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Boeing 737-200/-300 Air Conditioning Write-Ups
Jim Davidson, President
Davidson Engineering Resources, Inc., Tucson, AZ
Phone (520) 977-9824
Fax (520) 232-3660

-------------------------------May 2006-------------------------------

Part 1: Boeing 737-200/-300 Air Conditioning System: Description and Troubleshooting

This figure shows the Boeing 737-300 Air Conditioning component locations withing the airframe

The figure above indicates the airflow in the Boeing 737-300.


This write up is the first in a series that address the Boeing 737-200/-300 Air Conditioning System. The author has experienced about 6 years of the day-to-day line maintenance operations as well as supporting 737 heavy maintenance functions for a major airline. The major daily focus was on the reliability and performance improvements of the 200 or so 737-200/-300/-400 aircraft ATA 21 systems. In addition the author has approximately 6 years design experience of Cabin Pressure Systems for a major aircraft component vendor, with primary emphasis on the older pneumatically controlled systems.

Troubleshooting these ATA 21 systems, with the large number of valves, control systems, and system interrelationships were at first, overwhelming. Daily exposure to the ATA 21 system faults (those morning maintenance calls were murder sometimes!), the effective solutions, the experience gained from the aircraft maintainers and the continuous study of the ATA 21 system interrelationships eventually morphed into a basic knowledge of the systems and what to expect from them over time. The author also has 10 years experience as a consultant FAA DER with both Part 23 and Part 25 delegations in air conditioning and pneumatics, and electrical systems and equipment. Although certification issues of aircraft modifications and certification issues related to the design of the ATA 21 systems will not be addressed in these write ups, it will given time, appear at a later date, and be the topic of another set of write-ups.

My hope is that these write ups, describing the ATA 21 systems and providing a few personal experiences, will benefit readers and add to your existing body of knowledge of the ATA 21 systems. This first set of write ups address the Boeing 737-200/-300 Air Conditioning Systems.

This figure is a high-level schematic representation of the Boeing 737-300 Air Conditioning System.

NOTE: These write ups are not to be used for aircraft maintenance! Only approved maintenance documents are to be used for aircraft maintenance. The information provided is only for reference and for your academic purposes. Perform all aircraft maintenance per the approved and current aircraft AMM.

Even though an aircraft maintainer may have performed maintenance on the 737 ATA 21 systems for years, and could probably do many of the maintenance functions blindfolded, it is always “good form” and ”required” that you have up to date maintenance documents at arms reach while performing maintenance actions. Safety, knowledge, attention to detail, and self-checking are and need to be, the norm.

I sincerely hope you enjoy these write ups and can take away a bit of knowledge form them. Here we go…

This is the Air Conditioning Control Panel. It is located on the P5 Overhead, just right of centerline.

Air Conditioning System Overview:

This figure shows the systems that will be discussed - except the Temperature control System.

There are four major systems that make up the air conditioning / cabin pressurization system on the Boeing 737-200/-300 aircraft. They are designed to work together to perform the function of providing conditioned (proper temperature and humidity) air to the aircraft.

Another system, not discussed in the write up, is the Cabin Pressurization Control System (CPCS). The CPCS is designed to function in concert with the Air Conditioning system to provide the cabin pressurization required for high altitude flight and passenger safety. The CPCS will be the topic of another write up. The main Air Conditioning Systems are:

Air Cycle System:

The air cycle system functions by using air from either the engines or the Auxiliary Power Unit (APU). Airflow from the engines, as bleed air, or APU to the Air Cycle Machine (ACM or Pack) is controlled by various Engine Bleed valves and System Control Valves. The bleed airflow is modulated by Pack Flow Control Valves, and the air is thermally adjusted by means of air-to-air heat exchangers and Mix Valves.

This is the Air Cycle Machine - There is a compressor on one side and a turbine on the other.

