1.8.1 Cruise Regime
An aircraft can be flown in a wide range of altitudes and speeds. The cruise regime is limited by the following boundaries (Fig. 1):
Maximum operating speed and altitude
The maximum operating speeds (VMO and MMO) and maximum operating altitude are certificate limits and must not be exceeded.
Maximum cruise thrust limited speed
At high weights and/or high altitudes the maximum attainable speed is determined by the (maximum) available cruise thrust. At a given altitude, this thrust limited speed increases with decreasing weight, and decreases with increasing outside air temperature (except for the flat-rated part of cruise thrust).
Buffet onset load factor
The buffet onset load factor is the load factor (g) at which, for a given altitude, buffet onset occurs. The buffet onset load factor decreases with increasing weight.
For aircraft with a good altitude capability this load factor may restrict the maximum cruise altitude at a given weight.
The authority prescribed minimum value for the buffet onset load factor is 1.30 g. This minimum value has been implemented in the company computer flightplanning system.
Minimum cruise speed
A minimum stall margin must, obviously, always be maintained.
However, except for the linear holding (‘retarded cruise’) or pattern holding, the ‘Long Range’ speed (refer to 1.8.3) was until recently considered the minimum practical cruise speed with sufficient speed stability characteristics.
Speed stability
Speed stability is the ability of an aircraft to restore automatically inadvertent speed losses due to turbulence, wind shear, etc.
Adequate speed stability means that an aircraft is able to regain the stabilized cruise speed within reasonable time. Speed stability results from the fact that at a speed below the original cruise speed the required thrust is less than the available thrust so that some excess thrust is provided for accelerating the aircraft to the original cruise speed.
Figure 2 shows that at lower cruise speed (dashed condition) the amount of excess thrust, available to recover a certain speed loss, is less due to the curvature of the thrust required curve. Consequently the speed stability at lower speeds will be less.
Near the minimum drag speed (approx Max Range speed) there is little speed stability and at even lower speeds, the speed stability becomes negative.
Trials carried out in summer 1976 confirmed that all aircraft of the fleet did have adequate speed stability when operating at Long Range speeds. This means that in day-to-day operation no undue attention from the part of the flight crew is required to operate at Long Range speeds for prolonged periods of time.
With the introduction of aircraft with autothrottle systems designed for use in cruise, speed stability is no longer an operational constraint and the speed regime between Long Range and Maximum Range can be utilized (refer to 1.8.3) since with autothrottle speeds down to Maximum Range are fully flyable.
1.8.2 Cruise Optimization
Within the boundaries of the cruise regime (see 1.8.1) a flight can be optimized for minimum time, minimum fuel (= maximum payload) or minimum costs. Normally, minimum costs is the main objective.
Minimum cost cruise system
For the discussion of a “minimum cost” cruise system, the total (direct operating) costs for completing a trip may be split as follows:
- Fixed costs per trip (e.g. handling costs).
- Time-related costs (dependent on duration of flight and hourly cost factor).
- Fuel costs (proportional to consumption kg/NM).
Item 1 need not be taken into account as cruise speed does not affect fixed costs per trip. Consequently the cruise schedule should be such that per flight the sum of 2 and 3 is a minimum.
Hourly cost factor
The hourly cost factor in item 2 consists of:
- Commercial factors. These are hard to quantify. Time table aspects (connections) may sometimes dictate the cruise speed to be used.
- Crew costs. Many flights are non-critical from crew work and rest time point of view and on these flights crew costs do not vary with small changes in flight time. On other flights a small increase in flight time has a significant effect on crew costs due to exceedance of flight time limitations.
- That part of maintenance costs that varies with small cruise speed induced flight time variations. These costs have a tendency of becoming lower because maintenance is more and more done on basis of condition and cycles rather than on hours.
The sharp increase in fuel prices in 1974 and 1979 has resulted in item 3 to become an overriding factor in determining minimum cost cruise speeds in most cases.
Primary minimum cost cruise system
The sum total of fuel costs and time-related costs were calculated for a range of cruise speeds. It appeared that flights which are non-critical with regard to schedule requirements, crew turnaround or aircraft turnaround, achieve minimum costs when operating between Long Range speeds (see 1.8.3) and Maximum Range speeds. Since Long Range speeds were also considered the minimum acceptable speeds from speed stability point of view (see 1.8.1), Long Range cruise was selected as the primary minimum cost cruise system.
