MEA vs. MOCA: The 22-NM Rule, GPS at the MOCA, Chart Symbols, and When a Lower Altitude Is Actually Useful
Updated: 3 days ago
When flying IFR, altitude selection is never arbitrary. Every published altitude exists to ensure obstacle clearance, navigation signal reliability, and safe separation. Two of the most commonly misunderstood altitudes on IFR en route charts are the MEA (Minimum En Route Altitude) and the MOCA (Minimum Obstruction Clearance Altitude). They often appear together on a segment, sometimes only a few hundred feet apart and sometimes thousands — and the difference between them is not obstacle clearance (they provide the same) but navigation signal coverage. Understanding exactly what each guarantees, why the MOCA’s 22-mile limit exists and whom it applies to, and when a GPS-equipped aircraft can use the lower altitude is what separates memorizing two acronyms from actually using them.
This post covers MEA and MOCA in practical depth: what each guarantees, the regulation that governs them, how they’re depicted on the chart, why the 22 NM limit exists and why it’s VOR-specific, how GPS changes the picture (including GNSS MEAs), why the two altitudes can differ so much, when the MOCA is actually useful, the lost-communications rule, and a worked example.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
What the MEA Is
The Minimum En Route Altitude is the lowest published altitude between two fixes on an airway or route segment that provides BOTH obstacle clearance AND reliable navigation signal coverage over the entire segment.
The two guarantees:
Obstacle clearance — at least 1,000 feet above the highest obstacle in non-mountainous areas, 2,000 feet in designated mountainous areas, within the airway’s protected width
Navigation signal coverage — you can receive and use the navaids that define the segment, along its entire length
Why both:
Clearing the terrain is useless if you can’t tell where you are
Receiving the VOR is useless if the terrain is above you
The MEA is the altitude where both problems are solved at once
The default IFR altitude for the segment
Key characteristics:
Published on IFR en route low- and high-altitude charts
Applies to a specific segment (between two fixes), and can change at each fix
Guarantees full navaid reception along the whole segment — so a long segment far from its VORs will have a high MEA driven by signal range, not terrain
The MEA does not guarantee two-way radio communications coverage with ATC — only navigation signal coverage and obstacle clearance. Communications are often available at the MEA, but that’s a separate question (and part of why comm-coverage gaps exist in mountainous areas)
Why it’s the default:
ATC normally assigns altitudes at or above the MEA when you’re on your own navigation
Pilots plan cruise altitudes at or above it
It’s the altitude used in the lost-communications rule (the “M” in MEA/Expected/Assigned)
The predictable, no-surprises altitude
What the MOCA Is
The Minimum Obstruction Clearance Altitude is the lowest published altitude on a segment that provides obstacle clearance — with the SAME clearance standard as the MEA — but guarantees VOR navigation signal reception only within 22 NM of the VOR.
What the MOCA guarantees:
Obstacle clearance — the same 1,000 / 2,000 feet as the MEA, over the same protected width
VOR signal reception — only within 22 nautical miles of the VOR
Nothing beyond that about signal coverage
Key characteristics:
Published below the MEA on the chart, with an asterisk (e.g., *3200)
Lower than the MEA — sometimes by a few hundred feet, sometimes by thousands
Equal obstacle clearance; reduced signal guarantee
Not published for every segment (only where it differs from the MEA and designers chose to publish it)
The core distinction, in one line:
The MEA says: “you’re clear of terrain AND you can navigate, everywhere on this segment”
The MOCA says: “you’re clear of terrain everywhere on this segment — but you can only count on the VOR within 22 miles of it”
Same ground clearance, different signal promise
That’s the whole difference
(Preserve the existing MEA-vs-MOCA comparison table image here.)
The Regulation: 91.177(a)(1)
The rule that governs when you may descend from the MEA to the MOCA, quoted closely:
“If both a MEA and a MOCA are prescribed for a particular route or route segment, a person may operate an aircraft below the MEA down to, but not below, the MOCA, provided the applicable navigation signals are available. For aircraft using VOR for navigation, this applies only when the aircraft is within 22 nautical miles of that VOR.”