A pair of thermal sensors, thermocouple devices, (390º F Compressor Outlet Temperature, 210º F Turbine Inlet Temperature) located within the Pack ducting monitors specific temperatures and provides signals to shutdown the pack if system temperatures exceed specified limits.

Ram Air System:

The Ram Air System consists of Ram Air Control System. Low and forward of the wing root on each side of the aircraft there are Ram Air Inlets with Ram Air Inlet Doors. The doors modulate between fully open and some percentage closed based on a specified temperature feedback to the Controllers. The Ram Air Inlet and Door configuration is designed to provide specific levels of cooling ambient air (ram air) into the ram air side of the bleed air to the Primary and Secondary heat exchangers.

This is one view of the Ram Air linkages. the system has many more components which will be addressed in following sections.

The inlet and doors are also designed to minimize the drag penalty on the aircraft at cruise altitudes and speeds. The Ram Air Doors are controlled by a Ram Air Actuator, and a Ram Air Door Controller. When the aircraft is on the ground, or in flight with flaps extended, such as during takeoff and landing, there is not enough ram air available to provide the necessary bleed air cooling across the heat exchangers. During this aircraft configuration, a Turbofan is operated that pulls air through the ram air side of the bleed air heat exchangers. The Turbofan is actuated by a Turbofan Control Valve.

35º F System:

The 35º F System is a closed loop control system that maintains the ACM output airflow, down stream of the Water Separator, at approximately 35º F and with low levels of humidity. The thermodynamic properties of rapidly expanding gasses (air) result in a reduction of the gas temperature. The reduction in temperature from the expanding air can be well below zero for a given set of thermodynamic conditions.

In the 737-200/-300 Pack System, the air expanded from a properly operating ACM turbine is normally below 32º F and does contain a certain amount of entrained water (humidity). At 32º F or below the entrained water may freeze and clog the Water Separator. On an MD-80 flight during the initial descent phase, the author heard “marbles” rolling in the passenger overhead ducting. Most likely this was the result of frozen pellets of water due to a faulty air conditioning system.

This figure shows the 35 Degree System interfaces as well as several of the sensors that are discussed in following sections.

This will result in a severe reduction in cabin inflow or the excess water can create fog or “rain in the plane”. The 35º F System mixes warm air with the cold air from the ACM turbine outlet air as a function of the 35ºF control system set point temperature. The amount of the additional warm air introduced into the cold air stream in the Water Separator is controlled via the 35º F Controller, Sensor and Valve.

Temperature Control System:

The Temperature Control System senses, by various thermal sensors, and controls the temperature within the aircraft cabin based on the flight crew aircraft cabin temperature settings. The conditioned air exiting a properly operating Water Separator is at approximately 35º and with a relatively low humidity. In this state the air is not adequate for introduction into the passenger cabin. Further temperature adjustments are required and the Temperature Control System components accomplish the task.

This is a figure of the Temperature Control Panel, located on th P5 overhead, just above the Air Conditioning Control Panel.

There are temperature sensors located at various locations in the air distribution ducting, 140º F Duct Limiter, 190º F High Duct Limiter, 250º F Thermal Switch, a Thermal Anticipator, and a Cabin and Cockpit Temperature Sensors. All these sensors provide temperature information to the Cabin Temperature Controller which then provides drive signals to the Mix Valve or, in the event of excessive temperatures, cease the ACM operation.

Overview Conclusion:

The interaction of the four main systems described briefly above provides thermodynamically adjusted air for passenger comfort. In the event any one or a combination of the above systems fails to perform or becomes unstable, the intended function of the Temperature Control System is reduced and the air thermodynamic properties are adversely affected. Problems resulting from an improperly operating air conditioning system range from excessive humidity in the cabin (rain in the plane or fog), excessively low or high cabin temperatures, and/or a reduction in cabin inflow.

It should be noted that the reduction in cabin inflow can also adversely affect the performance of the Cabin Pressurization System that may result such anomalies as loss of pressurization. Again, the CPCS will be discussed in another write up.

