Direct Answer — The Four Safety-Avionics Rules and Their Records
Part 135 mandates several safety-avionics systems by aircraft type and seat count. 14 CFR §135.154 requires a Terrain Awareness and Warning System (TAWS) on turbine-powered airplanes — Class A (TSO-C151) for 10 or more passenger seats, Class B as a minimum for 6 to 9 passenger seats. 14 CFR §135.175 requires approved airborne weather radar on large, transport category aircraft in passenger-carrying operations. 14 CFR §135.173 requires approved thunderstorm-detection equipment or airborne weather radar on aircraft with 10 or more passenger seats. 14 CFR §135.180 requires an approved traffic alert and collision avoidance system (TCAS) on turbine-powered airplanes with a 10-to-30-seat configuration. The equipment is only legal if the records prove the installation: a Form 337 for a major alteration, the §43.9 maintenance entry, any STC and Airplane Flight Manual supplement, and — for TAWS — the terrain/obstacle database revision that keeps the system performing as approved. An inspector who cannot find those records treats the equipment as unproven.
The thresholds are different on purpose — read each rule by its own trigger
TAWS keys off turbine airplanes and seat count; weather radar keys off large, transport category aircraft; thunderstorm detection keys off the 10-or-more-seat configuration with weather radar as an alternative; TCAS keys off turbine airplanes in the 10-to-30-seat band. It is easy to assume one threshold covers all four — it does not. Confirm each rule against the live Part 135 Subpart C text for your specific aircraft and operation.
The Four Safety-Avionics Rules — Each With Its Own Trigger
These live in Subpart C of Part 135 (Aircraft and Equipment). They are independent requirements with independent applicability thresholds. An operator must read each one against its own aircraft and mission — not assume that meeting one covers the rest.
14 CFR §135.154 applies to turbine-powered airplanes and sets two floors by passenger-seat configuration (excluding any pilot seat):
- 10 or more passenger seats: a TAWS meeting the requirements for Class A equipment in TSO-C151
- 6 to 9 passenger seats: as a minimum, equipment meeting the requirements for Class B equipment in TSO-C151
- For airplanes manufactured after March 29, 2002 the requirement applied at that point; airplanes manufactured on or before March 29, 2002 had until after March 29, 2005 to comply
The rule also requires that the Airplane Flight Manual contain appropriate procedures for the use of the TAWS and proper flightcrew reaction in response to its audio and visual warnings. Note this is an airplane rule — it does not, by its terms, impose TAWS on Part 135 helicopters (helicopter terrain-awareness obligations arise under other rules, including the air-ambulance framework discussed in our HEMS records guide).
14 CFR §135.175 requires that no person operate a large, transport category aircraft in passenger-carrying operations unless approved airborne weather radar equipment is installed in the aircraft. The operational restriction is the key part for dispatch:
- No person may begin a flight under IFR or night VFR conditions when current weather reports indicate that thunderstorms or other potentially hazardous weather detectable with airborne weather radar may reasonably be expected along the route, unless the radar is in satisfactory operating condition
- If the radar becomes inoperative en route, the aircraft must be operated under the procedures in the manual required by §135.21
- Geographic carve-outs (for example, operations solely within Hawaii, within Alaska, or a defined part of western Canada) and training, test, and ferry flights are excepted
- An alternate electrical power supply for the radar is not required
Because §135.175 is keyed to a large, transport category aircraft, many smaller turbine charter airplanes meet their storm-detection obligation through §135.173 instead — the next rule.
14 CFR §135.173 applies to aircraft with a passenger seating configuration of 10 seats or more (excluding any pilot seat), and lets the operator choose its means of compliance:
- The aircraft must be equipped with either approved thunderstorm detection equipment or approved airborne weather radar equipment — they are alternatives at this level
- No person may begin a flight under IFR or night VFR conditions when current weather reports indicate thunderstorms or other potentially hazardous weather may reasonably be expected along the route, unless the installed equipment is in satisfactory operating condition
- Helicopters operating under day VFR conditions are carved out, and the same geographic and training/test/ferry exceptions apply
In short: a 10-or-more-seat turbine airplane that is not a large, transport category aircraft can satisfy storm detection with an approved thunderstorm-detection unit (a Stormscope-type detector) rather than installing full weather radar — but whichever it installs, the records must show that exact equipment is approved and serviceable.
