TM 1-1500-204-23-1 (8) Mixing units.  Mixing units combine four motions  from  the  cockpit  (pitch,  roll,  yaw,  and  thrust) and    convert    them    into    two    outputs.        Inspect    and maintain in accordance with the applicable maintenance manual. (9) Drive chains.  Replace defective links in chains    and    maintain    in    accordance    with    applicable maintenance manual. (10) Speed rigs.  Speed rigs are fitted in the cable  assembly  for  the  purpose  of  making  rapid  minor adjustments. 9-13.  Environmental Control Systems.   nvironmental control  systems  heat,  cool,  and  remove  moisture  from the air supplied to the crew or passenger compartments. a. Types.    The    forced    air    heating    system, combustion heater, and pressurized system are used by Army   aircraft.      Each   is   explained   in   the   following paragraphs. (1) Forced air heating system.  Heated air is  provided  for  crew  comfort,  deicing,  defrosting,  and anti-icing  Air  is  forced  into  muffs  or  jackets  installed around  the  tailpipes  The  air  routed  through  the  muff  or jacket  picks  up  heat  by  convection  through  the  tailpipe material.    The  heated  air  is  then  routed  to  an  air-to-air heat   exchanger   before   going   on   to   perform   various heating functions.  Maintenance to this type of system is limited, and will consist of the following: WARNING Drycleaning    solvent    is    flammable and  solvent  vapors  are  toxic  Use  P- D-680,   Type   II   Solvent   in   a   well- ventilated area Keep away from open flames.        Avoid    prolonged    solvent contact with skin. (a) Wash all metal parts of heating system  in  drycleaning  solvent,  Federal  Specification  P- D-680.   Wash   flexible   tubing   with   mild   soap,   Federal Specification P-S-620, and water. (b) Lubricate valve cranks and push-pull  levers  with  a  small  amount  of  molybdenum- disulfide technical powder, MIL-M-7866. (c) Pressure-test   any   repair   made to  heat  exchanger  tube  before  installation.  A  repaired tube  is  considered  a  temporary  measure  and  will  be replaced when parts become available. NOTE • Heaters  with  an  engine  exhaust heat source require an operational test for carbon monoxide levels upon installation     of     the     heater     or following extensive heat exchanger maintenance, modification, or suspected malfunction. • The test will be performed at heat duct opening with the heater full on  and  the  engines  operating  at full military power.  An indication of 50 parts per million (ppm/0.005 percent) or greater requires corrective  action  prior  to  heater use.    Suggested  test  equipment is Detector Kit, Carbon Monoxide, Colorimetric, NSN 6665-00-618-1482    and    Indicator Tubes,   Carbon   Monoxide,   NSN 6665-00-276-7545. (2) Combustion heater. The combustion  heater  provides  heat  by  routing  outside  air past a burner section.  The outside air picks up heat by convection from the burner section and is then routed to perform   various   heating   functions.      Maintenance   of heater    between    overhauls    will    normally    consist    of inspection,     cleaning,     or     replacement     of     parts     as explained in the following procedures: WARNING Drycleaning    solvent    is    flammable and solvent vapors are toxic.  Use P- D-680,   Type   II   Solvent   in   a   well- ventilated    area.    Keep    away    from open     flames.          Avoid     prolonged solvent contact with skin. CAUTION Do  not  use  a  metal  instrument,  as  it may damage core passage. (a) Clean    core    tip    and    body    of spray    nozzle    with        drycleaning        solvent,        Federal Specification P-D-680. Use a wooden stick to scrape off any dirt remaining on spray nozzle. 9-29