ETSO-C57a - EASA - Europa

Deviations requests #52 for an ETSO approval for CS-ETSO applicable to
Headsets, Speakers and Aircraft Microphones (ETSO-C57a, C58a)
Consultation Paper
Introductory note
The hereby presented deviation requests shall be subject to public consultation, in
accordance with EASA Management Board Decision No 7-2004 1 products certification
procedure dated 30 March 2004, Article 3 (2.) of which states:
“2. Deviations from the applicable airworthiness codes, environmental protection
certification specifications and/or acceptable means of compliance with Part 21, as well
as important special conditions and equivalent safety findings, shall be submitted to the
panel of experts and be subject to a public consultation of at least 3 weeks, except if they
have been previously agreed and published in the Official Publication of the Agency. The
final decision shall be published in the Official Publication of the Agency.”
ETSO-C57a – Headsets and Speakers
ETSO-C57a#7 – Headsets and Speakers
Deviate from RTCA DO-214 2.3.3.1 and provide a higher Sound Pressure Level (SPL) of
90 -5/+25 dB instead of 90 +/- 5 dB SPL (re: 20 µPa) out of 1 mW (RMS) input signal.
Industry:
It has been accepted by EASA as deviation to ETSO-C58a using RTCA DO-214 as the
Minimum Operational Performance Standard (Deviation publication ETSO-C58a#1).
To allow better harmonisation with the used side tone adjustments industry has received
demands from operators to increase the output sound pressure to values above 95 dB
(re: 20 µPa) when providing 1 mW (RMS) to the headset or handset. Today technology
allows providing higher sound pressure levels out of 1 mW (RMS) without producing
distortions.
A volume control is allowed to meet that requirement. Not in all cases a volume control at
the headset or handset is practicable and there is demand designing headsets or
handsets without internal volume control function. A volume control is provided at the
intercom control unit in most cases. Adding resistors would be requested to meet the
requirement by artificially reducing the efficiency of the unit. The user does not accept
this fixed reduction as he considers the headset / handset to be muted.
The user likes a system providing more loudness without increasing the amplifier
adjustments. This can be reached by providing greater SPL values than 95 dB (re: 20
µPa) per 1 mW of input signal. Instead of requesting an internal (adjustable) volume
control it should be allowed to use a higher range for the Sound Pressure Level
produced out of 1 mW (RMS). The original requirement does not specify a dedicated
maximum value as a volume control may be used. The original requirement is already
ambiguous.
1 Cf. EASA Web: http://www.easa.europa.eu/ws_prod/g/doc/About_EASA/Manag_Board/2004/mb_decision_0704.pdf
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The user may use the loudness control function provided in the intercom or in case of the
handset by using a greater distance between handset and ear to receive the same
loudness as previously. The main advantage is the higher efficiency of the headset /
handset.
In consequence the system is able to provide a higher maximum SPL level at maximum
input. To avoid damage to the ear, a limitation to maximum 125 dBA is implemented in
the headset / handset, which works independent from the provided input power.
Equivalent Level of Safety is provided by delivering more loudness in reference to the
same input signal and by limiting the absolute generated pressure. The generated sound
levels are equivalent to those generated by a system having an integrated volume
control.
EASA
We accept the deviation provided sufficient protection against the hearing damage risk is
provided. Substantiation is requested from each applicant for this deviation in respect to
the means in place limiting the hearing damage risk.
ETSO-C57a#8 – Headsets and Speakers
Deviate from RTCA DO-214 2.3.2.1 and measure the Distortion criteria using an input
signal of 10 mW (RMS) generating a sound pressure level of at least 110 dB (re: 20 µPa)
instead of measuring the Distortion with a sound pressure level of 110 dB (re: 20 µPa)
Industry:
It has been accepted by EASA as deviation to ETSO-C58a using RTCA DO-214 as the
Minimum Operational Performance Standard (Deviation publication ETSO-C58a#1).
When using deviation ETSO-C57a#7 to RTCA DO-214 2.3.3.1 and designing a more
effective headset being able providing a higher Sound Pressure Level (SPL) of more
than 90 +/- 5 dB SPL (re: 20 µPa) out of 1 mW (RMS) input signal the Distortion test
criteria has to be adopted as well.
In general the distortion test is performed with 10 mW (RMS) input generating the
requested 110 dB (re: 20 µPA) sound pressure level. This is an operation under typical
load conditions. In case the system is more effective and able to generate higher sound
pressure levels than requested out of 1 mW the distortion measurements shall be
performed at 10 mW input signal as well generating higher sound pressure levels than
required. The reason is that the unit is operated under the same load conditions – 10
mW input signal – when performing the distortion test.
Equivalent Level of Safety is provided by using the same input signal conditions than the
original test condition requested leading to a higher sound pressure level for the test.
Consequently the test is considered more severe.
EASA
We reject the deviation. The original test condition shall be used defining a sound
pressure level for the test which is considered to be the one used under typical
operational conditions. The use of an input power defining the test condition will not
produce comparable results, as the speaker devices may have differences in their
efficiency. If designing the efficiency to the extreme the generated sound pressure level
for this test could become already dangerous for the ear. Further, it is not defined in the
original requirement if the input level is increased or the potential available volume
ETSO.DevP.52 2/4

