EMERGENCY
AIRWORTHINESS DIRECTIVE

FAA
Aviation Safety

www.faa.gov/aircraft/safety/alerts/

DATE: July 23, 2020
AD #: 2020-16-51
Emergency Airworthiness Directive (AD) 2020-16-51 is sent to owners and operators of The
Boeing Company Model 737-300, -400, -500, -600, -700, -700C, -800, -900, and -900ER series
airplanes.
Background
This emergency AD was prompted by four recent reports of single-engine shutdowns due to
engine bleed air 5th stage check valves being stuck open. Corrosion of the engine bleed air 5th stage
check valve internal parts during airplane storage may cause the valve to stick in the open position. If
this valve opens normally at takeoff power, it may become stuck in the open position during flight
and fail to close when power is reduced at top of descent, resulting in an unrecoverable compressor
stall and the inability to restart the engine. Corrosion of these valves on both engines could result in a
dual-engine power loss without the ability to restart. This condition, if not addressed, could result in
compressor stalls and dual-engine power loss without the ability to restart, which could result in a
forced off-airport landing.
FAA’s Determination
The FAA is issuing this AD because the agency evaluated all the relevant information and
determined the unsafe condition described previously is likely to exist or develop in other products of
the same type design.
AD Requirements
This AD requires inspections of the engine bleed air 5th stage check valve on each engine and
replacement of the engine bleed air 5th stage check valve if any inspection is not passed.
Authority for this Rulemaking
Title 49 of the United States Code specifies the FAA’s authority to issue rules on aviation
safety. Subtitle I, Section 106, describes the authority of the FAA Administrator. Subtitle VII,
Aviation Programs, describes in more detail the scope of the Agency’s authority.
The FAA is issuing this rulemaking under the authority described in Subtitle VII, Part A,
Subpart III, Section 44701, General requirements. Under that section, Congress charges the FAA
with promoting safe flight of civil aircraft in air commerce by prescribing regulations for practices,
methods, and procedures the Administrator finds necessary for safety in air commerce. This
regulation is within the scope of that authority because it addresses an unsafe condition that is likely
to exist or develop on products identified in this rulemaking action.

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 Presentation of the Actual AD
The FAA is issuing this AD under 49 U.S.C. Section 44701 according to the authority
delegated to me by the Administrator.
2020-16-51 The Boeing Company: Project Identifier AD-2020-01032-T.
(a) Effective Date
This emergency AD is effective upon receipt.
(b) Affected ADs
None.
(c) Applicability
This AD applies to all The Boeing Company Model 737-300, -400, -500, -600, -700, -700C,
-800, -900, and -900ER series airplanes, certificated in any category.
(d) Subject
Air Transport Association (ATA) of America Code 36, Pneumatic.
(e) Unsafe Condition
This AD was prompted by four recent reports of single-engine shutdowns caused by engine
bleed air 5th stage check valves stuck in the open position. The FAA is issuing this AD to address
corrosion of the engine bleed air 5th stage check valves for both engines, which could result in
compressor stalls and dual-engine power loss without the ability to restart, which could result in a
forced off-airport landing.
(f) Compliance
Comply with this AD within the compliance times specified, unless already done.
(g) Definition
Any airplane that, for 7 or more consecutive days, has not been operated in flight is
considered to be in “storage.”
(h) Inspections and Corrective Actions
(1) For any airplane that is in storage on or after the date of receipt of this AD, and any
airplane that, as of the date of receipt of this AD, has been operated for 10 or fewer flight cycles since
returning to service from the most recent period of storage: Before further flight, do the inspections
specified in paragraphs (h)(1)(i) and (ii) of this AD on the engine bleed air 5th stage check valve on
each engine. If any engine bleed air 5th stage check valve fails any inspection, replace that engine
bleed air 5th stage check valve before further flight. For each engine bleed air 5th stage check valve
that passes both inspections specified in paragraphs (h)(1)(i) and (ii) of this AD, do the actions

