All HEATING 1. DESCRIPTION The cabin air heating system is covered in this section. Serials 1005 thru 138: Heated air is available at floor level for all occupants. Positive pressure inside the engine cowling causes airflow from the inlet at the rear engine baffle to the engine exhaust muffler. The airflow is then ducted to the hot air valve mounted to the forward RH side of the firewall. When the hot air valve damper is closed, the heated air exits into the engine compartment and is exhausted overboard with the engine cooling airflow. When the hot air valve damper is open, heated air flows from the hot air valve into the air mixing chamber. The heated air is conditioned in the air mixing chamber by mixing it with fresh air (if desired) from the fresh air inlet on the right wing. From the air mixing chamber, the conditioned air is ducted to either the defroster or passengers. Defroster outlet holes are located in the glareshield just aft of the windshield. Forward passenger conditioned air enters the cabin through outlets in the kick panel at each position. Conditioned air for rear passengers is ducted to outlets on the sidewalls near floor level. The amount of heated air allowed into the air mixing chamber is controlled by rotating the Cabin Heat Control, located inboard of the Cabin Air Selector. The control is mechanically linked to a damper in a hot air valve between the heat exchanger and air mixing chamber. Rotating the control full counterclockwise (HEAT OFF) bypasses heated air from the heat exchanger into the engine compartment. Rotating the control clockwise opens the door in the hot air valve allowing heated air to enter the mixing chamber. The proportion of heated air to ventilation air mixed in the air mixing chamber is regulated by manipulating the Cabin Heat Control and Cabin Cold Control to obtain the desired temperature and flow. The pilot and co-pilot control the conditioning (mixing) of the heated and fresh air. Serials 139 thru 1885: Ram air from the upper RH cowl inlet flows through the upper cowl and is ducted to a heat exchanger surrounding the RH exhaust muffler. The heated air is routed to a hot air valve mounted to the forward side of the firewall. The hot air valve controls the entry of hot air into the cabin distribution system. When the hot air valve damper is open, the heated air flows into the air mixing chamber and is mixed with fresh air from the RH wing air inlet. When the hot air valve damper is closed, the heated air exits into the engine compartment and is exhausted overboard with the engine cooling airflow. Cabin heat is regulated by controlling the volume of hot air admitted into the distribution system s air mixing chamber. The proportion of heated air to fresh air is accomplished using the temperature selector mounted on the RH instrument panel. Conditioned air can be directed to passengers and/or the windshield diffuser by manipulating the cabin vent selector mounted on the RH instrument panel. The conditioned air enters the cabin through adjustable air vents located in each kick plate and through non-adjustable, floor level vents located in the rear cabin trim side panels. Conditioned air can also be distributed to the windshield diffuser in the glareshield. Serials 188 & subs: Ram air from the upper RH cowl inlet flows through the upper cowl and is ducted to a heat exchanger surrounding the RH exhaust muffler. The heated air is then routed to the air mixing chamber mounted to the forward side of the firewall. In the air mixing chamber, the heated air is mixed with fresh air from the NACA vent on the RH lower cowl. The mixed air is then distributed by either ram air or by optional blower fan to the distribution manifold mounted to the center, aft side of the firewall. The distribution manifold uses butterfly valves to control airflow to the floor and defrost vents. Cabin heat is regulated by controlling the volume of hot air admitted into the distribution system s air mixing chamber. The proportion of heated air to fresh air is accomplished using the temperature selector mounted on the RH instrument panel. Conditioned air can be directed to passengers and/or the windshield diffuser by manipulating the cabin vent selector mounted on the RH instrument panel. The conditioned air enters the cabin through adjustable air vents located in each kick plate and through non-adjustable, floor level vents located in the rear cabin trim side panels. Conditioned air can also be distributed to the windshield diffuser in the glareshield. Page 1
