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Jet engine thrust ratings.
Article · September 2012
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Nihad E. Daidzic
AAR Aerospace Consulting, LLC
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TURBOFAN TECHNOLOGY
Embraer Legacy 600 operated by TAG Aviation takes off from GVA (Geneva, Switzerland).
Takeoff thrust is the highest thrust that the engine will deliver and is typically limited to 5 min.
Pilots must adhere to approved thrust or power ratings
for jet engines to work reliably and as advertised.
ATP, CFI-IA, MEI, CFI-G, AGI, IGI.
T
Certified ratings
The only 2 thrust ratings that airplane jet engines are certified for are
maximum takeoff (MTO) and maximum continuous thrust (MCT). All
108 PROFESSIONAL PILOT / September 2012
structural and performance limiting
weights—field length limited takeoff
weight (FLTOW), climb limit (WAT),
obstacle clearance, obstacle and
drift-down cruise, and landing limitations based on maximum landing
weight (MLW) and landing/ap -
Design limit
Bump rating
Thrust
Part
throttle
Flat Rating
EPR
Fn
it
e lim
ttl e
ro ur
th at
ll er
Fu mp
Te
hrust is the most important
engine operational parameter
for pilots. The installed static
takeoff thrust is often the maximum
rated thrust that, for example, a
modern high-bypass turbofan will
generate during normal operations.
Thrust limits are set by the maximum combustor/burner inlet static
pressure (p b or p s3), turbine inlet
temperature (TIT or Tt4) and maximum spool rpm. Bump and design
thrust ratings/limits are typically not
available in normal operations. The
airplane forward motion will effectively reduce installed gross thrust
due to momentum drag—especially
so in modern high-bypass turbofans, where most of the propulsive
force comes from the cold bypass
air. The inlet RAM effect becomes
noticeable only at much higher
TAS, but it is not so significant for
subsonic flight.
other thrust ratings—such as
(ground/flight) idle, climb and cruise
thrust—are only figures recommended or set by the engine manufacturer. The maximum allowable
airplane takeoff weight will be
determined by the most restrictive
%N1 corr
EPR
%N1
N1
N1c
ISA+15°C
By Nihad Daidzic
-50°C
Thrust break
Temperature TBT
Assumed
Temperature
OAT (°C)
Flat takeoff rating for a typical modern turbofan. The combustor inlet pressure (CIP) limits thrust
at low temperatures while TIT limits thrust at high outside air temperature (OAT) or total air temperature (TAT). Note that diagram is not to scale.
Photo by Jack Sykes
Jet engine thrust ratings
1/2
TOW µ( TODA x Fn x Sref x CL,tkoff x d x OATref )
Max rated takeoff thrust
x OAT–1/2
Part throttle
flat rating
TOW µ1/√OAT
TIT/ITT limit
TOW
Reduced thrust
Max assumed
temperature
TOW µ√Fn /OAT
e
ttl
ro
th it
ll m
Fu IT-li
T
25% thrust reduction
Min TOW
Assumed
temperature
TOW
ISA+15°C
Thrust Fn (lbf or kN or daN) or
Weight/mass (lb or kg)
Max TOW
–50°C
OAT (°C)
PA-corrected takeoff thrust and TOW versus OAT. Note that a similar chart could be constructed
for WAT-limit TOW if the flap settings were included on the vertical axis.
proach climb using maximum available certified takeoff thrust. Typ ically, MTO thrust is limited to 5
min, although some manufacturers
allow for an optional 10-min limit
and automatic power/thrust reserve
when one engine inoperative (OEI).
Helicopter turboshaft engines
have more certified power/torque
ratings. For example, the GE CT7-8
FADEC-equipped turboshaft, certified by FAA in 2004, powers medium-lift helicopters such as the
AgustaWestland EH101, NHIndustries NH90, Sikorsky S92 and MH60K. Maximum SL/ISA all engines
operating (AEO) takeoff power is
2634 hp, with specific fuel consumption (SFC) of 0.452 lb/shp-hr.
Thirty-minute maximum power is
2544 shp and maximum continuous
is 2157 shp. For OEI operations, the
FAA power ratings are 2769 shp for
30 sec, 2606 shp for 2 min, and
continuous OEI power of 2544 shp,
or almost 18% higher than for AEO
max continuous power per engine.
