Midcontinent MD41-244 Technical Document

INSTALLATION MANUAL AND
OPERATING INSTRUCTIONS
MD41-244/248 RELAY UNIT
MD41-244 14vdc 24 Pole, 14 volt operation
MD41-248 28vdc 24 pole, 28 volt operation
Mid-Continent Instruments and Avionics Manual Number 7019029
9400 E. 34th Street N., Wichita, KS 67226 USA Rev. C Oct. 12, 2020
Phone [phone]  Fax [phone]

Manual Number 7019029 2 Rev. C Oct. 12, 2020
REVISION DETAIL
ECO Rev. Date Detail
1 08/22/97 Revised DO-160C section 5.0 environmental test data. Test
category C is now category B.
5566 B 03/09/11 Revised Sec. 1.1 & 1.2.4 to indicate relay pairing and interlock
failure monitoring by means of power being applied to activating
coils. Added section 2.4 Limitations to describe MOD 0 & MOD
1. Revised Fig. 3.3 to reflect MOD 1 schematic.
7283 C 10/12/20 Deleted marketing statement in Sec 1.1. Updated trademark
symbol on cover page, edited spelling, added to footer.

Manual Number 7019029 3 Rev. C Oct. 12, 2020
TABLE OF CONTENTS
SECTION 1 GENERAL DESCRIPTION
1.1 INTRODUCTION
1.2 SPECIFICATIONS, TECHNICAL
1.2.1 PHYSICAL CHARACTERISTICS
1.2.2 ENVIRONMENTAL CHARACTERISTICS
1.2.3 SPECIFICATIONS, ELECTRICAL
1.2.4 INTERFACE
1.2.5 EQUIPMENT LIMITATIONS
1.2.6 MAJOR COMPONENTS
SECTION 2 INSTALLATION CONSIDERATIONS
2.1 COOLING
2.2 EQUIPMENT LOCATION
2.3 ROUTING OF CABLES
2.4 LIMITATIONS
SECTION 3 INSTALLATION PROCEDURE
3.1 GENERAL INFORMATION
3.2 UNPACKING AND INSPECTING
3.3 MOUNTING THE MD41-244/248
3.4 INSTALLATION LIMITATIONS
SECTION 4 POST INSTALLATION CHECKOUT
4.1 PRE-INSTALLATION TEST
4.2 OPERATING INSTRUCTIONS
FIGURE N0. LIST OF ILLUSTRATIONS
3.1A, B, C OUTLINE DRAWING, MOUNTING
3.2 MATING CONNECTOR LAYOUT
3.3 SCHEMATIC, MD41-244/248
APPENDIX
ENVIRONMENTAL QUALIFICATION FORM

Manual Number 7019029 4 Rev. C Oct. 12, 2020
SECTION 1 GENERAL DESCRIPTION
1.1 INTRODUCTION
The MD41-244/248 is a bulkhead mounted remote transfer relay unit specifically designed
for switching a CDI or HSI Indicator between two aircraft radio navigation systems. The
remote switching unit is designed to be used with the companion MD41-1XXX series
Annunciation Control Unit (ACU). This pertains to GPS systems where the user desires to
share a common CDI or HSI between an existing NAV system and a GPS system.
The 24 pole unit features eight relay pairs consisting of 3 poles per relay pair. Each relay
pair can be activated individually by either voltage or ground keying or as a group by
ground keying.
Other design features include:
 Interlocking of transfer relays utilizing one pole per relay pair for failure
monitoring. (This feature provides indication that all relays have power applied to
their activating coils. It does not monitor all relay contacts or coils.)
 A special ILS override that will automatically force the MD41-244/248 to switch
to NAV mode when the NAV (VOR) receiver is tuned to an ILS frequency
(optional connection).
 An adjustable dimming regulator is included to provide dimmed voltage to an
external annunciation (750ma max).
Two versions of the MD41-244/248 series relay units are available:
 MD41-244, 24 pole relay for 14 volt operation.
 MD41-248, 24 pole relay for 28 volt operation.
1.2 SPECIFICATIONS, TECHNICAL
1.2.1 PHYSICAL CHARACTERISTICS
Mounting: Bulkhead
Width: 2.86 Inches
Height: 1.52 Inches
Depth: 2.60 Inches
Weight: 0.75 lbs.

Manual Number 7019029 5 Rev. C Oct. 12, 2020
1.2.2 ENVIRONMENTAL CHARACTERISTICS
TSO Compliance TSO C129
Applicable documents: RTCA DO-160C, DO-208
Operating temperature range: -55C to +70C
Humidity: 95% non-condensing
Altitude range: 0 to 55,000 ft.
Vibration: Cat. M and N
Operational shock: Rigid mounting, 6 G operational
15 G crash safety
1.2.3 SPECIFICATIONS, ELECTRICAL
Design All solid state
MD41-244 (14VDC) 2 amps max.
MD41-248 (28VDC) 2 amps max.
Relay contact current rating: 2 amps DC, not to exceed 30 volts
1.2.4 INTERFACE
ILS Override Receives a logic low from the NAV (VOR)
J1 Pin 16 receiver when tuned to an ILS frequency.
This will force the MD41-244/248 into NAV
mode, regardless of the NAV/GPS selection.
This connection is optional.
Dimming Output Provides an adjustable dim voltage to be used
J2 Pin 33 with a remote mounted annunciation unit.
750 ma max.
GPS annunciation Pins 16 and 17 will short together when all
interlock transfer relays are energized. This can
J2 Pins16 and 17 be used to complete the GPS annunciation circuit
to monitor that power has been applied to each
relay coil.

