Control your speed… in cruise

Many speeds are used to certify and fly an aircraft operationally. For every flight, the applicable characteristic speeds are computed automatically by the aircraft Auto Flight Systems (Flight Management System (FMS), Flight Guidance (FG) and Flight Envelope (FE)) and displayed on the PFD airspeed scale. They are extremely useful as target maneuvering and limit reference speeds to safely guide the pilots navigation decisions through the cruise phase.

Our objective is to highlight the design and operational considerations underlying all recommendations Airbus has issued to flight crews regarding the monitoring of these speeds in cruise.

Green Dot was presented already in the previous article dedicated to the climb phase. Nevertheless, it is important to have this speed in mind during the cruise phase as well, because it is a clearly visible reference speed on the PFD airspeed scale.
We will see hereafter why pilots should not routinely fly slower than GD in cruise.

For this reason, a recap of GD definition is provided hereafter, as well as the consequences of flying slower than GD in cruise.

Green Dot (GD): best lift-to-drag ratio speed

• Definition

GD speed is the engine-out operating speed in clean configuration. It corresponds to the speed that allows the highest climb gradient with one engine inoperative in clean configuration. In all cases (all engines operative), the GD speed gives an estimate of the speed for best lift-to-drag ratio.

It is represented by a green dot on the PFD speed scale and displayed only when the slats / flaps control lever is in the ‘0’ (CLEAN) position and landing gears are not compressed (fig.1).

(fig.1)
GD on the PFD speed scale

• What are the operational implications of flying below GD?

Point 3 is not displayed on the PFD airspeed scale. Only GD is shown.

The higher the aircraft, the lower the maximum thrust available. This means that at high altitude, close to REC MAX (RECommended MAXimum altitude), point 3 and GD are close to each other because the thrust margin is small. Therefore flying below GD in level flight could easily drive the aircraft slower than point 3 and eventually in a continuous deceleration.

Consequently, in clean configuration in cruise, the crew should not fly below GD.

Exceptionally, if flight slightly below GD is required for some reason, then vigilant monitoring is necessary to ensure that further uncommanded speed reductions are immediately checked and recovered from.

For a given weight, each aircraft has a minimum selectable speed (V_LS) and maximum speed (V_MAX) at a particular altitude. At the cruise altitude, there needs to be a safe margin in relation to these lowest and highest speeds, before the flight envelope protections activate.

V_LS: Lowest Selectable speed

• Definition

V_LS is the lowest selectable speed with A/THR engaged. Even if the target speed is below V_LS, the A/THR will continue to target V_LS.
V_LS is indicated by the top of the amber line on the PFD speed scale (fig.3).

• How is V_LS determined?

V_LS is a characteristic speed computed by the AFS as a function of the aircraft weight (dependent on the Zero Fuel Weight (ZFW) inserted in the FMS during flight preparation).

(fig.3)
VLS on the PFD speed scale

Note: the 1.23 factor is applicable to fly-by-wire aircraft (1.3 for the others).
This formula means that V_LS is higher when the speed brakes are extended, since speed brakes extension increases V_S1g.

• What are the operational implications of not respecting V_LS?

Deliberately flying below V_LS could either lead to an activation of the Angle-Of-Attack protection on a protected aircraft, or expose the aircraft to a stall if it is not protected, i.e. flying in a degraded law.

V_MO/M_MO: Maximum Operating speed/Mach number

• Definition

In cruise, in clean configuration, V_MO/M_MO is the higher limit of the aircraft speed envelope.
It is indicated by the lower end of the red and black strip along the PFD speed scale (fig.4).

(fig.4)
VMO on the PFD speed scale

• How is VMO/MMO determined?

V_MO/M_MO is established with regards to the aircraft’s structural limits and it provides a margin to the design limit speed/Mach number VD/MD. VD/MD must be sufficiently above VMO/MMO to make it highly improbable that VD/MD will be inadvertently exceeded in commercial operations. Several certification criteria exist. As a result, on Airbus aircraft, MD is usually equal to M_MO + 0.07 and VD approximately equal to V_MO + 35 kt.