14 CFR §135.180 provides that, unless otherwise authorized by the Administrator, after December 31, 1995, no person may operate a turbine-powered airplane that has a passenger seat configuration, excluding any pilot seat, of 10 to 30 seats unless it is equipped with an approved traffic alert and collision avoidance system.
- The rule reaches turbine-powered airplanes in the 10-to-30-seat band — it is not a more-than-30-seat rule
- If TCAS II is installed, it must be capable of coordinating with TCAS units that meet TSO C-119
- The Airplane Flight Manual must contain procedures for the use of, and proper flightcrew action with respect to, the TCAS I system required by this section, and an outline of all input sources that must be operating for the TCAS to function properly
A more capable TCAS II installation is permitted as long as it meets the coordination requirement — the rule sets the floor (an approved TCAS) and conditions the higher-capability option, it does not cap the equipment at TCAS I.
Side-by-Side: Who Each Rule Reaches
| Rule | System | Applies to | Key condition |
|---|---|---|---|
| §135.154 | TAWS | Turbine airplanes | Class A at 10+ seats; Class B min at 6–9 seats (TSO-C151) |
| §135.175 | Weather radar | Large, transport category aircraft | Passenger-carrying ops; satisfactory before IFR/night VFR in storms |
| §135.173 | Thunderstorm detection | Aircraft with 10+ seats | Detector OR weather radar; helicopter day-VFR carve-out |
| §135.180 | TCAS | Turbine airplanes | 10–30 seats; TCAS II must coordinate per TSO C-119 |
These equipment rules sit beside the rest of the required-equipment picture. For how inoperative equipment is handled under a Minimum Equipment List, see our MEL / CDL records guide, and for the broader survival-and-emergency-gear obligations on a charter aircraft, see Part 135 emergency & survival equipment records.
TAWS in Detail — Class A vs. Class B, and the Database
TAWS is the rule operators ask about most, because the Class A/Class B distinction and the terrain database create two recurring questions: did we install the right class for our seat count, and is the database current? §135.154 answers the first directly; the database question is a function of how the installed unit meets its TSO.
Turbine-powered airplanes configured with 10 or more passenger seats (excluding any pilot seat) must have TAWS meeting the Class A requirements of TSO-C151. Per FAA guidance, Class A is the more capable configuration, including a terrain situational-awareness display and the fuller set of alerting modes.
The regulation points to TSO-C151 for the performance standard; it assigns Class A to this seat category as the floor.
Turbine-powered airplanes configured with 6 to 9 passenger seats (excluding any pilot seat) must have, as a minimum, equipment meeting the Class B requirements of TSO-C151. Because the rule sets a minimum, an operator in this category may install the more capable Class A equipment — but may not install less than Class B.
Class B is a reduced-feature configuration relative to Class A, per FAA guidance.
The Terrain / Obstacle Database — A Records Question, Not a Stated Interval
§135.154 sets the equipment standard and the flight-manual procedures; it does not, in its own text, prescribe a database-update cadence. But the terrain, obstacle, and airport database is what the alerting computes against, and keeping it current is part of how the installed system continues to perform to its TSO approval. From a compliance-records standpoint, the practical implications are:
- The database revision loaded in the aircraft is part of the evidence that the TAWS is performing as approved — capture the revision identifier and the date it was loaded
- Database currency requirements are driven by the specific installed unit and its data provider — verify the cadence for your equipment with the provider and your repair station rather than assuming a generic interval
- A TAWS with an out-of-date or unsupported database is a finding waiting to happen even though the box still powers on — the record of the current revision is what closes that gap
The TAWS database is conceptually similar to the navigation-database currency that drives other avionics — for the parallel discipline on position/surveillance equipment, see our ADS-B Out compliance & PAPR records guide, and for the recurring altimetry tests that keep the altitude side honest, see RVSM altimeter & transponder inspection records.
Related Aviation Compliance Guides
How the Equipment Gets Into the Aircraft — and What That Generates
TAWS, weather radar, a thunderstorm detector, and TCAS reach an in-service aircraft either as part of its type design or — far more commonly for retrofits — through a major alteration, frequently under a Supplemental Type Certificate. Each path produces a specific record an inspector expects to find.