2. TROUBLESHOOTING Trouble Probable Cause Remedy Cabin heat inoperable. Hot air valve duct disconnected or collapsed. Control linkage misaligned or disconnected. Faulty hot air valve damper. Cabin floor duct disconnected or collapsed. Connect or replace duct. (Refer to ) Adjust and secure control linkage. (Refer to 21-0) Replace hot air valve. (Refer to ) Connect or replace duct. (Refer to 21-20) Defroster system inoperable. Cabin occupants become disoriented, tired, or nauseated. Complaints of headaches. Cabin floor vents closed, blocked, or faulty. Control linkage misaligned or disconnected. Defroster vent holes blocked. Hot air valve duct disconnected or collapsed. Exhaust gases leaking into heater muffler. Open, remove restriction, or replace cabin floor vent. (Refer to 21-20) Adjust and secure control linkage. (Refer to 21-0) Remove restriction. Connect or replace duct. (Refer to ) Perform Adjustment/Test - Heat Exchanger. (Refer to 78-20) Page 2 All
3. MAINTENANCE PRACTICES A. Hot Air Valve - Serials 1005 thru 138 (See Figure 1) (1) Removal - Hot Air Valve (a) Remove engine cowling. (Refer to 71-10) (b) Remove clamp securing duct to hot air valve. (c) Disconnect hot air valve control cable from hot air valve. (d) Remove screws and washers securing hot air valve to firewall. (e) Serials 1148 & subs: Remove gasket from hot air valve. (2) Installation - Hot Air Valve (a) Acquire necessary tools, equipment, and supplies. Description P/N or Spec. Supplier Purpose Cable Ties 8 inch Any Source Secure air duct. High Temperature Silicone Sealant RTV 73 Dow Corning Corporation Midland, MI 488-0994 989-49-4400 Provide air-tight seal between cabin and engine compartment. (b) Solvent clean hot air valve and firewall mating surfaces. (Refer to 20-30) (c) Apply sealant to back side of hot air valve. (Refer to 20-10) (d) Position gasket on back side of hot air valve. (e) Apply sealant to back side of gasket. (f) Position hot air valve to firewall and secure with screws and washers. (g) Perform Adjustment/Test - Hot Air Valve Control Cable. (Refer to 21-0) (h) Position duct to hot air valve and secure with clamp. (i) Install engine cowling. (Refer to 71-10) Serials 1005 thru 138 Page 3
B. Hot Air Valve - Serials 139 thru 1885 (See Figure 1) (1) Removal - Hot Air Valve (a) Acquire necessary tools, equipment, and supplies. Description P/N or Spec. Supplier Purpose Flashlight - Any Source Inspection. Inspection Mirror - Any Source Inspection. (b) Remove engine cowling. (Refer to 71-10) (c) Remove hose clamp securing duct to hot air valve. (d) Set temperature control knob to maximum HOT position. (e) At hot air valve, remove RTV sealant covering bolt heads. (f) Remove bolts and washers securing hot air valve and gasket to firewall. (g) Using inspection mirror and flashlight, observe orientation of linkage hook and flapper door eyelet. (h) Rotate hot air valve 90 clockwise to disengage linkage hook from flapper door eyelet. Remove hot air valve. (2) Installation - Hot Air Valve (a) Acquire necessary tools, equipment, and supplies. Description P/N or Spec. Supplier Purpose Tongue Depressor - Any Source Fixture. Clamp - Any Source Fixture. -inch Scale - Any Source Measure. Dow Corning Silicone Sealant RTV 73 Any Source Seal. (b) To facilitate hot air valve installation, secure flapper door to closed position. 1 Rotate hot air valve flapper door to closed position. 2 Position tongue depressor into hot air valve firmly against closed flapper door. 3 Using clamp, secure tongue depressor to inner surface of hot air valve. Verify flapper door is secured to closed position. (c) Position gasket to hot air valve. Orient hot air valve 90 clockwise to installation location. (d) Latch linkage hook to flapper door eyelet on hot air valve. (e) Rotate hot air valve 90 counter-clockwise to engage hook into eyelet. (f) Remove clamp securing tongue depressor to hot air valve. (g) At coupler, apply silicone sealant around inner circumference of firewall pass-through. (Refer to 20-10) (h) Position hot air valve and new gasket to firewall and secure with bolts and washers. (i) Position duct to hot air valve and secure with clamp. (j) To secure attaching hardware, apply silicone sealant to bolt heads following installation. (k) Install engine cowling. (Refer to 71-10) Page 4 Serials 139 thru 1885
