2. CRUISE (II)
Economic Mach number
Constant Mach number cruise
Maximum endurance
Buffet onset graph
Drift-down
3. ECONOMIC MACH NUMBER
MMRC or MLRC are only efficient in terms of fuel. But that is only one part
of the Direct Operative Costs. Thus, if we take into account these
DOCs, the economic Mach number (MECON) can be introduced.
DOC can be expressed as:
DOC = Cc + CF · ΔF + CT · ΔT
Fixed costs
Cost of fuel
unit
Trip fuel
Time related costs
per flight hour
Trip time
4. ECONOMIC MACH NUMBER
The economic Mach number (MECON) is the one that gives minimum
Direct Operative Costs:
Costs
M
Fixed
Time related
Fuel
DOC
MECON MMOMMRC
5. ECONOMIC MACH NUMBER
For a given altitude, MECON decreases if weight decreases (slightly).
For a given weight, MECON increases if altitude increases.
MECON also depends on the time and fuel cost ratio. This ratio is known
as COST INDEX (CI):
F
T
C
C
fuelofCost
timeofCost
CI ==
6. ECONOMIC MACH NUMBER
If fuel price increases, it becomes interesting to decrease fuel
consumption. CI will decrease, and also MECON (so as to be closer to
MMRC).
The extreme CI values are:
CI = 0: Flight time costs are null (fixed wages), so MECON = MMRC.
CI = CIMAX: Flight time costs are very high and the fuel price is low, so
in order to have a trip with the minimum flight time, MECON = MMO.
7. ECONOMIC MACH NUMBER
As it can be seen on the graph below, MECON is practically constant (it
decreases just a little bit) as weight decreases.
9. ECONOMIC MACH NUMBER
If an FMS is fitted, the pilot will introduce the cost index through the
MCDU (INIT PAGE), so the FMC is able to calculate optimum profiles.
Otherwise the pilot will use appropriate charts to find MECON.
10. CONSTANT MACH NUMBER CRUISE
Since MECON has a very little variation as weight decreases due to fuel
burn-off, the cruise phase of a standard airline flight is performed at a
constant Mach number, thus simplifying operation.
This type of cruise is known as Constant Mach Number Cruise.
11. MAXIMUM ENDURANCE
For jet aircraft, maximum endurance can be achieved flying at
minimum drag speed.
This type of flight is only useful during holdings, and maybe during
some maritime surveillance missions, etc.
There is an optimum altitude for maximum endurance flight, that will
be higher as weight decreases.
Since flying at MME makes speed unstable, in practise a higher speed is
used for holding patterns. This will avoid unnecessary thrust
adjustments.
12. BUFFETONSETGRAPH
A 1.3 load factor is
generally adopted as
a manoeuvring safety
factor when cruise
level is selected.
13. DRIFT-DOWN
If an engine fails during cruise phase, the remaining thrust is no longer
sufficient to maintain an adequate cruise speed. The thrust necessary
to fly at cruise altitude cannot be achieved with the remaining engines
at MCT.
The only solution is to descend to a more appropriate altitude, where
the aircraft is able to level-off. This descent procedure is known as
drift-down and it must meet some requirements, since a random
descent over mountainous areas may be dangerous.
14. DRIFT-DOWN
Drift-down speed corresponds to the minimum drag speed, since it
provides the minimum descent angle or the maximum climb angle,
depending on whether we have to descend or climb to the level-off
altitude.
This level-off altitude is often known as “altitude capability” of the
aircraft.
Since drift-down speed corresponds to minimum drag speed, it
decreases as weight decreases.
17. DRIFT-DOWN
If an engine fails, the pilot must:
1. Manage engine systems (ignition, auto-thrust, etc)
2. Select MCT on remaining engines
3. Decelerate to drift-down speed while maintaining altitude.
4a. Descend at drift-down speed until reaching drift-down ceiling
(obstacle strategy), or…
4b. Descend at high speed (300 kt / 0.78 M for A320) until reaching long
range level and reduce to long range speed (normal strategy).
18. DRIFT-DOWN
If we have applied the obstacle strategy, once the aircraft has reached
the level-off altitude three possibilities appear:
1. Maintain minimum drag speed and climb as the aircraft loses
weight. (due to obstacles).
2. Accelerate to long range speed (acceleration will be very slow).
3. Descend to reach long range speed quickly, if obstacle clearance is
not a problem any more.
19. DRIFT-DOWN
REQUIREMENTS
As far as obstacle clearance is concerned, a net flight path has been
established, and represents the gross flight path minus a mandatory
reduction.
Gradient penalty
two engines three engines four engines
Net flight path
(1 ENG OUT)
1.1% 1.4% 1.6%
Net flight path
(2 ENG OUT)
- 0.3% 0.5%
21. DRIFT-DOWN
The gradient of the net flight path must be positive at least 1,000 ft
above all terrain and obstructions along the route.
22. DRIFT-DOWN
At any point of a critical area on the route, it must always be possible
to escape while ensuring, during descent, the relevant obstacle
clearance margin of 2,000 feet on the net flight path.
23. DRIFT-DOWN
Obstacles affecting the previous requirements are those located on a
corridor with a lateral margin of 5 NM. JAR extends this margin to 10
NM if the navigational accuracy does not meet the 95% containment
level.
24. DRIFT-DOWN
An additional requirement has been established for an engine failure:
The net flight path must have a positive gradient at 1,500 ft above the
aerodrome where the landing is assumed to be made after an engine
failure.