FAA Form 337 — Major Alteration
Installing safety avionics that constitutes a major alteration is recorded on FAA Form 337 (Major Repair and Alteration). The Form 337 documents the alteration and the approved data it was performed under, and a copy belongs with the aircraft records. This is the document that shows the installation itself was an approved change to the aircraft.
§43.9 Maintenance Record Entry
Under 14 CFR §43.9, the person who performed the work makes a maintenance record entry containing a description of the work performed (or a reference to acceptable data), the date of completion, and the signature, certificate number, and kind of certificate held by the person approving the aircraft for return to service. This is the entry that ties the installation to a person and a date — distinct from the inspection entries governed by §43.11.
STC and Approved Data
Where the equipment was installed under a Supplemental Type Certificate, the STC and its approved installation data become part of the aircraft records. The STC is the approval basis for that specific make/model of TAWS, radar, detector, or TCAS in that specific airframe — keep it retrievable, because it is what an inspector cross-references against the installation.
Airplane Flight Manual / AFM Supplement
Each of these rules ties to flight-manual content — §135.154 and §135.180 require AFM procedures for the system and the proper flightcrew reaction. An STC installation typically adds an AFM Supplement covering the new equipment. The current AFM and its supplements must be aboard and must match the installed configuration.
Operational Test / Functional Check & Database Record
The return-to-service typically depends on a successful operational test of the installed system, recorded per your maintenance program. For TAWS specifically, the terrain/obstacle database revision loaded at installation — and at each subsequent update — is part of the ongoing record that the system performs as approved.
The owner/operator is on the hook for the records — even when a shop does the work
Under 14 CFR §91.405, the owner or operator is responsible for having required maintenance performed and for ensuring that maintenance personnel make the appropriate entries in the aircraft maintenance records. A perfect installation with a missing Form 337 or §43.9 entry is still a records deficiency — and the inspector cannot tell a missing record apart from work that was never done.
For the full mechanics of a compliant maintenance entry, see our §43.9 maintenance record entry requirements; for major alterations specifically, see FAA Form 337 major repair & alteration records; and for how all of this fits the certificate holder’s maintenance recordkeeping, see Part 135 maintenance recordkeeping (CAMP) requirements.
Record Discipline — What an Inspector Actually Looks For
The equipment is only as defensible as the records that prove it is approved, installed correctly, and still performing. Here is what a complete, audit-ready records set for these safety-avionics systems looks like — and the specific ways it falls apart.
Complete Record Set
- Form 337 for each safety-avionics major alteration, filed with the aircraft records
- §43.9 maintenance entry for each installation — work performed, date, signer, certificate number
- STC and approved data for each installed make/model (TAWS, radar, detector, TCAS)
- Current AFM and AFM supplements matching the installed configuration
- Operational/functional test results recorded at return to service
- TAWS terrain/obstacle database revision identifier and load date, kept current
How These Records Fail
- Equipment installed but the Form 337 was never filed with the aircraft records
- The wrong TAWS class for the seat count (Class B where Class A was required)
- An AFM supplement that no longer matches a since-modified configuration
- A TAWS database that is years out of date with no record of any update
- Treating §135.173 as satisfied by an unapproved detector with no approval data on file
- A §43.9 entry missing the signer’s certificate number or the return-to-service approval
Why the records matter beyond the ramp check
FAA civil penalties are adjusted for inflation. Under 14 CFR §13.301, for violations occurring on or after December 30, 2024, the maximum civil penalty under 49 U.S.C. 46301(a)(1)(A) or (B) is $75,000 in general, and $1,875 for an individual or small business concern. Beyond the dollars, an unproven safety-avionics installation can put the aircraft’s airworthiness — and the operation that depends on it — in question.
FileFlo as the Safety-Avionics Records Layer
FileFlo is a compliance document intelligence platform — a read-only proof layer that sits alongside your maintenance-tracking stack and completes it. It does not run, test, or certify the TAWS, weather radar, thunderstorm detector, or TCAS; it does not perform maintenance, hold the certificate, or replace any safety program. What it does is make the records that prove these installations instantly producible.
