Aircraft Limitations PDF
Aircraft Limitations PDF
Aircraft Limitations PDF
B. AIRCRAFT LIMITATIONS
1. FLIGHT LIMITATIONS
During aircraft operation, the airframe must endure the forces generated from
such sources as engine(s), aerodynamic loads, and inertial forces. In still air, when
the aircraft is maneuvering, or during in flight turbulence, load factors (n) appear and
thereby increase loads on the aircraft. This leads to the establishment of maximum
weights and maximum speeds.
Lift
nz =
Weight
Except when the lift force is equal to the weight and nz=1 (for instance in
straight and level flight), the aircraft’s apparent weight is different from its real
weight (mg):
In some cases, the load factor is greater than 1 (turn, resource, turbulence). In
other cases, it may be less than 1 (rough air). The aircraft's structure is obviously
designed to resist such load factors, up to the limits imposed by regulations.
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AIRCRAFT LIMITATIONS Getting to Grips with Aircraft Performance
Consequently, load factor limits are defined so that an aircraft can operate within
these limits without suffering permanent distortion of its structure. The ultimate loads,
leading to rupture, are generally 1.5 times the load factor limits.
For all Airbus types, the flight maneuvering load acceleration limits are
established as follows:
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Getting to Grips with Aircraft Performance AIRCRAFT LIMITATIONS
SPEED VALUE
OPERATING
DEFINITIONS EXAMPLES
LIMIT SPEED
FOR THE A320-200
JAR / FAR 25.1505 Subpart G
VMO/MMO
Maximum VMO or MMO are the speeds that may not be
V = 350 kt (IAS)
operating deliberately exceeded in any regime of flight MO
MMO = M0.82
limit speed (climb, cruise, or descent).
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AIRCRAFT LIMITATIONS Getting to Grips with Aircraft Performance
In the determination of VMCG, assuming that the path of the aeroplane accelerating
with all engines operating is along the centreline of the runway, its path from the point
at which the critical engine is made inoperative to the point at which recovery to a
direction parallel to the centreline is completed, may not deviate more than 30 ft
laterally from the centreline at any point.”
Engine failure
Vmcg
Determination of V MCG:
lateral deviation under 30 ft
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Getting to Grips with Aircraft Performance AIRCRAFT LIMITATIONS
• The aeroplane in the most critical take-off configuration existing along the
flight path after the aeroplane becomes airborne, except with the landing
gear retracted; and
• The aeroplane airborne and the ground effect negligible
(d) During recovery, the aeroplane may not assume any dangerous attitude or require
exceptional piloting skill, alertness, or strength to prevent a heading change of more
than 20 degrees.”
5° max
(g) For aeroplanes with three or more engines, VMCL-2, the minimum control speed
during approach and landing with one critical engine inoperative, is the calibrated
airspeed at which, when a second critical engine is suddenly made inoperative, it is
possible to maintain control of the aeroplane with both engines still inoperative, and
maintain straight flight with an angle of bank of not more than 5 degrees. VMCL-2 must
be established with [the same conditions as VMCL, except that]:
• The aeroplane trimmed for approach with one critical engine inoperative
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AIRCRAFT LIMITATIONS Getting to Grips with Aircraft Performance
(h) In demonstrations of VMCL and VMCL-2, … lateral control must be sufficient to roll
the aeroplane from an initial condition of steady straight flight, through an angle of 20
degrees in the direction necessary to initiate a turn away from the inoperative
engine(s) in not more than 5 seconds.”
20º
5º max
During the flight test demonstration, at a low speed (80 - 100 kt), the pilot pulls
the control stick to the limit of the aerodynamic efficiency of the control surfaces. The
aircraft accomplishes a slow rotation to an angle of attack at which the maximum lift
coefficient is reached, or, for geometrically-limited aircraft, until the tail strikes the
runway (the tail is protected by a dragging device). Afterwards, the pitch is
maintained until lift-off (Figure B4).
Two minimum unstick speeds must be determined and validated by flight tests:
- with all engines operatives : VMU (N)
- with one engine inoperative : VMU (N-1)
In the one-engine inoperative case, VMU (N-1) must ensure a safe lateral control
to prevent the engine from striking the ground.
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Getting to Grips with Aircraft Performance AIRCRAFT LIMITATIONS
Air velocity over the wing increases with the angle of attack, so that air
pressure decreases and the lift coefficient increases.
Air pressure Ì
Angle of Attack Ê Ö Air velocity over the wing Ê Ö
Lift coefficient Ê
Therefore, the lift coefficient increases with the angle of attack. Flying at a
constant level, this lift coefficient increase implies a decrease of the required speed.
