• Andreas

Beyond Vmo/Mmo: A closer look at high-speed certification rules

Updated: Jun 17

It is every pilot’s duty to respect aircraft limitations. However, this fact does not relief us from getting familiar with the looming hazards, should we ever exceed a limit. A basic understanding of the design rules for speeds beyond the “barber pole” can be helpful to judge the implications of an upset condition. In this article, we look at design considerations for that speed regime, covering flight physics and certification aspects.


Figure 1: F/A-18 in transonic flight

Aerodynamics – Drag rise

Jet transport aircraft are designed to operate in the transonic regime. While the cruise speed is still subsonic, certain parts of the airflow are supersonic. The freestream Mach number, at which any part of the airflow first reaches Mach 1, is called the critical Mach number (Mcrit) [1]. The typical cruise speed lies slightly above Mcrit. Consequently, a shock wave will begin to form. A small shock wave will generally be tolerated by the designers. Further speed increase will quickly lead to a pronounced drag rise. This is referred to as the Drag Divergence Mach number MDD [2] and represents a “natural” obstacle when trying to accelerate further. This is typically associated with buffeting and degradation of stability [3].


Key point: When accelerating through MDD the drag rises significantly.


A classical solution to increase MDD has been to sweep the wings, and that is still what many designs incorporate. For that purpose, it does not matter if the wings are swept forward or backward, but obviously there are other constraints which favor the sweep back, such as stability and structural aspects [2]. The relation between Mcrit and MDD is depicted in Figure 2 below. Note, that an increase in lift coefficient lowers MDD.


Figure 2: Critical Mach number and Drag Divergence Mach number, based on [2] [3]

If the aircraft is operated at a higher lift coefficient, the curve shifts up and left. On the other hand, a wing sweep will increase Mcrit and MDD and therefore shift the curve to the right [2] [3].


Performance – How much thrust is needed?

The aerodynamic considerations above immediately raise the following question: How much thrust should the airplane be furnished with, in order to fulfill its mission without being “overpowered” and thus commercially inefficient? The airplane design engineers very carefully choose the powerplant to match rest of the airplane, as shown in Figure 3.


Figure 3: Typical speed capability of an airliner, based on data from [4]

At the typical cruise altitude, the excess thrust has reduced to almost zero. In other words: The airplane is equipped with just enough thrust to do, what it needs to do. The physics dictate, that the same powerplant will deliver a higher thrust at lower altitudes and thus, there is more “excess thrust” available at lower altitudes. In fact, it may be enough to significantly exceed VMO and under certain conditions even VD in level flight [4].


Key point: For a typical jet airplane, there is sufficient thrust to significantly exceed VMO in level flight at lower altitudes.


Atmospheric variations or a descending flight path may still cause exceedances at high altitudes. Obviously, there are rules for the designers how to deal with that, as we shall see in the next paragraph.


Certification rules

The certification requirements for high-speed flight can be quite overwhelming at times. While it is not the goal of this article to fully replicate the content of the applicable certification specifications, some of the most important aspects have been summarized in the following graphs. The interested reader is directed to [5] for a detailed description.

In jet and turboprop aircraft, the “barber pole” starts at VMO/MMO [5]. Figure 4 below provides the context of that with respect to other design and certification relevant speeds.


Figure 4: Design speeds overview

The manufacturer may select VMO/MMO up to VC. The aircraft needs to provide certain stability characteristics up to VFC. The design dive speed VD/MD is determined according to a dedicated procedure described in Figures 5 to 7. The speed achieved during the flight test campaign is referred to as VDF/MDF.


Figure 5: Design dive speed selection options in CS-25

CS-25 permits several options for selecting VD/MD. Apart from a fixed factor, the manufacturer may choose to apply the greater of the “upset criterion” and a minimum margin, as specified in CS 25.335(b). The values and procedures are depicted in Figures 5 to 7.


Figure 6: Minimum margin option acc. CS 25.335(b)(2)

If the manufacturer elects to apply the upset criterion, a distinction is made between aircraft with and without speed protection. See Figure 7.


Figure 7: CS-25 upset criterion

Notice, that the maneuvers described above are significantly different from a typical trajectory of an airline flight. This underlines the margins that manufacturers have to consider for operations above VMO/MMO.


Operational considerations

Despite all precautions, VMO/MMO may sometimes be reached or slightly exceeded. At high altitude this will mainly be caused by external factors, such as wind gradients and most probably occur during descent [4]. Manufacturers provide guidance how to react in such a case and what follow-up actions to take [4]. Keep in mind that the certification rules ensure sufficient controllability and structural integrity as described above. A hasty control input will usually create more harm than anything else, especially if it involves a significant load factor increase [6].


Case study

A typical scenario is described here, for details refer to the full report [6]. The crew of a B737 accepted an ATC instruction for a “high-speed descent”. The First Officer was pilot flying and selected the speed accordingly. There were significant wind variations during the descent and the guidance mode was changed several times between LVL change and V/S. Around 17000 ft, the tailwind component decreased rapidly. During this period, the guidance mode was changed to V/S, causing A/T to revert to MCP speed mode. However, as the engines were already at idle, the A/T did not have effective control over the speed in this mode. The aircraft had an automatic mode reversion logic to LVL change, if the speed was approaching VMO/MMO, however this was hindered by the manual intervention of the crew. All these factors combined led to an upset event involving injuries. See Figure 8 for the recording.


Figure 8: Recording of B737 overspeed and pitch up event [6]

The captain reacted by pulling back aggressively on the control column, causing the autopilot to disconnect and creating a significant increase in load factor. This led to injuries in the cabin. This is typical, as the very nature of this kind of event involves high speeds, leading to increased load factors for a given pitch rate. Notice, that the speed exceedance was relatively small (VMO 340 kt). At no point during the upset did the crew use the speedbrakes. Use of LVL change mode and speedbrakes would have resulted in less load factor change and a slight exceedance of VMO/MMO with only minor consequences [6].


Key point: Be familiar with manufacturer procedures for overspeed prevention and recovery.


References:

[1] BOEING, Jet transport performance methods, 2009.

[2] J. Anderson, Fundamentals of aerodynamics, McGraw-Hill, 2nd ed. 1984.

[3] H. Hurt, Aerodynamics for naval aviators, Naval air systems command, 1965.

[4] AIRBUS, «Control your speed,» Safety First, January 2016.

[5] EASA, «Easy access rules for large airplanes CS-25,» amdt 26, 2021.

[6] Australian Transport Safety Bureau (ATSB), «Overspeed and pitch up resulting in cabin crew injury involving B737, VH-VUE,» ATSB, Canberra, 2020.