- Classifies uploaded Form 337s, §43.9 entries, STCs, and AFM supplements and links them to the aircraft and the governing rule (§135.154 / §135.175 / §135.173 / §135.180)
- Captures the TAWS terrain/obstacle database revision and load date so an out-of-date database is visible before an inspector finds it
- Tracks the records and review dates that matter and surfaces alerts so a missing or stale document never surprises you
- Flags configuration mismatches — for example, an AFM supplement that no longer matches what is installed — for your team to resolve
- Generates an inspector-format records binder on demand, organized by aircraft, rule, and document type
FileFlo classifies 600+ compliance document types and manages records across Part 91, Part 135, and Part 145 operators in a single platform. Pricing: Starter $89/mo, Professional $299/mo. 5-day free trial, no credit card required. FileFlo keeps the documents that prove compliance audit-ready — it does not provide or run a safety management system (SMS), dispatch system, or flight operations system, and it claims no live integration with any avionics or maintenance system.
Same Equipment, Different Accountability by Operator
The installation records (Form 337, §43.9, STC, AFM supplement, database) are the same documents regardless of how the aircraft is operated. What changes is who is accountable for keeping them and the consequence when they go missing.
Who is responsible
Owner/operator
Owner is directly responsible under §91.405 for the records and for keeping the TAWS database current. Most exposed to a quietly stale database between updates.
Who is responsible
DOM via the maintenance program
Installation and database records fold into the certificate holder’s maintenance program and recordkeeping; a Principal Maintenance Inspector reviews them during surveillance.
Who is responsible
The installing shop
The repair station that performs the installation produces the Form 337 and §43.9 entry and retains its own work records — which the operator must also be able to produce.
These avionics records sit alongside the other airworthiness obligations an inspector cross-checks — see airworthiness directive compliance records (ADs frequently touch avionics), the Part 91 §409 annual inspection requirements, and how all of it is examined in a Part 135 FAA surveillance audit. For who actually holds the authority to dispatch the aircraft these systems protect, see what operational control means in Part 135.
Frequently Asked Questions
Which Part 135 airplanes must have a Terrain Awareness and Warning System (TAWS)?
Under 14 CFR §135.154, turbine-powered airplanes configured with 10 or more passenger seats (excluding any pilot seat) must be equipped with a TAWS that meets the requirements for Class A equipment in Technical Standard Order (TSO)-C151, and turbine-powered airplanes configured with 6 to 9 passenger seats (excluding any pilot seat) must have, as a minimum, equipment meeting the requirements for Class B equipment in TSO-C151. The rule is a turbine-airplane rule keyed to passenger-seat count — it is the seat configuration, not the aircraft weight, that sets the Class A versus Class B floor.
What is the difference between TAWS Class A and Class B?
14 CFR §135.154 itself does not spell out the functional differences between the two classes — it points to the performance standards in TSO-C151 and assigns Class A to the larger (10+ seat) turbine airplanes and Class B as the minimum for the 6-to-9-seat turbine airplanes. In practice, per FAA guidance, Class A is the more capable equipment (it includes a terrain display and additional alerting modes), while Class B is a reduced-feature configuration. Because the regulation only sets a floor, an operator may install more capable equipment than the minimum required for its seat category, but it may not install less.
Does §135.154 require a current terrain or obstacle database?
The text of 14 CFR §135.154 sets the equipment standard (Class A or Class B TSO-C151 equipment) and requires the Airplane Flight Manual to contain procedures for use of the system and proper flightcrew reaction to its warnings; it does not, in its own words, prescribe a database-update interval. The terrain, obstacle, and airport database that drives the alerting is part of how the installed system meets its TSO and continues to perform as approved, so keeping it current is a function of the equipment approval and your maintenance program — verify the specific currency requirement for your installed unit with its data provider and your repair station. From a records standpoint, the database revision in the aircraft is part of the evidence that the TAWS is performing as approved.
When is airborne weather radar required under Part 135?