The author’s experience with troubleshooting the Boeing 737-200/-300 air conditioning is that once the individual systems and their interrelationships with each other are fully understood, problems can be identified quickly and maintenance actions taken in time to prevent worsening problems (again, those morning maintenance calls referencing Delays and Cancellations were just a killer) . The author has seen more than a few aircraft that are “Rogue” aircraft, they just keep having problems each day. Each night the maintenance crews try something different to solve the problem and the air conditioning system just keeps on acting up. A great deal of cost and time can be expended in the process of nightly troubleshooting and “shot-gun” maintenance procedures. Of course, the author fully understands the pressure to get the aircraft fixed and back in revenue flight condition. Knowledge of the systems and their interrelationships go a long way to preventing “Rogue” aircraft.

The approach to this paper is to go through each system and detail the operation of each system component, and then look at how a failure or combination of failures affects the air conditioning system as well as the other systems.

Quick Checks for Systems Interactions:

• Supply Air:

› On the 737-200 if engine bleed pressure follows throttle movements without hesitation prior to the 8th stage bleed valve switch over point from the 5th stage bleed valve, the area of interest is the engine bleed air system. More on the JT8 engine bleed valve configuration later.

› On the 737-300 if at approximately 33 PSI duct pressure, as displayed on the Duct Pressure Indicator (dual needle display) there should be a slight drop in duct pressure during the transition from the 9th stage bleed to the 5th stage bleed source. More on the CFM-56 engine bleed valve configuration later.

› Air conditioning pack flow can be determined by Auxiliary Power Unit (APU) Exhaust Gas Temperature (EGT) drop – procedure will be detailed later.

› If the Turbo Fan runs when it is not supposed to, inspect the Air Conditioning Pack Valve closed limit switch for proper operation.

• Ram Air:

› Ram air doors and ram air exit louvers are full open on the ground. Any position less than full open indicates a Ram Air System problem.

› Due to the location of the ram air inlets on the 737, on the lower portion of the fuselage, check for debris in the inlets. These inlets are like vacuums and will suck up paper, baggage tags, and even your hat. The author has even seen one heat exchanger with a very unlucky bird plastered on the ram air inlet side of the heat exchanger. You guessed it, the aircraft had air conditioning problems, but that bird had even bigger problems!

› The operation of the ram air doors is accomplished by a cabling system, so check for correct cable tension of the actuation cables.

› In flight the ram air door Open light should not illuminate. If this occurs there is a problem with the ram air inlet door actuation system.

› The 230º F sensor located in the ACM compressor inlet ducting, is used for the actuation of the ram air doors. The sensor only comes into play when the aircraft is in flight. Proper sensor operation is required for proper ram air door operation.

• Air Cycle Cooling System:

› A faulty 390º F ACM compressor inlet temperature sensor may be the cause for the illumination of the Pack Trip annunciation during flight. When the 230º F sensor is in the closed position, the Pack Trip annunciation will not illuminate. A ground check of the operation of the 230º F sensor will assist with troubleshooting the occurrence of an in flight Pack Trip annunciator illumination. Similar checking of the 210º F turbine inlet and 250º F supply duct temperature sensors for proper operation will also assist with troubleshooting.

› If a Pack Trip annunciator illumination occurs right after switching a pack to the operational condition, the removal of the 390º F sensor connector and then the 210º F sensor connector for a brief time. This will simulate an Open sensor and will determine which sensor is faulty (faulted in the closed position). Remove each connector only long enough for the annunciator to extinguish, because leaving the connector off of each will result in a system overheat condition eventually. The same procedure can be used with the 250º F supply duct temperature sensor connector to assist with the troubleshooting.

› While performing the connector removal procedures, have someone monitor the duct temperature indicator to verify that an overheat condition does not exist.