Reading it:
You MAY fly below the MEA, down to the MOCA, on a segment where both are published
The condition is that “the applicable navigation signals are available”
For VOR navigation, that’s defined as being within 22 NM of the VOR
For other navigation (GPS), the signal is available throughout, so the 22 NM limit doesn’t apply (next section)
What the regulation does NOT require:
It does not require ATC authorization to use the MOCA (though in controlled airspace you’re flying ATC’s assigned altitude, so you’d request it)
It does not require a special chart note
The regulation itself is the authorization
A pilot-usable altitude under its conditions
What it does require:
A published MOCA for the segment (if none is published, the MEA is the floor)
Navigation signal availability (the 22 NM rule for VOR)
And, in controlled airspace, an ATC clearance to that altitude (you can’t just descend from your assigned altitude)
Conditions, not permission
The neighboring paragraph — 91.177(a)(2):
When NO minimum altitude is prescribed (an off-airway direct route, for example), (a)(2) requires 1,000 feet above the highest obstacle within 4 NM of your course (non-mountainous) or 2,000 feet (mountainous)
That’s the off-route rule, distinct from the MEA/MOCA provision in (a)(1)
Two paragraphs, two situations
Know which one you’re under
Why 22 Nautical Miles?
The number isn’t arbitrary — it comes from VOR signal physics at low altitude.
VOR is line-of-sight:
VOR signals are VHF, which travel in straight lines and don’t bend over the horizon or around terrain
At low altitude, the usable range is limited by the curvature of the Earth and intervening terrain
The closer you are, the more reliably the signal reaches you
Distance and altitude trade off
The 22 NM assumption:
The FAA’s airway design assumes that at the MOCA — which is only 1,000 or 2,000 feet above the obstacles — reliable VOR reception can be assured only within about 22 NM of the facility
Beyond that, at such a low altitude, the signal may be blocked by terrain or lost over the horizon
So 22 NM is the design assumption for low-altitude VOR coverage
A conservative, standardized figure
Why the MEA solves it:
The MEA is raised, where necessary, until the VOR can be received along the ENTIRE segment
On a long segment between distant VORs, that can mean an MEA thousands of feet above the terrain-driven MOCA — purely for signal reasons
The extra altitude buys line-of-sight to the VOR
Signal, not rocks
The practical insight:
When the MEA is far above the MOCA, the gap is almost always SIGNAL coverage, not terrain
The terrain clearance is the same at both altitudes
The MEA is “high” because the VOR is far away, not because the mountains are tall
Understanding this tells you what you’re actually giving up at the MOCA
The “Which VOR?” Question — and the 44 NM Trick
A nuance that examiners like, and that makes the MOCA more useful than it first appears.
Within 22 NM of WHICH VOR?
The regulation says “within 22 nautical miles of that VOR” — the VOR you’re navigating by
On a segment between two VORs, you navigate FROM one and then TO the other
Each VOR has its own 22 NM zone
The 44 NM consequence:
A segment up to 44 NM long between two VORs can be flown at the MOCA the ENTIRE way by VOR navigation
Use the departure-end VOR (tracking FROM it) for the first 22 NM
Switch to the arrival-end VOR (tracking TO it) for the last 22 NM
You’re always within 22 NM of the VOR you’re using
The switch is the key:
You must actually change the VOR you’re navigating by at the switch point
Staying tuned to the first VOR past 22 NM at the MOCA means you’ve left the guaranteed coverage
Retune, re-identify, and track inbound
The MOCA stays legal because the signal stays guaranteed
Longer segments:
On a segment longer than 44 NM, there’s a middle stretch where you’re more than 22 NM from BOTH VORs
The MOCA is not usable by VOR there
Climb to the MEA for that stretch, or navigate by GPS (below)
The gap in the middle
The chart hint — where to switch:
If no changeover point is depicted on the segment, switch VORs at the midpoint
Where a changeover point symbol (the bent line with mileages to each VOR) IS depicted, the switch has been moved off the midpoint — usually because terrain or signal geometry favors one VOR longer — and you switch at the depicted point instead
Use whichever applies as your switch point at the MOCA
The chart tells you when to retune
MEA, MOCA, and GPS Navigation
Here’s where modern equipment genuinely changes the answer — and where the regulation is more permissive than many pilots assume.