The flat-rated MTO thrust, engine
pressure ratio (EPR) and N 1 for a
typical turbofan engine are illustrated in the chart on p 01. Since inflight thrust measurement is unreliable, a thrust setting/rating parameter (TSP/TRP) such as EPR is often
used. EPR is the ratio of the exit (station 5, 7 or 9) and inlet (station 2)
total (or stagnation) pressures. Highbypass turbofans have relatively low
EPRs. Therefore, fan-speed N1 and
N1-corrected (N1,corr = N1 /√θ) is
2.40
2.30
EPR(-)
2.25
+10,000 ft and above
+3,000 ft
+2,000 ft
2.4
P&WJT8D-17
Eng 1&3/B727-200
Max takeoff—5 min
AC bleed ON
3,000 ft
2.2
+1,000 ft
SL ft
2.1
-1,000 ft
2.0
2.10
2.05
1.9
2.00
1.8
1,000 ft
SL ft
1.95
1.90
-50
P&WJT8D-17
35,000 ft Eng 1&3/B727-200
Max continuous
AC bleed ON
20,000 ft
2.3
2.20
2.15
10,000 ft
EPR(-)
2.35
often used as TSP instead (θ = OAT/
OATref).
While EPR is directly proportional
to inflight thrust for any OAT, the N1
increases for constant flat-rated
thrust until reaching a thrust break
temperature (TBT) of ISA +15°C and
then decreases again with increasing OAT to comply with the TIT limitation. However, the temperaturecorrected fan-speed N1,corr is flatrated indeed. The reason N 1 de creas es with colder OAT to meet
constant EPR/thrust is simply
because less “pumping” is needed
from the fan/compressor to deliver
the required constant air mass flow
rate, which is otherwise augmented
at higher densities (lower temperatures). The mass flow rate for constant calibrated airspeed (CAS) will
increase as a square root of dimensionless density-ratio (σ = ρ/ρref).
Although thrust/EPR stays constant
for any OAT between, say, –50°C
and TBT, the maximum airplane
TOW will increase slowly for constant takeoff distance available
(TODA) with lower OATs, as shown
in the upper chart on this page. The
reason is that, for constant TAS,
dynamic air pressure will increase
with lower OAT, which will be
“understood” by the wing as more
lift potential. So, although takeoff
IAS/CAS speeds will be greater at
higher TOWs, the TODR = TODA
will not change. Once in full-throttle TIT-limit range, TOWs decrease
steeper as thrust itself is also decreasing with increasing OATs. All
PA-dependent thrust can practically
merge into a single curve, or PAscaled corrected-thrust (F n,corr =
Fn/δ, where δ = P/Pref).
Temperature TAT (°C)
-30
-10
10
Temperature OAT (°C)
30
50
1.7
-60
-40
-20
0
20
40
60
Maximum takeoff and maximum continuous EPRs as a function of OAT (or TAT) and PA for a PW JT8D-17 turbofan. The exact EPR setting is dependent
on engine/airframe integration.
PROFESSIONAL PILOT / September 2012 109
106
105
+9,000 ft
103
104
101
102
99
100
98
95
96
+7,000 ft
93
+5,000 ft
91
85
-50
94
+3,000 ft
89
87
N1corr (%)
N1 (%)
97
92
+1,000 ft
CFM56-3C-1
Eng 1&2/B737-300
AC Packs ON/AUTO
SL ft
90
88
-30
-10
10
30
Temperature OAT (°C)
For example, MTO and MCT
EPRs, as a function of OAT/TAT and
pressure altitude (PA) for an older
PW JT8D-17 low-bypass turbofan
are shown in the lower pair of
charts on p 02. Typically these values are provided to the crew in a
tabular AFM form, but this visual
presentation shows clearly what is
meant by flat-rating.
MTO thrust flat-rating for a
CFM56-3 turbofan is shown in the
chart above. The physical N1 speed
is not flat, but the corrected N 1
almost is. Once the TIT limit is
reached, different PA curves merge
into one, as shown in the lower
charts on p 02 and the upper chart
on this page. For example, in a
CFM56-7B26 turbofan, N 1 will
increase 1.5% for every 10°C up to
TBT = ISA+15°C, and then steadily
decrease by about 1.5% for
every 10°C up to maximum
95
operating temperatures (50–
54°C).