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 specified in paragraph (h)(2) of this AD on that engine bleed air 5th stage check valve before further
flight.
(i) Rotate the flapper plates by hand at least 3 times. If the flapper plate moves smoothly,
without signs of binding or sticking, from the fully closed position to the stop tube using gravity force
alone, the engine bleed air 5th stage check valve has passed this inspection.
(ii) Measure the clearance between the flapper bushings at both locations on each engine
bleed air 5th stage check valve. If the clearance between the flapper bushings is a minimum of 0.004
inch (0.102 mm) at both locations, the engine bleed air 5th stage check valve at that location has
passed this inspection.
(2) For each engine bleed air 5th stage check valve that passes the inspections specified in
paragraphs (h)(1)(i) and (ii) of this AD, do the inspections specified in paragraphs (h)(2)(i) through
(iii) of this AD before further flight on the engine bleed air 5th stage check valve on each engine. If
any engine bleed air 5th stage check valve fails any of the inspections specified in paragraphs (h)(2)(i)
through (iii) of this AD, replace that engine bleed air 5th stage check valve before further flight.
(i) Do a general visual inspection of the flapper bushings for signs of cracks, fractures, and
missing bushing heads. If the flapper bushings do not show any signs of cracks, fractures, or missing
bushing heads, the engine bleed air 5th stage check valve has passed this inspection. Signs of
corrosion are not a cause for replacing the engine bleed air 5th stage check valve if the engine bleed
air 5th stage check valve did not fail any of the inspections specified in paragraph (h)(1) of this AD.
(ii) Using only hand pressure, try to rotate the flapper bushings in the flapper plates. If the
bushings do not rotate in the flapper plate, the engine bleed air 5th stage check valve has passed this
inspection.
(iii) Do a general visual inspection of the check valve for signs of the flappers rubbing against
the valve body. If the flappers do not show any signs of rubbing against the valve body, the engine
bleed air 5th stage check valve has passed this inspection.
(i) Minimum Equipment List Relief for Certain Airplanes
For airplanes that have operated 10 or fewer flight cycles since the most recent period of
storage prior to receipt of this AD, as an alternative to compliance with paragraph (h): If allowed by
the operator’s FAA-approved Minimum Equipment List, the airplane may be dispatched with one
engine’s engine bleed air high stage valve locked closed. Thereafter, within 5 additional flight cycles,
inspect the engine bleed air 5th stage check valve on both engines as required by paragraph (h) of this
AD.
(j) Special Flight Permit
Special flight permits may be issued in accordance with 14 CFR 21.197 and 21.199 to operate
the airplane to a location where the airplane can be inspected, provided one engine’s engine bleed air
high stage valve has been locked closed. This option is only available if the operator’s FAA-approved
Minimum Equipment List allows dispatching the airplane with one engine’s engine bleed air high
stage valve locked closed.

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 (k) Alternative Methods of Compliance (AMOCs)
(1) For Boeing Model 737-300, -400, and -500 series airplanes, the Manager, Los Angeles
ACO Branch, FAA, has the authority to approve AMOCs for this AD, if requested using the
procedures found in 14 CFR 39.19. In accordance with 14 CFR 39.19, send your request to your
principal inspector or local Flight Standards District Office, as appropriate. If sending information
directly to the manager of the certification office, send it to the attention of the person identified in
paragraph (l)(1) of this AD. Information may be emailed to: 9-ANM-LAACO-AMOCRequests@faa.gov.
(2) For Boeing Model 737-600, -700, -700C, -800, -900, and -900ER series airplanes, the
Manager, Seattle ACO Branch, FAA, has the authority to approve AMOCs for this AD, if requested
using the procedures found in 14 CFR 39.19. In accordance with 14 CFR 39.19, send your request to
your principal inspector or local Flight Standards District Office, as appropriate. If sending
information directly to the manager of the certification office, send it to the attention of the person
identified in paragraph (l)(2) of this AD. Information may be emailed to: 9-ANM-Seattle-ACOAMOC-Requests@faa.gov.
(3) Before using any approved AMOC, notify your appropriate principal inspector, or lacking
a principal inspector, the manager of the local flight standards district office/certificate holding
district office.
(4) An AMOC that provides an acceptable level of safety may be used for any repair,
modification, or alteration required by this AD if it is approved by The Boeing Company
Organization Designation Authorization (ODA) that has been authorized by the Manager, Seattle
ACO Branch, FAA, to make those findings. To be approved, the repair method, modification
deviation, or alteration deviation must meet the certification basis of the airplane, and the approval
must specifically refer to this AD.
(l) Related Information
(1) For Boeing Model 737-300, -400, and -500 series airplanes, for further information about
this AD, contact Serj Harutunian, Aerospace Engineer, Propulsion Section, FAA, Los Angeles ACO
Branch, 3960 Paramount Boulevard, Lakewood, CA 90712-4137; phone: 562-627-5254; fax: 562627-5210; email: serj.harutunian@faa.gov.
(2) For Boeing Model 737-600, -700, -700C, -800, -900, and -900ER series airplanes, for
further information about this AD, contact Rajendran Mohanraj, Aerospace Engineer, Propulsion
Section, FAA, Seattle ACO Branch, 2200 South 216th St., Des Moines, WA 98198; phone and fax:
206-231-3621; email: rajendran.mohanraj@faa.gov.

Issued on July 23, 2020.

Lance T. Gant, Director,
Compliance & Airworthiness Division,
Aircraft Certification Service.

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