C. Air Mixing Chamber - Serials 188 & subs (See Figure 2) (1) Removal - Air Mixing Chamber (a) Acquire necessary tools, equipment, and supplies. Description P/N or Spec. Supplier Purpose Flashlight - Any Source Inspection. Inspection Mirror - Any Source Inspection. (b) Remove engine cowling. (Refer to 71-10) (c) Remove clamp securing fresh air duct to air mixing chamber. (d) Remove clamp securing heat exchanger duct to air mixing chamber. (e) Remove bolt and wire stop securing temperature control cable to actuation arm on air mixing chamber. (f) Remove bolt, washers, and nut securing temperature control cable and clamp to bracket on air mixing chamber. (g) Set temperature control knob to maximum HOT position. (h) At air mixing chamber, remove RTV sealant covering bolt heads. (i) Remove bolts and washers securing air mixing chamber and gasket to firewall. (j) Using inspection mirror and flashlight, observe orientation of linkage hook and flapper door eyelet. (k) Rotate air mixing chamber 90 clockwise to disengage linkage hook from flapper door eyelet. Remove air mixing chamber from airplane. (2) Installation - Air Mixing Chamber (a) Acquire necessary tools, equipment, and supplies. Description P/N or Spec. Supplier Purpose Tongue Depressor - Any Source Fixture. Clamp - Any Source Fixture. -inch Scale - Any Source Measure. Dow Corning Silicone Sealant RTV 73 Any Source Seal. (b) (c) (d) (e) (f) (g) (h) To facilitate air mixing chamber installation, secure flapper door to closed position. 1 Rotate hot air valve flapper door to closed position. 2 Position tongue depressor into hot air valve firmly against closed flapper door. 3 Using clamp, secure tongue depressor to inner surface of air mixing chamber. Verify flapper door is secured to closed position. Position gasket to air mixing chamber. Orient air mixing chamber 90 clockwise to installation location. Latch linkage hook to flapper door eyelet on air mixing chamber. Rotate air mixing chamber 90 counter-clockwise to engage hook into eyelet. Remove clamp securing tongue depressor to hot air valve. At coupler, apply silicone sealant around inner circumference of firewall pass-through. (Refer to 20-10) Position air mixing chamber and new gasket to firewall and secure with bolts and washers. Serials 188 & subs Page 5
(i) Position temperature control cable and clamp to bracket on air mixing chamber and secure with bolt, washers, and nut. (j) Position temperature control cable and wire stop to actuation arm on air mixing chamber and secure with bolt. (k) Position fresh air duct to air mixing chamber and secure with clamp. (l) Position heat exchanger duct to air mixing chamber and secure with clamp. (m) To secure attaching hardware, apply silicone sealant to bolt heads following installation. (n) Install engine cowling. (Refer to 71-10) Page Serials 188 & subs
D. Cabin Air Coupler - Serials 139 thru 1885 (See Figure 1) (1) Removal - Cabin Air Coupler (a) Remove engine cowling. (Refer to 71-10) (b) Remove RH kick plate. (Refer to 25-10) (c) Remove clamp securing duct to coupler. (d) Remove nut, washers, and spacer securing hot air valve linkage to coupler. (e) Remove nut, washers, and spacer securing vent, fan, or evaporator linkage to coupler. (f) Remove nut, washers, and spacer securing distribution manifold linkage to coupler actuation arm. (g) Remove bolt, washer, and clamp securing distribution manifold linkage to coupler mounting plate. (h) Remove bolts and washers securing coupler and gasket to firewall. (2) Installation - Cabin Air Coupler (a) Position coupler and new gasket to firewall and secure with bolts and washers. (b) Position distribution manifold linkage to coupler actuation arm and secure with spacer, washers, and nut. (c) Position distribution manifold linkage to coupler mounting plate and secure with bolt, washer, and clamp. (d) Position vent, fan, or evaporator linkage to coupler and secure with spacer, washers, and nut. (e) Position hot air valve linkage to coupler and secure with spacer, washers, and nut. (f) Position duct to coupler and secure with clamp. (g) Install RH kick plate. (Refer to 25-10) (h) Install engine cowling. (Refer to 71-10) Serials 139 thru 1885 Page 7
E. Cabin Air Coupler - Serials 188 & subs (See Figure 2) (1) Removal - Cabin Air Coupler (a) Set BAT 1, BAT 2, and AVIONICS switches to OFF positions. (b) Pull COMPRESSOR/CONTROL PANEL circuit breaker. (c) Remove engine cowling. (Refer to 71-10) (d) Remove RH kick plate. (Refer to 25-10) (e) Disconnect coupler drive assembly electrical connectors. (f) Remove clamp securing duct to coupler. (g) Remove nut, washers, and spacer securing air mixing chamber linkage to coupler. (h) Remove bolts and washers securing coupler and gasket to firewall. Remove coupler from airplane. (2) Installation - Cabin Air Coupler (a) Position coupler and new gasket to firewall and secure with bolts and washers. (b) Position air mixing chamber linkage to coupler and secure with spacer, washers, and nut. (c) Position duct to coupler and secure with clamp. (d) Connect coupler drive assembly electrical connectors. (e) Install RH kick plate. (Refer to 25-10) (f) Install engine cowling. (Refer to 71-10) (g) Reset COMPRESSOR/CONTROL PANEL circuit breaker. Page 8 Serials 188 & subs