Indeed, the lift has to balance the aircraft weight, which can be considered as
constant at a given time.
Angle of Attack Ê Ö CL Ê
ρ = constant
S = constant CL Ê Ö TAS Ì
Lift = constant
The speed cannot decrease beyond a minimum value. Above a certain angle
of attack, the airflow starts to separate from the airfoil (Figure B5).
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AIRCRAFT LIMITATIONS Getting to Grips with Aircraft Performance
CL
n = 1g
C L MAX
n < 1g
Angle of Attack
CAS
V S1g VS
Figure B6: CL versus Angle of Attack
VCLMAX
VSR ≥
n zw
Where:
VCLMAX = [speed of maximum lift coefficient, i.e. VS1g]
nzw = Load factor normal to the flight path at VCLMAX”
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Getting to Grips with Aircraft Performance AIRCRAFT LIMITATIONS
VS = 0.94 x VS1g
FAR 25 doesn’t make any reference to the 1-g stall speed requirement.
Nevertheless, Airbus fly-by-wire aircraft have been approved by the FAA, under
special conditions and similarly to JAA approval, with VS1g as the reference stall
speed.
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AIRCRAFT LIMITATIONS Getting to Grips with Aircraft Performance
• Zero Fuel Weight (ZFW) : The weight obtained by addition of the total
traffic load (payload including cargo loads, passengers and passenger’s
bags) and the dry operating weight.
• Landing Weight (LW) : The weight at landing at the destination airport. It is
equal to the Zero Fuel Weight plus the fuel reserves.
• Takeoff Weight (TOW): The weight at takeoff at the departure airport. It is
equal to the landing weight at destination plus the trip fuel (fuel needed for
the trip), or to the zero fuel weight plus the takeoff fuel (fuel needed at the
brake release point including reserves).
Weight
Taxi Weight
taxi fuel
TakeOff Weight (TOW)
trip fuel
catering
newspapers
Operational Empty Weight (OEW)
cabin equipment
crews
propulsion
systems
structure
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Getting to Grips with Aircraft Performance AIRCRAFT LIMITATIONS
L L L
L
2 2 2
2
mWFg mWFg
mg mg
Figure B8: wing bending relief due to fuel weight
The takeoff fuel is the sum of the trip fuel and the fuel reserves. Consequently:
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AIRCRAFT LIMITATIONS Getting to Grips with Aircraft Performance
The minimum weight is the lowest weight selected by the applicant at which
compliance with each structural loading condition and each applicable flight
requirement of JAR/FAR Part 25 is shown.
Usually, the gusts and turbulence loads are among the criteria considered to
determine that minimum structural weight.
4. ENVIRONMENTAL ENVELOPE
“JAR/FAR 25.1527
The extremes of the ambient air temperature and operating altitude for which
operation is allowed, as limited by flight, structural, powerplant, functional, or
equipment characteristics, must be established.”
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Getting to Grips with Aircraft Performance AIRCRAFT LIMITATIONS
5. ENGINE LIMITATIONS
The main cause of engine limitations is due to the Exhaust Gas Temperature
(EGT) limit (Figure B10).
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AIRCRAFT LIMITATIONS Getting to Grips with Aircraft Performance
- The TakeOff (TOGA) thrust represents the maximum thrust available for
takeoff. It is certified for a maximum time of 10 minutes, in case of engine failure at
takeoff, or 5 minutes with all engines operative.
- The Go Around (TOGA) thrust is the maximum thrust available for go-
around. The time limits are the same as for takeoff.
- The Maximum Continuous Thrust (MCT) is the maximum thrust that can be
used unlimitedly in flight. It must be selected in case of engine failure, when TOGA
thrust is no longer allowed due to time limitation.
- The Climb (CL) thrust represents the maximum thrust available during the
climb phase to the cruise flight level. Note that the maximum climb thrust is greater
than the maximum cruise thrust available during the cruise phase.
Figure B11 shows the influence of pressure altitude and outside air
temperature on the maximum takeoff thrust, for a given engine type.
Tref
Thrust
(Tref depends on engine type)
(daN) Tref (PA = 0)
23000
22000
21000
20000
19000
PA = 0 ft
18000
PA = 2000 ft
17000
PA = 8000 ft
16000
OAT (°C)
15000
-10 -5 0 5 10 15 20 25 30 35 40
Figure B11: TOGA thrust versus OAT and PA for a given engine type
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