The applicable V_MO/M_MO are indicated in each Aircraft Flight Manual. For example, V_MO/M_MO and VD/MD are given in the following table.

These concepts involve understanding the maximum structural speed and Mach of the aircraft VD/MD.

VD is a Calibrated Air Speed (CAS). During test flights, VD/MD are reached by test pilots with the objective to demonstrate that the aircraft structural integrity is not put at stake at these speeds, and that the aircraft remains safely recoverable at all times. The article “High-altitude manual flying” that was published in the 20th issue of this magazine provides a good explanation of the maneuver performed by test pilots to determine these speed and Mach.

Key points to remember are:

• Reaching VD is much easier than reaching MD,
• At high altitude, reaching the aircraft’s structural limit is almost impossible,
• At lower altitudes (i.e. below the crossover altitude), reaching VD is possible because the available thrust is higher, and drag due to Mach is lower.

•  What are the operational implications of not respecting VMO/MMO?

The JAR / FAR 25 rule dictates that VMO or MMO may not be deliberately exceeded in any regime of flight. The parameter VMO/MMO basically sets upper boundaries to the aircraft speed envelope.

Crews should keep in mind that

• At high altitude, whilst it is important to always respect MMO, a slight and temporary Mach increase above that value will not lead the aircraft into an immediate hasardous situation.
• At lower altitudes, exceeding VMO by a significant amount is a real threat and can dramatically affect the integrity of the aircraft’s structure.

Although intentional VMO / MMO exceedance cases are rare, this limit speed can typically be overshot when the aircraft is subject to unusual wind and/or temperature gradient. Prevention is therefore essential.

• Definition

V_{α PROT} is the speed corresponding to the maximum Angle-Of-Attack (AOA) at which Alpha Protection becomes active. It is only displayed in normal law and corresponds to the top of the black and amber strip along the PFD speed scale (fig.5).
In practice, the AOA value of the Alpha Protection decreases as the Mach number increases. When the AOA value of the Alpha Protection decreases, the Alpha Protection strip on the PFD moves upward.

V_{α MAX} is the maximum Angle-Of-Attack speed. It is the speed corresponding to the maximum Angle-Of-Attack the aircraft can fly at in normal law. It corresponds to the top of the solid red strip along the PFD speed scale (fig.5).

α _MAX is a function of the Mach number: it decreases when the Mach increases (fig.6).

(fig.5)
V_{α PROT} and V_{α MAX} on the PFD speed scale

(fig.6) Evolution with the Mach number of the AOA value triggering the α Protection and α Maximum, in clean configuration

• How are V_{α PROT} and V_{α MAX} determined?

Contrary to GD and VLS, V_{α PROT} and V_{α MAX} are not based on the aircraft weight, as inserted in the FMS during flight preparation through the ZFW.

Vα PROT (resp. V_{α MAX}) as displayed on the PFD is a prediction of what the aircraft speed would be if it flew at an Angle-Of-Attack (AOA) equal to α _PROT (resp. α _MAX). In fact, both speeds are calculated on the basis of the aircraft longitudinal equilibrium equation, along with the actual aircraft speed and AOA.

On the A320 Family, V_{α PROT} and V_{α MAX} can have different numerical values on both PFDs because VC comes from different sources for left and right PFDs.

On A330/A340, A350 and A380 Families, V_{α PROT} and V_{α MAX} have the same numerical values on both PFDs.

In order to avoid a fluctuating V_{α PROT} and V_{α MAX} display, AOA and VC values are filtered so that fast AOA variations (for example during turbulence) do not pollute the PFD speed scale.

As a result of this filtering, a little delay can be observed; therefore during a dynamic maneuver, the aircraft may enter into a protection law with the IAS not yet below the displayed V_{α PROT}.

• What are the operational implications of flying below V_{α PROT}?