F. Baffle Duct - Serials 139 & subs (See Figure 1) (1) Removal - Baffle Duct (a) Remove lower engine cowling. (Refer to 71-10) (b) Remove bolt, washers, nut, and clamp securing duct to engine mount. (c) Remove clamp securing duct to engine baffling. (d) Remove clamp securing duct to heat exchanger. Remove duct from airplane. (2) Installation - Baffle Duct (a) Position duct to heat exchanger and secure with clamp. (b) Position duct to engine baffling and secure with clamp. (c) Position duct to engine mount and secure with clamp, bolt, washers, and nut. (d) Install lower engine cowling. (Refer to 71-10) Serials 139 & subs Page 9
G. Hot Air Valve Duct - Serials 139 thru 1885 (See Figure 1) (1) Removal - Hot Air Valve Duct (a) Remove lower engine cowling. (Refer to 71-10) (b) Remove clamp securing duct to heat exchanger. (c) Remove clamp securing duct to hot air valve. Remove duct from airplane. (2) Installation - Hot Air Valve Duct (a) Position duct to hot air valve and secure with clamp. (b) Position duct to heat exchanger and secure with clamp. (c) Install lower engine cowling. (Refer to 71-10) Page 10 Serials 139 thru 1885
H. Air Mixing Chamber Duct - Serials 188 & subs (See Figure 2) (1) Removal - Air Mixing Chamber Duct (a) Remove lower engine cowling. (Refer to 71-10) (b) Remove clamp securing duct to heat exchanger. (c) Remove clamp securing duct to air mixing chamber. Remove duct from airplane. (2) Installation - Air Mixing Chamber Duct (a) Position duct to air mixing chamber and secure with clamp. (b) Position duct to heat exchanger and secure with clamp. (c) Install lower engine cowling. (Refer to 71-10) Serials 188 & subs Page 11
I. Cabin Air Coupler Duct - Serials 139 & subs (See Figure 1) (1) Removal - Cabin Air Coupler Duct (a) Remove RH kick plate. (Refer to 25-10) (b) Remove sidewall air duct trim. (Refer to 25-10) (c) Remove clamp securing duct to coupler. (d) Serials 139 & subs w/o Fan: Remove clamp securing duct to vent assembly. (e) Serials 139 & subs w/ Fan: Remove clamp securing duct to fan assembly. (f) Serials 205 & subs w/ Air Conditioning: Remove clamp securing duct to evaporator. (g) Remove cable ties securing duct to fuselage. Remove duct from airplane. (2) Installation - Cabin Air Coupler Duct (a) Position duct to vent assembly and secure with clamp. (b) Serials 139 & subs w/o Fan: Position duct to vent assembly and secure with clamp. (c) Serials 139 & subs w/ Fan: Position duct to fan assembly and secure with clamp. (d) Serials 205 & subs w/ Air Conditioning: Position duct to evaporator and secure with clamp. (e) Position duct to coupler and secure with clamp. (f) Position duct to fuselage and secure with cable ties. (g) Install sidewall air duct trim. (Refer to 25-10) (h) Install RH kick plate. (Refer to 25-10) J. Defrost Diffuser The defrost diffuser is integral to the lower windshield trim assembly. For maintenance practices pertinent to the lower windshield trim assembly, see Chapter 25, Cabin. (Refer to 25-10) K. Heat Exchanger For maintenance practices pertinent to the heat exchanger, see Chapter 78, Noise Suppressor. (Refer to 78-20) Page 12 19 Sep 2010 All
CABIN AIR PLENUM 2 3 4 1 FIREWALL 5 HEAT EXCHANGER 7 3 4 1 2 5 Serials 128 thru 132, 134 thru 138. 7 HEAT EXCHANGER NOTE Serials 1005 thru 1147 : Gasket not installed on these airplanes. Serials 1005 thru 127. LEGEND 1. Hot Air Valve 2. Gasket 3. Screw 4. Washer 5. Hot Air Valve Duct. Clamp 7. Baffle Duct Serials 1005 thru 138 Serials 1005 thru 138. Figure 1 Hot Air Valve Installation - Serials 1005 thru 138 (Sheet 1 of 2) SR20_MM21_1135D Page 13
2 8 9 4 9 4 2 1 ENGINE BAFFLE 7 FIREWALL 5 HEAT EXCHANGER LEGEND 1. Hot Air Valve 2. Gasket 4. Washer 5. Hot Air Valve Duct. Clamp 7. Baffle Duct 8. Cabin Air Coupler 9. Bolt Serials 133, 139 thru 1877, 1879 thru 1885. SR20_MM21_2321B Page 14 Figure 1 Hot Air Valve Installation - Serials 139 thru 1885 (Sheet 2 of 2) Serials 139 thru 1885
4 9 2 8 ENGINE BAFFLE 9 4 4 3 9 4 1 2 FIREWALL 5 ENGINE MOUNT 7 HEAT EXCHANGER LEGEND 1. Air Mixing Chamber 2. Gasket 3. Nut 4. Washer 5. Air Mixing Chamber Duct. Clamp 7. Baffle Duct 8. Cabin Air Coupler 9. Bolt SR20_MM21_2557 Serials 188 & subs Figure 2 Air Mixing Chamber Installation - Serials 188 & subs Page 15
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