90
Turboprop engines are also
85
flat-rated in power/torque. For
example, MTO torque (AEO),
80
expressed as a percentage, for
the Pratt & Whitney Canada
75
PW121 turboprop, which
powers the successful ATR 4270
500, among others, is shown
in the chart on the right.
65
If the thrust available is
greater than required to meet
takeoff run available (TORA)
and TODA limitations, it is
70
recommended to use reduced takeoff thrust. Reducing takeoff thrust is
good operating practice. It also prolongs engine life significantly and
lowers maintenance and operational cost. For example, a Boeing
747-400’s PW4056 engines will
experience about 60°C lower EGTs
(T5) for a 25% takeoff thrust reduction. This may not seem much, but
at already high EGTs (about 780 K
or 500°C) where turbine blade
material lifetime decreases exponentially with higher temperatures,
such a 7% temperature reduction
may result in a decrease in maintenance material cost of about 25%.
For example, the PW4056 transpiration-cooled HP turbine blades may
be dealing with TIT of 1300°C
(2350°F) or more. There is a direct
correlation between the hot-section
lifetime and the number of times the
particular TIT/ITT/EGT has been
exceeded.
Although seemingly the same,
important differences exist between
derated takeoff (D-TO) and reduced
takeoff (R-TO) thrust. For example,
FAA’s AC 25-13 defines procedures
for D-TO and R-TO thrust. JAR AMJ
25-13 does the same thing for operations under jurisdiction of the
European JAA. Typically, thrust cannot be reduced below 75% of maximum, although, for example, up to
40% of max thrust reduction is possible for the Boeing 777-300ER/
200LR/200F using GE90 engines.
Derated thrust method
Derated thrust can be fixed (set by
manufacturer) or variable (changed
by operator) and is selected
through the FMS or by using
the thrust management con-1,000 ft
trol panel (TMCP). Derated
SL ft
takeoff thrust (eg, D-TO1, D+1,000 ft
TO2) now becomes TSP/TRP
PA= +20,000 ft
operational limitation (N1 or
+3,000 ft
EPR)
for which separate and
+5,000 ft
clearly
distinguishable takeoff
+7,000 ft
data must exist in the AFM.
Thrust should not be
increased beyond derate durP&W Canada 121
ing takeoff except in dire
Eng 1&2/ATR 42-500
emergency and/or imminent
Np=100%@CAS≤50kt
ground contact. Accordingly,
60
-50
-30
-10
10
30
50
airplane control could be
jeopardized because V mcg
Temperature SAT (°C)
will increase when increasing
MTO torque for the PW121 free-turbine turboprop, expressed as a asymmetric thrust. Although
seemingly paradoxical, derate
percentage.
Takeooff torque (%)
To reduce or to derate?
50
Actual (solid) and corrected-N1 (dashed) MTO
fan speed for a GE/SNECMA CFM 56-3 (-B1/
B2/C1) engine as installed on the venerable
Boeing 737-300.
110 PROFESSIONAL PILOT / September 2012
TO
Static thrust
Turbojet
Thrust
CLB
CLB1
–50°C
ISA + 15°C
CLB2
Reduced takeoff
thrust required
OAT (°C)
Assumed temperature method
The assumed temperature method
(ATM) for setting takeoff thrust (RTO or FLEX TO) provides more flexibility and does not in itself constitute an operating limitation. The
High-bypass
turbofan
0.7
Fn= Fn,SL,ISA . d0.7 .(1.0 – 5.0 x 10–4 x TAS + 4.0 x 10–7 x TAS2)
θ
AT2 AT1 AT
Maximum and derated takeoff and climb thrust. Diagram is not to
scale.
thrust can actually increase TOW
on short runways. Short runways
will require low V 1 to meet the
accelerate-stop distance available
(ASDA) limitation. Low V 1 will
require lower Vmcg which may not
be possible with full-rated asymmetric thrust thus prohibiting takeoff at
a given TOW. Using derate thrust
will result in lower V mcg and V 1
making takeoff possible. With wingpod engines, derate thrust will also
require more nose-up trim. An illustration of fixed takeoff and climb
derates is shown in the left hand
chart above. The issue with fixed
derates is that excess thrust above
that required could still exist.