GPS and the MOCA:
91.177(a)(1) conditions the MOCA on “the applicable navigation signals” being available, and restricts the 22 NM limit to “aircraft using VOR for navigation”
An aircraft navigating the airway by IFR-approved GPS has its navigation signal available along the ENTIRE segment
So a GPS-navigating aircraft may fly at the MOCA anywhere on the segment, not just within 22 NM of a VOR
The 22 NM limit is a VOR problem, and GPS doesn’t have it
What GPS does NOT change:
The MOCA’s obstacle clearance applies only within the airway’s protected width — you must still be ON the airway
Off the airway, the MOCA means nothing; the 91.177(a)(2) off-route rule (1,000/2,000 feet within 4 NM of course) or the OROCA applies
You still need an ATC clearance to the altitude in controlled airspace
GPS removes the signal limit, not the other conditions
GNSS MEAs (a separate thing):
Some airways publish a SECOND, lower MEA for GPS-equipped aircraft, shown on FAA charts with a “G” suffix (e.g., “4500G” beneath the conventional MEA)
This is a GNSS MEA: the lowest altitude at which an RNAV/GPS aircraft is assured obstacle clearance and — because GPS doesn’t depend on ground stations — effectively a lower en route altitude
It exists because the conventional MEA was raised for VOR signal reasons that don’t apply to GPS
A GNSS MEA is an MEA, not a MOCA — it’s a full en route altitude for GPS aircraft
The two GPS paths to a lower altitude:
A published GNSS MEA (“G”) — fly it as your MEA if GPS-equipped
The MOCA — usable throughout the segment by GPS per 91.177(a)(1)
Both are legitimate; the GNSS MEA is the more formal, chart-designed option
Either way, GPS lets you use the terrain-driven altitude rather than the signal-driven one
The equipment caveat:
“GPS” here means IFR-approved GPS (TSO-C129/C145/C146/C196) used as the primary navigation source for the segment
A handheld or VFR-only GPS doesn’t qualify
Non-WAAS units need RAIM availability
The signal must be legally usable, not just present
The honest summary:
GPS does not “eliminate MEA requirements” — but it does remove the 22 NM MOCA limitation, and it may unlock a published GNSS MEA
The MEA remains the default; the MOCA (or GNSS MEA) is the GPS pilot’s legitimate lower option on the airway
Know which navigation source you’re actually using
The regulation follows the signal
How MEA and MOCA Appear on the Chart
Reading the en route chart correctly is half the skill.
The MEA:
Printed in bold above or beside the airway segment
One number per segment (between fixes)
Example: 5000 on the segment
The MOCA:
Printed in smaller type with an asterisk, below the MEA
Example: *3200 beneath the 5000
Not every segment has one published
Directional MEAs:
Some segments have DIFFERENT MEAs in each direction, shown with arrows: “7000 →” and “← 5000”
Usually because terrain rises in one direction (you need to be higher sooner), or because of signal geometry
Use the MEA for YOUR direction of flight
A common misread
The GNSS MEA:
A second MEA with a “G” suffix (e.g., 4500G)
The MEA for GPS-equipped aircraft
Lower than the conventional MEA where VOR signal drove the conventional one up
The changeover point:
A bent-line symbol with mileages to each VOR, depicted only when the changeover is NOT at the segment midpoint
No symbol = switch at the midpoint
Symbol = switch at the depicted point
The MEA-change bar:
A “T”-shaped bar across the airway at a fix marks where the MEA changes
Signals you to check the new MEA for the next segment (and whether an MCA applies)
Easy to miss on a busy chart
Look for it at every fix
Related symbols at the fix:
“X” flag — Minimum Crossing Altitude (cross the fix at or above it)
“R” flag — Minimum Reception Altitude (needed to receive an off-route navaid for the fix)
“MEA GAP” — a stretch with no assured navaid reception even at the MEA
The reading habit:
For every segment: bold number = MEA; asterisk number = MOCA; G number = GNSS MEA; arrows = direction-specific; bent line = changeover point
At every fix: look for the T-bar, the X, and the R
Thirty seconds of chart reading before the segment
Prevents the surprise
Why the MEA and MOCA Can Differ So Much
Sometimes they’re 300 feet apart; sometimes 4,000. The gap tells a story.
