1
A Process for Real-Time Performance Monitoring of a Turboshaft
Engine
Eric Bechhoefer
1
, and Fatemeh Hajimohammadali
2
1
GPMS International Inc., Waterbury, VT
eric@gpms-vt.com
2
Department of Energy, Systems, Territory and Constructions Engineering
University of Pisa, Pisa, Tuscany, Italy
fatemeh.hajimohammadali@phd.unipi.it
ABSTRACT
For helicopters engaged in sling loads or heavy lift, there is a
need to report current turboshaft engine health (e.g., margin)
and contingency power available from the engine in real-
time. Displaying this information allows the pilot in
command of the aircraft to make more informed decisions
about the safety of continuing a mission. For engine margin,
when aircraft parameter data is recorded by a health and
usage monitoring system (HUMS) or flight data monitoring
system (FDM), this functionality allows maintainers to be
notified of the engines’ degraded performance to initiate an
inspection/maintenance action to restore the engine to its
designed performance. However, this does not help the pilot
make mission-critical decisions during the flight. The paper
covers the method to use HUMS/FDM data to calculate, in
real-time, the power available to the pilot.
1. SOME BACKGROUND ON HELICOPTERS AND
TURBOSHAFT ENGINES
Turboshaft engines are ubiquitous in aerospace applications
where high power and reliability are needed in a low-weight
package. Most helicopters incorporate turboshaft engines. All
turboshaft-equipped aircraft have power assurance checks to
ensure the engine can achieve the minimum specification for
power. However, these checks seldom are automatically
collected, nor do they provide information during an actual
mission (flight) to indicate current engine health or power
available. In many cases, the check looks at the engines’
measured gas temperature (MGT) vs. an idealized MGT
model to determine if the engine is operating properly.
Engines degrade over time, and assessing when maintenance
is required is essential for the safe and efficient operation of
the aircraft. For many operational missions, knowing the
current engine performance will allow the pilot in command
to make a go/no-go decision about continuing with the
mission.
Turboshaft engines are for their weight and power and are
remarkably reliable. For example, the M250C47B engine on
the Bell 407 aircraft (from which this data was measured as
part of a Health and Usage Monitoring System – HUMS),
weighing a mere 273 lbs., can provide a continuous 804
horsepower (HP) of power. The engine has an overhaul
period on the turbine of 2000 hours, while the compressor
and gearbox are essentially on condition.
As noted, helicopters perform periodic tests to ensure the
engine, compared to a nominal healthy condition, is operating
at its design specification. For example, for the Bell 407, the
flight manual (BHT-407-FM-3, 2018) states that
periodically, power assurance checks need to be performed
and that if the measured MGT is greater than or equal to some
nominal temperature value, then maintenance is required. In
this case, the modeled MGT is a function of pressure altitude
(PA), outside air temperature (OAT), and the measured
torque. However, this check does not determine the power
available, e.g., torque, in real-time.
In the case of power assurance checks, which compare the
operational MGT to the notional/modeled MGT. This is a
go/no-go criterion. That is, if the HIT check MGT is greater
than the notional MGT, maintenance is performed. Often, to
make this trendable over time, an engine factor is calculated.
𝑒𝑛𝑔𝑖𝑛𝑒%𝑓𝑎𝑐𝑡𝑜𝑟 =
!"#$%&"#$
!"#$
𝑥%100 (1)
Where nMGT is the notional MGT.
A positive temperature factor indicates the engine is
operating as designed. Often, an engine fresh out of rework
will have a 5 to 7 percent positive engine factor. Other power
assurance checks for different engines may compare engine
performance metrics such as torque or expected compressor
RPM (Ng). In any case, the ratio of the difference of
measured to expected performance indicator can be
Eric Bechhoefer et al. This is an open-access article distributed under the
terms of the Creative Commons Attribution 3.0 United States License,
which permits unrestricted use, distribution, and reproduction in any
medium, provided the original author and source are credited.