For example, a CFM56-7B26
engine, which powers the Boeing
737-800 at a full rating of 26,000
lbs (27,000 lb bump rating), can be
derated to 24,000 or 22,000 lbs.
The older CFM56-3C-1 was produced as a common engine for all
737 “classics” (737-300/400/500) at
discrete max thrust ratings from
18,500–23,500 lbs set by a programming data plug. The PW4000
series of turbofans also employ a
programming EEC/FADEC data plug
to set thrust ratings.
In addition, AC 25-13 requires
that operators perform periodic
checks to ensure that the engines
are capable of producing full takeoff
thrust without exceeding any engine
operating limitations.
Low-bypass
turbofan
Thrust Fn (lbf or kN)
D-TO1
D-TO2
TAS (kts)
Approximate thrust dependence on TAS for pure turbojet, low and
high-bypass turbofan.
idea behind ATM is simple—ignore
the actual TAT-probe measurement
and enforce a fictitious higher
(assumed) air temperature (AT)
instead. This will “trick” the FADEC
into thinking the air is warmer than
it really is, scheduling less fuel to
generate less thrust.
This works only if AT > TBT, as the
FADEC will otherwise increase N1
to compensate for apparent higher
OAT up to TBT. However, what ATM
cannot account for is that existing
air density is really higher because
the real OAT is lower than the AT.
Therefore, airspeed entering the
engine diffuser will be lower than it
would be in the case of actually
higher OAT. Accordingly, reduced
thrust produced using ATM at an
actually lower OAT is higher than if
the actual OATs were elevated and
equal to the AT. So the assumed
temperature method is conservative
in the sense that it provides thrust
margins compared to operating in
actually higher OAT.
This fact is often misused and misunderstood by those who say that
ATM creates “additional margins.”
In fact, this is not possible. Any
thrust reduction brought about by
using an AT higher than actual OAT
and, say, TBT = ISA+15°C, will
result in longer TORR, TODR and
ASDR. Operators desire lower TIT,
which will result in longer TODR (≤
TODA) at a given TOW. Reduced
thrust using ATM can also be combined with derates, provided that no
more than 25% overall thrust reduction exists, as shown in the left hand
chart above.
To illustrate the theory, let us
assume that the actual OAT is 15°C
at sea level and that our fictitious
twin-engine airplane TOW is
340,000 lbs (MSTOW = 400,000
lbs). Let’s assume performance limiting takeoff weight to be 380,000
lbs at OAT = 15°C and given flap
setting. Since our intention is not to
increase TOW (although we could),
we can use R-TO thrust for all the
good reasons we discussed before.
Say that from the runway analysis
chart we find FLTOW of 340,000 lbs
at an actual OAT of 40°C. We could
now just set AT of 40°C in the
TMCP/FMS.
If we also assume an imaginary
high-bypass turbofan in the class of
the PW 4056, GE CF6-80C2 or RR
RB211-514G/H, our fictitious engine will generate static TO thrust of
62,500 lbs at SL up to, let’s say, ISA.
At SL ISA and an airspeed of 140
KCAS, our fictitious turbofan will
produce 58,615 lbf of thrust. If OAT
= 40°C, each engine will produce
55,156 lbf of thrust. However, if we
set AT to 40°C (for OAT = 15°C), the
thrust produced is 55,292 lbf, or
about 136 lbf more per engine than
when OAT actually is 40°C.
The average thrust reductions of
140 lbf per degree Celsius OAT and
135 lbf per degree Celsius AT are
valid for temperatures above 15°C
and at 140 KCAS. Above thrust versus TAS calculations are based on
results obtained from an RR RB211514G turbofan. (See the right hand
chart above). Using an AT lower
than actual OAT will not magically
increase TOW as we cannot “trick”
the atmosphere. (With FADEC we
can actually trick the engine!)