NOTE: To keep short distance operations of the DC-9 practical, 280 kt IAS/M.76 has been selected as primary cruise system. This system approximates Long Range speeds and fuel economy under average conditions.
FMS/PMS and Cost Index
For aircraft equipped with Flight Management System (FMS, A310) or a Performance Management System (PMS, 747 retrofit) having an autothrottle-in cruise feature, the system continuously computes, updates and commands the minimum cost cruise speed as a function of weight, altitude, wind component and Cost Index (CI = Time cost/hr / Fuel cost/kg) (see Fig. 3 a/b).
1.8.3 Cruise Systems
Maximum Range cruise
Maximum range (= maximum NM/kg) is obtained at a particular angle of attack. This implies that the speed for Maximum Range, at a given altitude, decreases with decreasing weight (see Fig. 3).
Maximum Range cruise speeds may give rise to serious speed stability problems for aircraft not equipped with autothrottle systems approved for use in cruise.
Long Range cruise
The speed at which 99% of maximum range is achieved, is the so-called “Long Range” speed. As a result of the flat optimum of specific range vs Mach curves, the Long Range speeds are significantly faster (10-15 kt) than the Maximum Range speeds.
Long Range cruise speeds are characterized by decreasing speed with decreasing weight at a given altitude.
Fixed Mach/IAS Cruise
In the past Fixed M/IAS systems (e.g. M.80/300 kts) were mostly applied. Such systems are easy to recall, they do not require consultation of tables or graphs but they may be uneconomical, especially at lower altitudes.
Minimum Cost Cruise
As discussed in 1.8.2, the minimum cost speed depends on weight, altitude, cost factor and wind component. Such cruise systems are automatically flown by aircraft equipped with PMS/FMS, having a minimum cost mode. When Cost Index = 0 is entered, the aircraft is operated for maximum range. At a high Cost Index, max. thrust limited high speed operation results.
Accounting for wind component may be regarded a “second order optimization” with a potential saving of 0.03% (BEF. 300,000.- per year for the total company network).
Fig. 3 shows that for min cost operation the speed reduction at below optimum altitudes is less than for Long Range and moreover, since the minimum cost speeds vary significantly with wind component an added advantage of operating in the Minimum Cost mode is less variation in achieved blocktimes on a given stretch.
For aircraft not equipped with FMS/PMS and disregarding the effect of wind component, a composite Minimum Cost cruise system may be laid down.
1.8.4 Cruise Altitudes
For each cruise system curves can be plotted showing cost/NM vs altitude for various gross weights, see Fig. 5.
These curves show that the optimum altitude (lowest cost/NM) will gradually increase with decreasing weight (dashed line). Due to ATC reasons such a climbing cruise is not feasible and a fixed altitude or a step must be used instead.
Optimum altitude
The step schedules specified on the Maximum Cruise Altitude charts provide the best approximation of the climbing cruise for steps of 4000 ft or 2000 ft.
Step schedules
The excess consumption resulting from 4000 ft resp. 2000 ft steps as compared with climbing cruise is approx. 0.7% resp. 0.24%.
Fig. 5 also shows that the cost/NM increase progressively at altitudes well below the optimum.
The optimum altitudes for faster cruise systems associated with Higher Cost Factors are lower (higher TAS) because the value of time has an overriding effect.
Short distance flights
To avoid impractical high cruise altitudes on short distance flights, a rule has been introduced specifying a minimum level flight segment of 5 minutes. The computer flightplanning system ensures that this minimum level segment is observed.
1.8.5 Effects of Temperature and Wind
01. EFFECTS OF TEMPERATURE
Effect of Temperature on Specific Range
The specific range (NM/kg) is affected only slightly by temperature. This is because fuel flow does vary with temperature in almost the same way as TAS (NM/kg = TAS/Fuel Flow). At 20° C above standard specific range is approx. 1% less than at standard temperature.
In flightplanning however, this effect is neglected.
NOTE: The fuel correction for off-standard temperatures as given on the Time-Fuel Chart is required, because the fuel consumed is read from the Time-Fuel Chart at the flight planned time (till top of descent). This time varies with TAS, which varies with temperature (1 kt per 2° C).