A small gap (a few hundred feet):
The VOR is close to the whole segment, so signal coverage barely raises the MEA
Both altitudes are driven by terrain
Common on short segments near a VOR
Little practical difference
A large gap (thousands of feet):
The segment is long, or the VOR is distant, or terrain blocks the signal at low altitude
The MOCA sits at terrain + 1,000/2,000
The MEA has been raised well above that to guarantee VOR line-of-sight for the whole segment
The gap is pure signal margin
What a large gap means for you:
At the MOCA, you have exactly the same terrain clearance as at the MEA
What you lack is the VOR beyond 22 NM
If you’re on GPS, that gap is altitude you can legitimately use
If you’re on VOR, that gap is the signal you’d lose
The mountain example:
A segment in the Rockies: MOCA *11,000, MEA 15,000
Terrain plus 2,000 feet is 11,000 — that’s the obstacle floor
The MEA is 15,000 because the VORs are 50 NM apart and the ridges block the signal below that
A GPS aircraft can fly the segment at 11,000 (on the airway, with a clearance); a VOR-only aircraft needs 15,000 for the middle portion
When the MOCA Is Actually Useful
The original frames the MOCA as a limited-use altitude. True — but the uses matter, and they’re worth naming.
Escaping icing:
The single most common real-world reason to want the MOCA
You’re at the MEA in cloud, picking up ice, and warmer air (or clear air) is below
The MOCA lets you descend to terrain-clearance altitude where it’s legal — within 22 NM by VOR, anywhere by GPS
Request it: “Request descent to the MOCA, 3,200, for icing”
Getting below a layer:
Descending below a cloud base to VMC for comfort, visibility, or to cancel IFR
The MOCA is the floor on the airway
Same request
Turbulence or headwinds:
Lower altitude may be smoother or have less headwind
The MOCA is as low as the airway allows
Worth asking for
Non-radar environments and emergency descents:
The MOCA is NOT the lost-comms altitude (that’s the MEA — below)
But in a genuine emergency where you must descend, the MOCA is the published floor that keeps you clear of terrain on the airway
Know it as the last published floor
Why ATC rarely assigns it unprompted:
ATC’s minimum altitudes (MVA/MIA) are their own charts, and they typically assign the MEA for aircraft on their own navigation
The MOCA is a pilot-requested altitude
Ask for it when it helps
It’s there for you
The MOCA and Lost Communications
A rule with no gray area.
The lost-comms altitude (91.185):
Fly the HIGHEST of: the MEA for the segment, the altitude ATC told you to expect, or your last assigned altitude
The MEA — not the MOCA
The MOCA plays no role in the lost-comms rule
MEA / Expected / Assigned, take the highest
Why the MEA:
Lost comms means you’re navigating on your own with no ATC backstop
The MEA guarantees you can receive the navaids and clear the terrain for the whole segment
The MOCA’s 22 NM limit is exactly the kind of uncertainty you don’t want when you can’t talk to anyone
The conservative, signal-guaranteed altitude
The common error:
“I’ll descend to the MOCA since I’m within 22 NM” during lost comms — NO
The rule says MEA (as the minimum of the three)
Using the MOCA during lost comms is a regulatory error and a safety one
MEA, every time
A Worked Example
Putting the rules together on a realistic segment.