control is adjusted to reach the request sound pressure level. The potential available
volume control could be set to a value producing less sound pressure level as required.
Aim is to have a limited distortion over the frequency at a typical volume defined by the
sound pressure level.
ETSO-C58a – Aircraft Microphones (Except Carbon)
ETSO-C58a#5 – Aircraft Microphones (Except Carbon)
Deviate from RTCA DO-214 Test procedures as detailed in 2.6.2.1, 2.6.2.3, and 2.6.2.5
and use a supply voltage of 12 to 16 VDC instead of 8 to 16 VDC for handsets.
Industry:
It has been accepted by EASA as deviation to ETSO-C58a using RTCA DO-214 as the
Minimum Operational Performance Standard (Deviation publication ETSO-C58a#1). DO-
214 provides in section 2.6 test procedures for aircraft microphones. The supply voltage
for the microphone is provided using a dedicated electrical interface as the audio signal
and the DC power supply share one line.
Instead of 8 to 16 VDC a supply voltage of 12 to 16 VDC shall be used for testing.
The limitation in respect to the low voltage supply allows the use of more complex
internal designs for the additional specific functionality integrated into the handset.
2.6.1.e already allows using different test procedures and equipment in case equivalent
information can be gathered.
It has to be understood when assessing the deviation request that the handset is used
for intercom functions within the aircraft cabin - the communication between cabin crew,
cockpit crew including the possibilities for announcements to the passengers. The
system is not used for radio communication and especially not for radio communication
in an emergency mode having potentially reduced intercom functionality - during the
emergency mode the cockpit microphone may directly be connected to the radio
bypassing the intercom. Installation instruction ensure that the handset is always
connected to an intercom providing sufficient supply voltage on the microphone line
ensuring proper operation of the unit under defined conditions.
This deviation has been accepted in the past for headsets and no occurrence report
indicates a problem with the modified supply voltage for the handset microphone.
Further it has to be considered that this requirement is provided only in the test
procedure section and not in the requirement section. The test procedures reflect a
technology at time of standard development - 1993. As the technology may change the
test setup may need to be adopted as well. We consider that this is the case for the
handset as the current technology provides more functionality like a counterpart selection
function and display backlight compared to the original pure microphone function. As all
other microphone performance is kept and modern technology is able providing the
needed supply voltage in the aircraft we consider that an equal level of safety is provided
as long as the requested supply voltage requirements are clearly stated in the installation
manual and allow for some degradation due to cable length.
EASA
We accept the deviation provided clear installation instructions are provided.
ETSO.DevP.52 3/4

ETSO-C58a#6 – Aircraft Microphones (Except Carbon)
Deviate from RTCA DO-214 2.2.9 and allow a lower Push to Talk switch current of 0.5 A
instead of 1A and a higher switch contact resistance of 400 mOhm instead of 50 mOhm
for Handsets.
Industry:
It has been accepted by EASA as deviation to ETSO-C58a using RTCA DO-214 as the
Minimum Operational Performance Standard (Deviation publication ETSO-C58a#1). The
Push to Talk (PTT) switch requirements are defined in 2.2.9.
The communication systems installed in modern aircraft have significantly lower PTT
currents than 1A and higher contact resistors are acceptable to the systems. To allow a
smaller size of the contact the PTT current has been limited to 0.5 A and a higher switch
resistor of 400 mOhm can be accepted.
The equal level of safety is ensured by highlighting the interface definition in the
installation instruction. The installer has to take care anyhow, that the PTT switch and the
PTT line requirements are matching. The handset is a fixed installed device and has less
possibilities of interchanging than a mobile PTT switch e.g. integrated into a headset or
microphone. Further the communication is performed mainly inside the aircraft and no
operation of radios is expected to be performed by the handset. Consequently the
interface from the handset is provided only to the intercom system and not directly to a
radio.
The increase in switch resistor by maximum 350 mOhm equals to additional 11 meter
cable length considering the use of AWG 20 wire having a typical resistor of 33 mOhm
per meter. This shows that the increased resistor is well within the conditions the
designer for an intercom is facing anyhow due to cable resistance.
EASA:
EASA is accepting the change of the interface specification for the PTT switch under the
following conditions:
A clear limitation is provided in the Declaration of Design and Performance and
the installation instruction, which limit the connection to compatible systems only.
The interface concerns a fixed installed unit like the handset and not mobile
equipment like headsets or hand-microphones.
The unit is not used for the emergency operation of radios.
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