› The duct temperature should read 43º F or less, but higher than 32º F when the system is commanded to full cold.

• 35º F System:

› If the aircraft cabin does not cool, or if there is excessive humidity (fog or liquid water coming from the passenger gaspers), the primary system to trouble shoot is the 35º F system. The author had the unique opportunity to sit in Row 7 of the Fokker 100 air craft and experience first hand the “rain in the plane” syndrome. No amount of peanut napkins would keep me dry, but being a non-rev, I had to just sit there and enjoy the experience. So, the problems addressed in this write up can and do occur on non-Boeing aircraft. This system is a closed loop system designed to control the air temperature exiting the water separator to a temperature of 35º F. Troubleshooting methods are discussed later in the write up.

• Temperature Control:

› If the cabin will not cool in manual or automatic control modes, the primary system to troubleshoot is the temperature control system. Troubleshooting methods are discussed later in the write up.

737-200 and -300 Air Conditioning System Sensors and Switches – Brief Description

The following are brief descriptions of the sensors and switches used in the air conditioning system to monitor and provide feedback to control systems.

A. Compressor Discharge Temperature Sensor
› Temperature Set Point: 390º F (198.9º C)
› Function: Pack Trip Function
› Location: Air Cycle Machine (ACM) Compressor Scroll

B. Pack Cooling Temperature Limit Switch (2)
› Temperature Set Point: 230º F (110.0º C)
› Function: Drives Ram Air Door Full Open
› Location: Inlet to Secondary Heat Exchanger

C. Turbine Inlet Overheat Switch (2)
› Temperature Set Point: 210º F (98.9º C)
› Function: Pack Trip Function
› Location: Inlet to Air Cycle Machine (ACM) Turbine

D. Duct Anticipator Sensor (2) (4)
› Temperature Set Point: NA
› Function: Automatic Control of Mix Valve (Senses Rate of Temperature Change)
› Location: Main Distribution Manifold

E. Duct Limit Sensor (2) (5)
› Temperature Set Point: 140º F(60.0º C)
› Function: Limit Automatic control of Mix Valve to Cancel Heat Demand if Duct Temperature Raises
› Location: Main Distribution Manifold

F. Duct Overheat Switch (2) (6)

› Temperature Set Point: 190º F(87.8º C)
› Function: Duct Overheat
› Location: Main Distribution Manifold

G. Duct Overheat Switch (2) (7)
› Temperature Set Point: 250º F(121.1º C)
› Function: Pack Trip Protection for Duct in the Event of a Mix Valve Failure
› Location: Main Distribution Manifold

H. Supply Duct Temperature Bulb (1)
› Temperature Set Point: 35º F to 200º F (1.7º C to 93.3º C)
› Function: Information to Air Temperature Indicator
› Location: Main Distribution Manifold, Right Side

I. Control Cabin Temperature Sensor (1)
› Temperature Set Point: NA, Values Vary
› Function: Automatic control of Mix Valve
› Location: Left Side of Control Cabin Bulkhead

J. Passenger Cabin Temperature Sensor (1)
› Temperature Set Point: NA, Values Vary
› Function: Automatic Control of Mix Valve
› Location: Passenger Cabin, Left Side in the Overhead Storage Bull Nose

K. Passenger Cabin Temperature Bulb (1)
› Temperature Set Point: NA, Values Vary
› Function: Information to Air Temperature Indicator
› Location: passenger Cabin, Right Side in the Overhead Storage Bull Nose


Pack Trip Function
(1) Pack Valve Closes
(2) Trip Annunciator ON
(3) Mix Valve Full Closed

Duct Overheat
(1) Duct Annunciator ON
(2) Mix Valve Full Closed (Must be reset)

On 737-300 Aircraft
(4), (5), (6), & (7) are
Located in Cabin Overhead
Duct, Right Side


Davidson Engineering Resources, Inc.      Phone (520) 977-9824      Fax (520) 232-3660     
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