Effect of Temperature on Cruise Altitude
The optimum altitude step schedule can be maintained up to temperatures well above standard. Details (for flightplanning purposes) are given on the Time-Fuel Chart (“Step delay for high temperatures”). At higher temperatures the steps may have to be delayed to a lower weight because of cruise thrust limitations.
During flight, the Maximum Cruise Altitude chart can also be consulted to determine the gross weight at which a step to a higher altitude can be made at a particular temperature condition.
02. EFFECTS OF WIND
Effect of Wind on Cruise Speed Schedules (Long Range)
In case of very strong headwinds (over 200 kt) the use of cruise speeds faster than the Long Range speed may give some gain in fuel. Minimum Cost Cruise speeds vary very significantly with wind component (see 1.8.3).
Effect of Wind on Cruise Altitude
The optimum altitude step schedules are affected in case of excessive wind gradients. This effect is incorporated in computer flightplanning and may be accounted for by delaying the steptime by 2 hours per 5 kt/1000 ft less favourable wind component.
For short distance flights the best altitude can be selected with the aid of the Time-and Trip Fuel tables.
1.8.6 Cruise Control
01. THRUST SETTING PARAMETERS
The primary thrust setting parameters in use for turbofan engines are Engine Pressure Ratio (EPR), or Fan Speed (N1) expressed as a percentage.
The most important feature of N1 as thrust setting parameter is, that the N1 speed required to produce a certain amount of thrust at a given altitude and speed, varies with the outside air temperature. The relationship is given by the rule-of-thumb: increase/decrease required N1 by 1% per 5° C increase/decrease in temperature.
02. CRUISE TECHNIQUE
The cruise procedure (levelling off; use of autopilot modes; setting of cruise thrust is discussed in detail in the relevant AOM under Flight Techniques. However, in stable conditions, resetting of the power lever every 15-20 min is warranted and recommended to compensate for the weight of fuel burnt off.
03. SPEED CONTROL
The following explanation is given for proper understanding of the phenomena that may occur in various non-stabilized cruise conditions.
Power Lever adjustments
When changing the power lever setting in still air conditions to a lower angle, the following train of events will occur:
- the EPR/N1 will be lower; thrust is reduced;
- the aircraft starts to decelerate;
- as the speed falls off, the TAT/RAT decreases (ram temperature rise decreases with decreasing TAS);
- the decreasing TAT results in an increasing EPR/N1;
- the aircraft will finally stabilize at an EPR/N1 reading somewhat higher than the EPR/N1 originally set.
It follows that power lever readjustments may be minimized by setting the power lever angle to an EPR/N1 indication somewhat below the chart setting in case of speed reduction and to an EPR/N1 indication somewhat beyond the chart setting in case of speed increase, i.e. by applying somewhat larger EPR/N1 adjustments than required according to the Cruise Setting tables.
Effect of changing temperature
Outside air temperature has a significant effect on cruise operation.
If for instance an aircraft enters an area of higher OAT and the power lever angle is not changed, the following will be noticed upon entering:
- TAS initially remains the same (because of inertia of the aircraft);
- Mach decreases (because sound velocity increases);
- TAS decreases (because density of the air decreases);
- EPR/N1 decreases (as TAT increases).
If the power levers are not adjusted timely the aircraft may experience considerable speed losses.
As the aircraft must be operated according to a defined Mach schedule the thrust should be increased to maintain the original Mach number. After a stabilized cruise condition is reached in the area of higher temperature the following differences can be noted as compared with the original cruise condition:
- same EPR/higher N1;
- higher fuel flow;
- higher TAS.
The specific range NM/kg will not have changed because Fuel Flow and TAS vary in approx the same ratio.
During transient flight conditions the TAS indicator and in particular the GS indicator (Doppler; INS) are very useful “tendency” instruments to determine, in case of suspect instrument indications, whether acceleration effects are involved or not.
Effect of changing winds
In cases of rapidly changing headwind or tailwind components abnormal airspeed indications may be expected until the aircraft is stabilized in the new air mass. This takes some time because of aircraft inertia effects.
Effect of sloping pressure surfaces
An aircraft flying pressure altitude may have a varying true altitude due to variations in height of the pressure surface. This results in differences of required thrust depending on the slope of the flight path. This effect may be noticeable in case of steep flight path slopes occurring in areas with strong crosswinds.