7. cruise ii

7. CRUISE (II)7. CRUISE (II)

Performance JAR 25Performance JAR 25

CRUISE (II)CRUISE (II)

Economic Mach number

Constant Mach number cruise

Maximum endurance

Buffet onset graph

Drift-down

ECONOMIC MACH NUMBERECONOMIC 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


The economic Mach number (MECON) is the one that gives minimum Direct Operative Costs:

Costs

M

Fixed

Time related

Fuel

DOC

MECON MMOMMRC


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

fuel of Cost

time of Cost CI


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.


As it can be seen on the graph below, MECON is practically constant (it decreases just a little bit) as weight decreases.


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.

CONSTANT MACH NUMBER CRUISECONSTANT 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.

MAXIMUM ENDURANCEMAXIMUM 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.

BU

FF

ET

ON

SE

T G

RA

PH

BU

FF

ET

ON

SE

T G

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A 1.3 load factor is generally adopted as a manoeuvring safety

factor when cruise level is selected.

DRIFT-DOWNDRIFT-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.


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.


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).


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.


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%


The gradient of the net flight path must be positive at least 1,000 ft above all terrain and obstructions along the route.


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.


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.


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.

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7. cruise ii

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