Under 14 CFR §135.175, no person may operate a large, transport category aircraft in passenger-carrying operations unless approved airborne weather radar equipment is installed. Separately, the rule prohibits beginning a flight under IFR or night VFR conditions when current weather reports indicate that thunderstorms or other potentially hazardous weather conditions detectable with airborne weather radar may reasonably be expected along the route, unless the radar is in satisfactory operating condition. The rule contains geographic carve-outs (for example, operations solely within Hawaii, within Alaska, and a defined part of western Canada) and excepts training, test, and ferry flights, and it states that no person may operate an aircraft solely on the basis that weather radar is not required by these geographic exceptions if airborne weather radar is in fact installed and usable.
How does airborne thunderstorm-detection equipment (§135.173) relate to weather radar?
Under 14 CFR §135.173, for an aircraft with a passenger seating configuration of 10 seats or more (excluding any pilot seat), the operator may satisfy the requirement with either approved thunderstorm detection equipment or approved airborne weather radar equipment — the two are alternatives at that level. The rule then prohibits beginning a flight under IFR or night VFR conditions when current weather reports indicate thunderstorms or other potentially hazardous weather may reasonably be expected along the route unless the installed equipment is in satisfactory operating condition. Helicopters operating under day VFR conditions and the same geographic and training/test/ferry carve-outs apply. So a smaller turbine airplane that is not a large transport-category aircraft may meet the storm-detection obligation with a Stormscope-type detector rather than a full weather-radar installation.
Does Part 135 require TCAS, and is it TCAS I or TCAS II?
Under 14 CFR §135.180, after December 31, 1995, no person may operate a turbine-powered airplane that has a passenger seat configuration, excluding any pilot seat, of 10 to 30 seats unless it is equipped with an approved traffic alert and collision avoidance system. The flight-manual provision in §135.180(b) refers to the TCAS I system required by this section, and the rule adds that if TCAS II is installed it must be capable of coordinating with TCAS units that meet TSO C-119. So §135.180 reaches the 10-to-30-seat turbine airplanes; it is not a more-than-30-seat rule, and a more capable TCAS II installation is permitted as long as it meets the coordination requirement.
What records prove the installation of TAWS, weather radar, or TCAS?
The equipment gets into the aircraft through a major alteration or as part of type design. A major alteration is recorded on FAA Form 337, and the aircraft maintenance records carry the entry made under 14 CFR §43.9 (description of the work, date of completion, and the signature, certificate number, and kind of certificate of the person approving the aircraft for return to service). If the equipment was installed under a Supplemental Type Certificate (STC), the STC and its approved data become part of the aircraft records and the Airplane Flight Manual Supplement is retained. Together the Form 337 (where applicable), the §43.9 maintenance entry, the STC/AFM supplement, and any required operational test results are the proof that the safety-avionics installation is approved and airworthy.
Who is responsible for keeping these avionics installation and database records, and does FileFlo run the equipment?
Under 14 CFR §91.405, the owner or operator is responsible for having required maintenance performed and for ensuring that maintenance personnel make the appropriate entries in the aircraft maintenance records. For a Part 135 operator, that recordkeeping folds into the certificate holder's maintenance program. FileFlo does not run, test, or certify the TAWS, weather radar, or TCAS, and it does not perform maintenance, hold the certificate, or replace any safety program. FileFlo is a compliance document intelligence platform — a read-only proof layer — that classifies and indexes the Form 337s, §43.9 entries, STCs, AFM supplements, and database-revision records, tracks the dates that matter, and produces an inspector-ready records set on demand. The equipment is installed, tested, and approved by appropriately rated maintenance providers; FileFlo keeps the documents that prove it.
Prove every TAWS, radar, and TCAS installation in one click
FileFlo classifies your Form 337s, §43.9 entries, STCs, and AFM supplements, captures the TAWS database revision, links each document to the governing rule, and generates a complete inspector-format records binder on demand. Starter plan $89/mo. Professional $299/mo. 5-day free trial — no credit card required.
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Written by Chad Griffith, Founder & CEO of FileFlo — compliance document intelligence. Reviewed June 13, 2026. FileFlo is a compliance document intelligence platform, not legal counsel, an A&P, or a repair station. Verify all equipment applicability thresholds, installation approvals, who-may-sign rules, and database-currency requirements with a certificated A&P or IA, your repair station, your avionics provider, and your FSDO.