TM 1-6625-724-13&P
0009 00
0009 00-5
The types of measurements performed for a particular aircraft are completely determined by the aircraft
configuration file. If the configuration file does not request a particular measurement it will not be made.
The AVA has been designed to provide a flexible system to measure vibration and track for the most
sophisticated helicopters. The DAU can interface to the following sensors:
(a) 14 Accelerometers of various types
(b) 2 Magnetic or optical tachometers
(c) 2 Universal Tracking Devices (UTDs)
(d) 1 Strobe Light
The measurement and diagnostic setups for particular helicopters are controlled by the individual aircraft
configuration file. This allows easy customization of measurement, display, and diagnostics to an
airframe. The AVA has basic measurement capabilities as shown in table 1, and discussed as follows:
Table 1. Measurement Capabilities
Sensor Measurements
Resolution
Range
Sample
Vibration
Power Average (sync):
400 lines
1/8 R to 50 R
128 Spectra
Power Average (async):
400 lines
2.0 Hz to 20 kHz
128 Spectra
Time Average:
128 lines
1/4 R to 32 R
512 Revs
Zoom Displays (power only)
6400 Lines
x1 x2 x4 x8 x16 x32
Vibration Amplitude:
2%
68 dB (100 g max.)
Vibration Phase:
1°
0 to 360°
Tachometer Frequency:
2 to 485 Hz
(120 to 29100 RPM)
Track:
2 mm
512 Revs
Lag:
2 mm
512 Revs
(1) Asynchronous Sampled Power Averaged Vibration Spectrum (ASPA) (ASPA Zoom): The
conventional power spectrum provides a measurement of vibration amplitude vs. frequency.
Normally 400 spectral points are provided, but an optional zoom parameter increases the spectral
resolution to 6400 (ASPA Zoom) spectral points. The AVA can collect a single vibration channel
at a time using this mode. Table 2 provides a list of the possible frequency ranges vs. their
respective measurement resolutions. The aircraft configuration file can setup the frequency
range, number of Fast Fourier Transforms (FFTs) to average, window type, channel setup,
accelerometer type, and output type. The asynchronous vibration measurements can be
collected on any of the available fourteen channels
(2) Synchronous Sampled Power Averaged Vibration Spectrum (SSPA)(Signature Ratio): This mode
provides a power spectrum measurement with sampling, which is based on the external
tachometer frequency. This type of measurement eliminates the spectral smearing commonly
found when using a conventional asynchronous spectrum with vibration components that
constantly change frequency. Normally 400 spectral points are computed, however there is an
optional zoom (SSPA Zoom) capability that increases the resolution to 6400 spectral points. The
AVA collects this type of measurement on a single channel at a time. The external tachometer
frequency range can vary between 2.0 Hz and 485 Hz. The setup in the aircraft configuration file
determines the number of FFTs to average, window type, overlap, output type and acceptable
frequency range. This is probably the method of choice for collecting vibration data that does not
have a harmonic relationship to the tachometer source, but does have a fixed frequency ratio.