At any time during cruise, the actual AOA is compared to α _PROT (or α _MAX) in real time. The difference of AOA is then converted to speed and applied on each PFD: the delta between current speed and V_{α PROT} (or V_{α MAX}) represents the actual margin against α _PROT (or α _MAX) (fig.7).

In normal law, on a protected aircraft, exceeding the AOA value of the α _PROT threshold would immediately trigger the high AOA protection, thus resulting in a nose down pitch rate ordered by the flight control laws. Further increasing the AOA by maintaining full back stick would eventually result in reaching the α _MAX threshold.

When flying in a degraded law, increasing the AOA would directly expose the aircraft to stall.

(fig.7)
PFD display of the available speed margin against α_PROT and α_MAX

Reading the first section of this article and understanding how V_MO/M_MO and VD/MD are determined highlighted that:

• At high altitude, reaching the aircraft’s structural limit Mach number is almost impossible (except in a steep dive with maximum thrust); therefore at high altitude, flying at high Mach number should not be viewed as the biggest threat to the safety of flight. Conversely, flying too slow (below Green Dot) at high altitude can lead to progressive reductions in speed until the protections are triggered. Should this speed reduction take place in a degraded law, it could lead to a loss of control due to stall. At and near the performance altitude limit of the aircraft, the range of available speeds between Green Dot and M_MO will be small. Speed decay at high altitude must be avoided as a result.
• At lower altitudes (i.e. below the crossover altitude), too large a speed decay can similarly lead a non protected aircraft (i.e. flying in a degraded law) to enter a stall. Nevertheless, at low altitude, the available envelope is greater and the thrust margin is much higher, thus providing flight crews a greater ability to safely control the airspeed and recover from a speed decay. On the other hand, at low altitude, reaching V_MO and VD is possible; therefore high speed should be viewed indeed as a significant threat to the safety of flight.

This chapter offers pilots background knowledge of available prevention means in order to properly manage the main threats to the airspeed, and eventually prevent an overspeed or a speed decay thanks to anticipation and use of dedicated procedures.

Clearly flight crews are expected to be able to rapidly scan the essential and relevant parameters, in every situation, in every flight phase, including dynamic ones. In most cases, speed excursion situations are due to rapid wind and temperature variations/evolutions.

Gaining a good awareness of weather

Weather is an important factor that influences aircraft performances. Be it a local flight or a long haul flight, decisions based on weather can dramatically affect the safety of the flight. As it turns out, the first external threat to airspeed comes from weather disturbances, such as turbulent areas that can lead to significant speed changes.

Common sense generally makes pilots avoid those areas; however, they sometimes end up in a situation where some solid turbulence is encountered, when dodging thunderstorms for example. At this point, the airspeed begins to fluctuate, thus making speed exceedance or speed decay more likely. Such situations need to be planned ahead and as far as possible, avoided through regular scanning of weather conditions and flight path adaptation.

The first key to preventing speed excursion events is gaining awareness of the available weather predictions along the forecasted route.

Before take-off, the weather briefing has to be as complete as possible. Pilots should check weather reports at alternate and destination airports and, depending on the weather context, this information needs to be updated in flight as often as necessary. Weather information can be communicated either by the Air Traffic Controllers or by the other crews flying in the area.

Once airborne, the weather radar is one powerful tool to help the crew make sound weather related decisions to avoid adverse weather and turbulence areas.

Altitude and wind gradients: the main contributing factors

On aircraft with no failure, and the A/THR engaged or the MAX CLB thrust applied in manual mode, a continuous speed decay during cruise phase may be due to:

• A large and continuous increase in tailwind or decrease in headwind, in addition to an increase in the Outside Air Temperature (OAT), that results in a decrease of the REC MAX FL, or
• A large or prolonged downdraft, when the flight crew flies (parallel and) downwind in a mountainous area, due to orographic waves. The downdraft may have a negative vertical speed of more than 500 ft/min. Therefore, if the aircraft is in a downdraft, the aircraft must climb in order to maintain altitude, and the pitch angle and the thrust values increase. Without sufficient thrust margin, the flight crew may notice that aircraft speed decays, but the REC MAX FL is not modified.

