilot reaction time after effective inherent or artificial speed warning occurs, it must be shown that the airplane can be recovered to a normal attitude and its speed reduced to VMO/MMO, without—
(i) Exceptional piloting strength or skill;
(ii) Exceeding VD/MD, VDF/MDF, or the structural limitations; and
(iii) Buffeting that would impair the pilot's ability to read the instruments or control the airplane for recovery.
Source: 14 CFR 25, §25.253(2)
How about a translation?
- The maximum operating limit speed (VMO/MMO) cannot be greater than the design cruising speed (VC). [14 CFR 25, §25.1505]
- There is a margin between design diving speed (VD/MD) and design cruising speed (VC/MC) of at least Mach 0.05 or 20%. [14 CFR 25, §25.335]
- VMO/MMO must be sufficiently below VD/MD or VDF/MDF to make it improbable you will ever get to VD/MD or VDF/MDF. [14 CFR 25, §25.1505]
- The aircraft was designed so it could be flown without exceptional piloting strength or skill to avoid exceeding VD/MD or VDF/MDF. [14 CFR 25, §25.253(2)]
- The narrowest margin between VMO/MMO and VD/MD occurs when VMO/MMO is set equal to VC/MC, in which case it must be at least Mach 0.05. [14 CFR 25, §25.335(b)]
Bottom line: VMO/MMO is a limiting speed, but the aircraft was designed to dive at least Mach 0.05 more than MMO.
Most manufacturers don't bother publishing the various speeds that go into determing VMO/MMO. We have a glimpse into the Airbus fleet, however, thanks to an article in their safety magazine, Control your speed . . . in cruise:
|Aircraft type||VMO (kt)||MMO||VD (kt)||MD|
Source: de Baudis, p. 8
Bombardier Global 8000
The Global 8000 has an MMO of Mach 0.94 and achieved Mach 1.015 in dive tests.
While the Global 8000's top operating speed is Mach 0.94, it has broken the sound barrier during flight tests. In May 2021, the jet repeatedly achieved speeds of Mach 1.015, accompanied by a NASA-operated Boeing F-18 fighter.
The G650 has a VMO/MMO of 340 KCAS / Mach 0.925 and achieved a MD of Mach 0.995.
Gulfstream says its first prototype G650 business jet has successfully passed high-speed flutter testing, a key hurdle in meeting the planned certification of the 7,000nm (13,000km) range, fly-by-wire, Rolls-Royce BR725-powered twinjet next year.
Flutter testing uses exciters to input disturbances ranging in frequency from 2Hz to 58Hz to the wings, tail and flight-control surfaces with the aircraft flying at various altitudes, speeds, weights and centre-of-gravity conditions. Aircraft are designed so that the components naturally dampen out the oscillations without actions by the pilots.
In the most recent flutter tests, which are ongoing, Gulfstream test pilots pitched S/N 6001's nose down to as much as 18° below the horizon to reach a speed of Mach 0.995 with the exciters activated. "The demonstrated flutter margins met or exceeded our expectations out to maximum speeds," says Pre Henne, Gulfstream's senior vice-president of programmes, engineering and test. "That's a good sign." The G650 is set to be the world's fastest production business jet, with a maximum cruise speed of M0.925.
Bodell, Luke, Bombardier Reveals New Global 8000 Jet Following Supersonic SAF Test flight, Simple Flying, May 27, 2022
Croft, John, Gulfstream proclaims success in G650 dive test, FlightGlobal, 1 September 2010
De Baudis, Lorraine, and Castaigns, Philippe, Control your speed . . . in cruise, Safety First, Airbus Product Safety Department, 21 January 2016
14 CFR 1, Title 14: Aeronautics and Space, Definitions and Abbreviations, Federal Aviation Administration, Department of Transportation
14 CFR 25, Title 14: Aeronautics and Space, Federal Aviation Administration, Department of Transportation