The segment:
V-airway from ABC VOR to XYZ VOR, 38 NM long
MEA 7,000; MOCA *4,500; no GNSS MEA published; no changeover point depicted (so the switch is at the 19 NM midpoint)
Scenario 1 — VOR navigation, at the MEA:
Fly 7,000 the whole way
Full signal coverage, full terrain clearance
No decisions needed
The default
Scenario 2 — VOR navigation, icing at 7,000, request the MOCA:
Request and receive “descend and maintain 4,500”
From ABC, track outbound on the ABC radial — within 22 NM, the MOCA is legal
At the midpoint (19 NM), retune and identify XYZ, track inbound — now within 22 NM of XYZ
The segment is 38 NM, under 44, so the MOCA is legal end-to-end with the switch
Terrain clearance is the same 1,000 feet it was at 7,000
Scenario 3 — GPS navigation (IFR-approved), icing at 7,000:
Request and receive 4,500
Navigate the airway by GPS; the signal is available throughout
No 22 NM concern, no VOR switch required
Stay on the airway centerline (the MOCA’s clearance applies only to the airway)
Scenario 4 — lost comms at 7,000:
Fly the highest of MEA (7,000), expected, assigned
Stay at 7,000 (or higher if expected/assigned was higher)
Do NOT descend to the MOCA
MEA rules
Scenario 5 — the segment is 60 NM, VOR navigation:
The middle 16 NM is more than 22 NM from both VORs
At the MOCA by VOR, you’d lose guaranteed reception there
Climb to the MEA for that stretch, or use GPS for the whole segment
The 44 NM limit in action
Common Pilot Mistakes
Assuming the MOCA is always acceptable because it’s published (it’s conditioned on signal availability — and in controlled airspace, on a clearance)
Forgetting the 22 NM limit when navigating by VOR
Forgetting to SWITCH VORs at the midpoint (or depicted changeover point) when flying a segment at the MOCA by VOR
Believing GPS requires special chart authorization to use the MOCA (91.177(a)(1) authorizes it when the signal is available — the 22 NM limit is VOR-only)
Confusing a GNSS MEA (“G” suffix) with a MOCA (it’s a full MEA for GPS aircraft)
Descending below the MEA on your own without a clearance in controlled airspace
Using the MOCA during lost communications (the rule is MEA)
Confusing the MOCA with the OROCA (the OROCA is a grid altitude for off-route flying with no navigation or communication guarantee; the MOCA is an airway altitude)
Using the wrong directional MEA on a segment with arrows
Missing the MEA-change T-bar at a fix (and the MCA that may come with it)
Leaving the airway centerline at the MOCA (its clearance applies only within the airway’s protected width)
Practical Takeaway
The MEA is your go-to en route altitude: terrain clearance AND navigation coverage, guaranteed
The MOCA has the SAME terrain clearance but guarantees VOR reception only within 22 NM — a signal limit, not a terrain limit
By VOR, a segment up to 44 NM can be flown at the MOCA if you switch facilities at the midpoint (or the depicted changeover point)
By IFR-approved GPS, the MOCA is usable along the whole segment (and some airways publish a lower GNSS MEA with a “G”)
The MOCA is a pilot-requested altitude — ask for it to escape icing, get below a layer, or find smoother air
Lost comms = MEA, never MOCA
When in doubt, fly the MEA
Understanding MEA versus MOCA isn’t just a checkride topic — it’s knowing exactly what you’re giving up when you descend, and exactly when you aren’t giving up anything at all.