The flight crew must be aware that at high altitude, the thrust margin (difference between the thrust in use and the maximum available thrust) is limited. The maximum available thrust decreases when there is an increase in altitude and/or outside temperature. The REC MAX FL indicated in the FMS decreases when the OAT increases. The nearer the aircraft is to the REC MAX FL, the smaller the thrust margin.

At any altitude, decreasing the speed too much will certainly lower the aircraft’s level of energy and decrease margins for maneuvering, thus potentially leading to a loss of control due to stall with an aircraft flying in a degraded law. It is important to understand and detect signs of a significant speed decay in order to be able to recover.

When speed decreases, pilots should be attentive to their speed trend vector as displayed on the PFD and take action if an unfavourable speed trend develops in order to remain above GD.

If the speed decreases further, then the Angle-Of-Attack (AOA) must be increased in order to increase the lift coefficient C_L, which keeps the forces balanced. However, it is not possible to indefinitely increase the AOA.

As per basic aerodynamic rules, the lift coefficient C_L increases linearly with the AOA up to a point where the airflow separates from the upper wing surface. If the AOA continues to increase, the point of airflow separation is unstable and rapidly fluctuates back and forth. Consequently, the pressure distribution along the wing profile changes constantly and also changes the lift’s position and magnitude. This effect is called buffeting and is evidenced by vibrations. Buffet is a clear sign of an approaching stall or even of the stall itself depending on its severity: it is created by airflow separation and is a function of AOA (fig.8).

• At buffet initiation, the pilot starts to feel airflow separation on wings upper surface.
• The buffet onset corresponds by definition to 1.3g (corresponding to 40° of bank angle in level flight).
• The “deterrent buffet” is so strong that any pilot will feel he/she needs to leave these buffet conditions. It corresponds to one of the definitions of stall.

(fig.8)
AOA effect on lift

When the AOA reaches a maximum value, the separation point moves further forward on the wing upper surface and almost total flow separation of the upper surface of the wing is achieved: this phenomenon leads to a significant loss of lift, referred to as a stall. Incidentally, stall is not a pitch issue and can happen at any pitch value.

These conditions should be avoided thanks to anticipation and regular scanning of both the weather conditions along the flown route, and of the speed trend on the PFD. Nevertheless, these conditions might be approached unintentionally. As soon as any stall indication is recognized – be it the aural warning “STALL + CRICKET” or buffet – the aircraft’s trajectory becomes difficult to control and the “Stall recovery” procedure must be applied immediately.

Using dedicated procedures
As soon as an unfavourable speed trend develops, pilots must take action and prevent a speed exceedance, following the operating techniques and recommendations detailed in the OVERSPEED PREVENTION  procedure.

In most cases, the use of this OVERSPEED PREVENTION procedure will effectively prevent exceeding V_MO/M_MO. Nevertheless, due to system design and limited authority, this may not be sufficient. For this reason, a OVERSPEED RECOVERY procedure was developed as well and implemented in the FCOM /QRH.

The OVERSPEED warning is triggered when the speed exceeds V_MO + 4 kt or M_MO + 0.006, and lasts until the speed is below V_MO/M_MO. In this case, the flight crew must apply the OVERSPEED RECOVERY procedure.

Maintaining the aircraft after a VMO/MMO exceedance

The flight crew must report any type of overspeed event (i.e. if the OVERSPEED warning is triggered). Indeed, in case of an overspeed, an inspection of the aircraft structure may be required.

Indeed, when an overspeed event occurs, the aircraft may experience a high load factor. Only an analysis of flight data allows to tell whether or not an inspection is required.

This supports the crucial need for flight crews experiencing an overspeed to report it! Then maintenance and engineering teams will judge whether or not further inspection is needed.