Normally, we need AT > TBT,
since thrust remains unchanged for
lower OATs. Reduction in thrust
comes from 2 physical principles—
PROFESSIONAL PILOT / September 2012 111
TAS as a function of OAT and PA at 140 CAS
TAS (kts)
PA/OAT
5°C (θ = 0.965)
15°C (θ = 1.000)
40°C (θ = 1.087)
137.55
140.00
145.95
5000 ft (δ = 0.8322)
150.78
153.47
159.99
15°
C
will be restricted by maximum
OATs. AT can be higher than the
environmental limit as it is only a
fictitious temperature. The chart
above shows the thrust of a typical
turbofan as a function of N1 and the
power/thrust lever angle (PLA). It is
possible to design thrust vs PLA as a
piecewise linear segmented partpower region to provide for different
sensitivities in idle, approach and
climb/cruise thrust.
Conclusions
En
vir
on
m
en
ta
l
21.00 inc Hg (9,000 ft)
Reduced thrust/TOW
–50°C
112 PROFESSIONAL PILOT / September 2012
View publication stats
Thrust Fn (lbf or kN)
PLA
Reduced-thrust takeoffs extend
engine life significantly. They also
lower operating and maintenance
costs. R-TO thrust can be used in
combination with improved-V 2
climbs. ATM and derate methods
can be combined, but total thrust
reduction cannot exceed 25% of the
maximum thrust. While not rebalancing the field length and not providing minimum
possible thrust reTBTs
duction for given
conditions, ATM is
simple. Increasing
V1 and VR simultaneously with thrust
TIT/ITT limit
reduction will rebalance the field,
which, of course,
will be longer than
re
the original BFL.
atu ust)
r
e r
A quick look at runway/takeoff
analysis performance charts for particular airports and airplane/engine
combinations will reveal limiting
TOWs at AT = OAT. Another ATM
benefit is that it doesn’t limit maximum available thrust. If experiencing engine failure and/or windshear, a pilot can simply firewall the
throttles.
Reduced-thrust takeoffs are not
permitted in the case of runway
contamination, except for approved
derates. Reduced-thrust takeoffs are
also not allowed when specific
power-management equipment is
missing or inoperative.
A company-specific aircraft AFM
should always be consulted for specific limitations on reduced thrust
use. It is possible to use correctedATM, which will exactly match the
reduced thrust to the minimum
required to meet regulatory takeoff
distance requirements. Accordingly,
payload can be increased somewhat or thrust reduced further, generating additional savings.
lim
it
23.00 inc Hg (7,000 ft)
PLA
Thrust as a function of N1 and PLA for a typical turbofan.
Part throttle
ISA + 15°C
TOW/TOM (lbm or kg)
25.00 inc Hg (5,000 ft)
N1 (%)
e
ttl
ro
th
ll
Fu
29.92 inc Hg (SL)
28.00 inc Hg (2,000 ft)
ISA
+
31.00 inc Hg (-1,000 ft)
Thrust Fn (lbf or kN)
the influence of the lower air density and the higher TAS for OAT =
40°C at constant CAS. With ATM we
are only capturing the former effect.
The 2nd effect is small at low
speeds, but nevertheless “generates”
a small amount of extra thrust. A
caution is required in calculating N1
with ATM. It is not the same N1 that
corresponds to OAT = AT. The
reduction will be calculated by
using the operating manuals simply
because air is not as rarefied as
implied. Actual air density is higher
than assumed and less compressor
pumping is required.
In the 1st approximation, we can
assume that the percentage of TOW
increase due to ATM is proportional
to excess thrust. So, in this particular
case, when AT = 40°C in 15°C air,
we can increase the weight of a
340,000-lb airplane by about 840
lbs due to 272 lbf excess thrust.
Alternatively, we could assume a
somewhat higher AT of about 41°C
(corrected-ATM) to reduce thrust to
minimum required by regulation,
which is 55,156 lbf per engine in
this particular case.
The conversion of 140 CAS to TAS
for various PAs and OATs is given in
the table at the top of this page. The
chart at the bottom of this page
shows an example of TOW as a
function of PA and OAT for a flatrated engine. Typically, the engine
Fan rotor speed N1 (%)
SL (δ = 1.000)
p th
em d
d t rate
e
m %
su 75
As it (
il m
AT
OAT (°C)
TOW as a function of
OAT. Modern jet engines
are typically flat-rated
to TBT = ISA+15°C.
Nihad Daidzic is
president of AAR
Aerospace
Consulting, located
in Saint Peter MN.
He has worked for
many years on the
US and European
space programs.
Daidzic is also a university professor of
aviation and mechanical engineering.