On the Written Test and Checkride
MEA and MOCA appear on every instrument written and oral. The most commonly tested topics:
What the MEA guarantees (obstacle clearance + navigation signal for the whole segment)
What the MOCA guarantees (obstacle clearance + VOR signal within 22 NM)
The 91.177(a)(1) rule for descending to the MOCA
That the 22 NM limit applies to VOR navigation (GPS may use the MOCA throughout)
Chart depiction (bold MEA, asterisk MOCA, arrows, “G” GNSS MEA, changeover point)
The lost-comms altitude (MEA, not MOCA)
MOCA vs. OROCA
Quick Reference
MEA — Minimum En Route Altitude:
Lowest altitude between fixes with BOTH obstacle clearance AND navigation signal coverage for the whole segment
Obstacle clearance: 1,000 ft (non-mountainous) / 2,000 ft (mountainous)
Does NOT guarantee communications coverage
Chart: bold number on the segment; may be directional (arrows); may have a GNSS version (G suffix)
The default IFR altitude; the lost-comms altitude
MOCA — Minimum Obstruction Clearance Altitude:
Lowest altitude with obstacle clearance (SAME as MEA) but VOR signal guaranteed only within 22 NM of the VOR
Chart: smaller number with an asterisk (*) below the MEA
Not published on every segment
A pilot-requested altitude
The Rule (91.177(a)(1)):
May descend below the MEA to the MOCA “provided the applicable navigation signals are available”
“For aircraft using VOR for navigation, this applies only when the aircraft is within 22 nautical miles of that VOR”
No special authorization needed — but a clearance to the altitude is required in controlled airspace
Only on the airway (the clearance applies within the airway’s protected width)
(Off-route, with no prescribed minimum: 91.177(a)(2) — 1,000/2,000 ft within 4 NM of course)
Why 22 NM:
VOR is line-of-sight; at the low MOCA altitude, reliable reception is assumed only within ~22 NM
The MEA is raised to guarantee reception for the whole segment
A large MEA–MOCA gap = signal coverage, not terrain
The 44 NM Trick (VOR):
Track FROM the first VOR for 22 NM, switch at the midpoint (or the depicted changeover point), track TO the second VOR for 22 NM
A segment up to 44 NM can be flown at the MOCA by VOR with the switch
Longer segments: climb to the MEA for the middle, or use GPS
GPS and the MOCA:
IFR-approved GPS has the signal throughout → MOCA usable along the ENTIRE segment (no 22 NM limit)
Still must be on the airway; still need a clearance in controlled airspace
GNSS MEA (“G” suffix): a separate, lower full MEA for GPS aircraft on some airways — an MEA, not a MOCA
GPS removes the signal limit, not the other conditions
Chart Symbols:
Symbol | Meaning |
Bold number | MEA |
*Number | MOCA |
Number with G | GNSS MEA (GPS aircraft) |
Arrows with numbers | Directional MEAs |
Bent line with mileages | Changeover point (shown only when NOT at the midpoint) |
T-bar at a fix | MEA changes here (check for an MCA) |
X flag | MCA (cross the fix at/above) |
R flag | MRA (to receive an off-route navaid) |
MEA GAP | No assured reception even at the MEA |
When the MOCA Is Useful:
Escaping icing (the #1 real reason)
Getting below a layer
Smoother air / less headwind
The published floor in an emergency descent on the airway
Request it: “Request descent to the MOCA, [altitude], for icing”
Lost Communications:
Fly the HIGHEST of MEA / Expected / Assigned
MEA, never MOCA
MOCA vs. OROCA:
MOCA: on an airway; obstacle clearance; VOR signal within 22 NM
OROCA: off-route grid altitude; obstacle clearance; NO navigation or communication guarantee
Key Principle:
The MEA guarantees terrain clearance AND navigation coverage for the whole segment; the MOCA guarantees the same terrain clearance but VOR reception only within 22 NM — a signal limit, not a terrain limit (91.177(a)(1)). By VOR, fly a segment up to 44 NM at the MOCA by switching facilities at the midpoint; by IFR-approved GPS, the MOCA is usable throughout. Request it to escape icing or get below a layer — but in lost communications, it’s always the MEA.
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.