FAR Ninety-One Point One Eighty-Five, the IFR Radio Failure Procedure Students Memorize as an Acronym, and the Clearance Limit Timing That Nobody Actually Gets Right
FAR 91.185 governs exactly what to do when your radio fails in IMC - route priority, altitude selection, clearance limit timing, and light gun signals.
When two-way radio communication fails in IFR flight, FAR 91.185 specifies exactly which route to fly, which altitude to select, and precisely when to begin an approach - all without speaking to anyone. Understanding this procedure isn’t just checkride preparation. ATC uses it in real time to build protection around your flight the moment they see your transponder change.
Is Your Radio Actually Dead?
Before declaring a full communications failure, spend 60 to 90 seconds confirming the radio is genuinely unresponsive. Check the volume knob, squelch, and audio panel position. Cycle to your backup comm radio. Switch from headset to speaker. Try a different frequency - including guard (121.5 MHz), which some controllers and nearby aircraft monitor.
If you’re low enough for cell service, call Flight Service. They can relay messages directly to ATC. If another aircraft is on frequency nearby, ask them for a relay.
A significant percentage of reported radio failures trace to a headset plug that worked loose, a bumped volume knob, or an audio selector switched to the wrong position. Check the obvious things first. Only after exhausting every option should you commit to the full radio failure procedure.
What Do You Squawk When the Radio Is Truly Gone?
Set your transponder to 7600 - the universal code for radio failure. The moment ATC sees that code on their scope, they pull up your filed flight plan and begin predicting your behavior based on what FAR 91.185 requires. They start clearing traffic from your route and sequencing the airspace around your expected arrival time.
This is precisely why following the regulation matters beyond the written test. ATC is building protection around you based on the assumption you’re executing the procedure correctly. Deviating from the required route or timing puts you outside the gap they created.
Which Route Do You Fly?
FAR 91.185 establishes a four-step route priority sequence: Assigned, Vectored, Expected, Filed - commonly remembered as AVEF. Always use the first one that applies.
Assigned means the last route ATC assigned in a clearance. If they cleared you direct Lansing via Victor 23, fly that - not the filed route, not what seems like a good idea. The last assigned route is the one ATC is watching for.
Vectored is a special case that trips up many students. If ATC was actively vectoring you when the failure occurred, you don’t lock onto the last heading and fly it indefinitely. Instead, plot a direct course from the point of failure to the fix, airway, or route named in the vector clearance, then proceed from there. You complete the vector yourself and rejoin the cleared route.
Expected means any route ATC told you to expect in a further clearance. If Center said “Expect direct Kalamazoo in fifteen miles” and the radio failed shortly after, that expected clearance becomes your route.
Filed is the fallback - the route in your actual flight plan. ATC has it in their system right now, and if you go quiet, this is the route they’ll watch for. File accurate routes. The flight plan is not a formality; it’s the last line of communication between you and the system if everything else fails.
What Altitude Do You Fly?
For each segment of the route, fly the highest of these three altitudes:
- The altitude assigned in your last clearance
- The minimum en route altitude (MEA) for that segment
- The altitude ATC told you to expect in a further clearance
In IMC, terrain and obstacles are the threats you cannot see and won’t be warned about. The MEA is your absolute floor. If ATC told you to expect a higher altitude before the radio died - before they issued the formal climb clearance - climb to the expected altitude. That is where they planned to put you, and that is the number built into their mental model of your flight.
A concrete scenario: You’re cruising at 6,000 feet on Victor 23. The MEA along the segment is 5,000 feet. Center says, “Expect 8,000, expect further clearance at the outer marker.” Three minutes later, complete silence. Your last assigned altitude is 6,000, the MEA is 5,000, ATC told you to expect 8,000. The highest of those three is 8,000 feet - climb to it, even without a formal clearance. That is where ATC is protecting you.
What Is the Clearance Limit - and Why Does Timing Matter?
Every IFR clearance includes a clearance limit - for most flights, the destination airport itself. But in busier airspace, ATC sometimes issues a clearance limit to a specific fix short of the airport: an outer marker, initial approach fix, named intersection, or VOR. That clearance limit, combined with your timing, is how ATC sequences you into the approach without speaking to you again.
The regulation handles two distinct cases:
Case 1: Your clearance limit is a fix from which an approach begins (an outer marker or initial approach fix). Commence descent and approach as close as possible to the expect-further-clearance (EFC) time ATC gave you. If you were never given an EFC time, start the approach as close as possible to your estimated time of arrival based on your filed or amended estimated time en route.
Case 2: Your clearance limit is not a fix from which an approach begins (a VOR or named intersection en route). Fly to that fix, hold there until your EFC time. If you received no EFC time, hold until your ETA, then depart the fix and proceed to begin the approach.
When you squawked 7600, a controller applied the 91.185 procedure to your filed flight plan and calculated when you should arrive at the final approach fix. They then moved other traffic around that window - holding aircraft behind you, vectoring others away. That gap in the sequence is centered on your expected arrival time. Show up eight minutes early and you arrive before the gap is fully cleared. Show up eight minutes late and aircraft that were moved may have been released back in.
Before every IFR flight, write down your ETA - not just a rough idea, but the actual number. If your groundspeed differs significantly from what you filed, update it. That figure is what ATC uses to protect you if you go silent without ever receiving an explicit EFC time.
How Do You Receive Clearance on the Approach?
At a towered airport, a controller who can see your aircraft - or track you on radar - can communicate via light gun signals:
| Signal | Meaning |
|---|---|
| Steady green | Cleared to land |
| Flashing green | Cleared to land, use caution |
| Steady red | Give way to other aircraft, continue circling |
| Flashing red | Airport unsafe - do not land |
| Alternating red and green | General danger, exercise extreme caution |
These signals are operational. Pilots have executed full approaches to towered airports without radios and received the steady green. Know them cold before any IFR flight.
What Happens After You Land?
FAR 91.185 requires you to contact ATC as soon as practicable after landing. Call the approach facility by landline or reach Flight Service - whatever is available. ATC has been actively managing airspace around your flight. That one call lets them release the holds and vectors they placed on your behalf. It takes two minutes and it matters to the controllers who worked your situation.
What Will the Checkride Examiner Expect?
On an instrument rating practical test, the examiner will probe FAR 91.185 in the oral. They want the route priority order and the reasoning behind it. They want the altitude selection logic. They want the clearance limit, the hold, and the timing.
The answer that earns real credit isn’t reciting AVEF - it’s explaining why the order is what it is. Assigned comes first because it reflects ATC’s most current, specific decision for your flight. Filed comes last because it’s the most general fallback. The altitude rule selects the highest because that’s where you’re expected to be and where terrain clearance is greatest. Explaining the logic demonstrates airman judgment - which is precisely what the ACS is measuring.
Key Takeaways
- Spend 60–90 seconds exhausting every option to restore communication before treating the failure as confirmed.
- Squawk 7600 immediately; ATC will begin predicting your route and protecting your airspace from that moment forward.
- Follow the AVEF route priority - Assigned, Vectored, Expected, Filed - using the first option that applies.
- Fly the highest of your last assigned altitude, the MEA, or the altitude ATC told you to expect; that is where ATC has placed you in their mental model.
- Your ETA and clearance limit determine when you begin the approach - timing is the mechanism by which you arrive safely when no one can talk to you.
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