Manual Number 7019029 6 Rev. C Oct. 12, 2020
1.2.5 EQUIPMENT LIMITATIONS
The conditions and tests required for TSO approval of this article are minimum
performance standards. It is the responsibility of those desiring to install this article either
on or within a specific type or class of aircraft to determine that the aircraft installation
conditions are within the TSO standards. The article may be installed only if further
evaluation by the applicant documents an acceptable installation and is approved by the
Administrator.
The MD41-244/248 relay unit MUST be installed with a TSO approved MD41-1XXX series
Annunciation Control Unit (ACU) in order to be approved as a complete TSO system. These
items will not be TSO’d if one is installed without the other.
NOTE: Any time the MD41-244/248 relay transfer unit is disconnected or removed from the
aircraft, the HSI/CDI will be inoperative in both NAV (VOR) and GPS modes.
1.2.6 MAJOR COMPONENTS
The system is comprised of two major components, the MD41-244/248 relay unit and the
MD41-1XXX series annunciation control unit.

Manual Number 7019029 7 Rev. C Oct. 12, 2020
SECTION 2 INSTALLATION CONSIDERATIONS
2.1 COOLING
No direct cooling is required. As with any electronic equipment, overall reliability may be
increased if the MD41-244/248 is not located near any high heat source or crowded next to
other equipment.
2.2 EQUIPMENT LOCATION
The MD41-244/248 is provided with mounting rails that will allow mounting on side, rear or
flat side down. Rails for end mount are an optional item. Order p/n 8011991 (2 required).
Locate an area large enough to facilitate mounting of unit with connector and back shell in
place. Ensure area is clear of mechanical obstructions and is not impeding movement of any
other aircraft systems. Note: Unlike previous versions of the MD41 Annunciation Control
Units (ACU), the transfer relays have been removed and are now remotely mounted in a
separate package designated as the MD41-244/248 Relay Unit. This has allowed for a smaller
size ACU which now provides more options for panel mounting.
2.3 ROUTING OF CABLES
Care must be taken not to bundle the MD41-244/248 logic and low level signal lines with any
high energy sources. Examples of these sources include 400 HZ AC, Comm, DME, HF and
transponder transmitter coax. Always use shielded wire when shown on the installation print.
Avoid sharp bends in cabling and routing near aircraft control cables.
2.4 LIMITATIONS
The MOD 0 relay unit interlock functionality has the capability of monitoring 1 contact per
relay to indicate power has been applied to the activating coil.
The MOD 1 relay unit interlock functionality has the capability of monitoring 1 contact per
relay pair to indicate power has been applied to the pair of activating coils.
In either MOD 0 or MOD 1 there is no guarantee of individual relay contact for the interlock
functionality. This is only an indicator that relays have power applied to their activating coils.
Refer to Fig. 3.3 for MOD 1 schematic diagram.

Manual Number 7019029 8 Rev. C Oct. 12, 2020
SECTION 3 INSTALLATION PROCEDURES
3.1 GENERAL INFORMATION
This section contains interconnect diagrams, mounting dimensions and other information
pertaining to the installation of the MD41-244/248. After installation of cabling and before
installation of the equipment, ensure that power is applied only to the pins specified in the
interconnect diagram.
3.2 UNPACKING AND INSPECTING EQUIPMENT
When unpacking equipment, make a visual inspection for evidence of damage incurred during
shipment. The following parts should be included:
1. MD41-244 (14volt, 24 pole)
MD41-248 (28volt, 24 pole)
2. J1 Connector Kit (50 pin, sockets) MCIA PN 7014509
3. J2 Connector Kit (50 pin, pins) MCIA PN 8012247
4. Vertical and horizontal mounting brackets MCIA PN 7018658 (2 required)
5. Installation Manual (this document) MCIA PN 7019029
Optional parts:
Mounting brackets for rear mount. MCIA PN 8012379 (2 required)
3.3 MOUNTING THE MD41-244/248
The MD41-244/248 is provided with mounting rails that will allow mounting on the side, rear
or with flat side down. Locate an area large enough to facilitate mounting of unit with
connector and back shell in place. Ensure area is clear of mechanical obstructions and is not
impeding movement of any other aircraft systems. Avoid mounting close to heater vents or
other high heat sources. Allow a clearance of at least 3 inches from front of unit for plug
removal.
3.4 INSTALLATION LIMITATIONS
Wire the aircraft harness according to figure 3-3. Use at least 24 AWG wire for all
connections. You MUST use shielded wire where shown. Avoid sharp bends and routing
cable near high energy sources. Care must be taken to tie the harness away from aircraft
controls and cables. Normal installation techniques should be applied. Also see equipment
limitations, Section 1.2.5.

Manual Number 7019029 9 Rev. C Oct. 12, 2020
FIGURE 3-1A OUTLINE DRAWING
VERTICAL MOUNT

Manual Number 7019029 10 Rev. C Oct. 12, 2020
FIGURE 3-1B OUTLINE DRAWING
HORIZONTAL MOUNT