Part 91 Series — Article 7IFR Operations Under Part 91: Flying in the System

Flying VFR is about seeing and avoiding.

Flying IFR is about trusting the system.

When you enter the clouds in a small aircraft — whether it’s a well-equipped Cessna 172 with a GPS navigator or a Piper Arrow on an instrument cross-country — you’re no longer relying primarily on visual separation. You’re operating within a structured air traffic system governed by very specific rules in FAR Part 91.

Under IFR, there’s no such thing as “I’ll just head that direction and see what happens.”

You need:

• A clearance.

• A flight plan.

• Proper fuel planning.

• Specific equipment.

• And a clear understanding of what to do if things go quiet on the radio.

In this seventh article of our 8-part Part 91 series, we’ll break down the core IFR operational rules that apply to private pilots flying small aircraft in instrument conditions.

Let’s climb into the clouds — legally.

The Core IFR Regulations Under Part 91

In this article, we’re covering:

• §91.167 — IFR fuel requirements

• §91.169 — IFR flight plan requirements

• §91.171 — VOR equipment checks

• §91.173 — ATC clearance and flight plan required

• §91.175 — Takeoff and landing under IFR

• §91.185 — IFR operations: Two-way radio communications failure

These sections form the backbone of IFR flight under Part 91.

§91.173 — Clearance and Flight Plan Required

You may not operate in controlled airspace under IFR unless:

• You have filed an IFR flight plan, and

• You have received an ATC clearance.

This applies even in VMC if you are operating under IFR.

IFR is not just about weather — it’s about operating under instrument flight rules.

No clearance? No IFR.

§91.167 — IFR Fuel Requirements

Before departure under IFR, you must have enough fuel to:

1. Fly to the first intended airport of landing,

2. Fly to the alternate airport (if required),

3. Then fly for 45 minutes at normal cruising speed.

Unlike VFR minimums, IFR fuel planning assumes contingencies.

The regulation is structured around “what if” scenarios.

§91.169 — IFR Flight Plan Requirements

An IFR flight plan must include:

• Aircraft identification

• Aircraft type and equipment suffix

• True airspeed

• Departure point

• Proposed departure time

• Cruising altitude

• Route

• Destination

• Estimated time en route

• Fuel on board

• Alternate airport (if required)

The well-known “1-2-3 Rule” determines whether an alternate is required:

If, from one hour before to one hour after your ETA, the forecast is at least:

• 2,000-foot ceiling, and

• 3 statute miles visibility,

…then an alternate is not required.

If not, you must file one — and carry fuel to reach it.

§91.171 — VOR Equipment Checks

If you plan to use VOR navigation under IFR, you must verify VOR equipment accuracy within the preceding 30 days.

The check must be logged, including:

• Date

• Place

• Bearing error

• Signature

While GPS navigation has reduced reliance on VORs, the regulation still applies if VOR equipment is used under IFR.

§91.175 — Takeoff and Landing Under IFR

This regulation governs approach and landing minimums.

You may not descend below:

• The published minimum descent altitude (MDA), or

• Decision altitude (DA),

…unless:

1. The required flight visibility is met, and

2. At least one of the specified visual references for the runway is distinctly visible and identifiable.

Examples of acceptable visual references include:

• Approach light system

• Threshold

• Runway markings

• Runway lights

Descending below minimums without required visual references is a violation — and a serious safety risk.

§91.185 — Lost Communications (IFR)

If you lose two-way radio communication under IFR, the regulation provides a structured response.

Route (AVEF):

Fly the route assigned by:

• Assigned,

• Vectored,

• Expected,

• Filed (in that order of priority).

Altitude (MEA):

Fly the highest of:

• Minimum altitude,

• Expected altitude,

• Assigned altitude.

These memory aids (AVEF and MEA) help pilots comply with regulatory expectations during lost comm scenarios.

At this level, you understand the regulatory framework for IFR operations.

But the practical application is where things get interesting.

Practical IFR Compliance & Common Pitfalls

Let’s examine how these regulations play out in real-world general aviation flying.

Filing vs. Activating

Filing an IFR flight plan is not enough.

You must receive an ATC clearance before entering controlled airspace under IFR.

At non-towered airports, this may require:

• Clearance delivery via radio,

• Telephone clearance,

• Or release times.

Launching into the clouds without a clearance is not just risky — it’s non-compliant.

Alternate Planning Misunderstandings

The 1-2-3 rule applies to forecast weather — not current conditions.

Also, alternate minimums may differ from standard approach minimums.

Some approaches require higher weather minimums when used as alternates.

Reading the fine print on approach plates matters.

Descent Below Minimums

A common misunderstanding:

Seeing “ground” is not sufficient.

You must see one of the specific visual references listed in §91.175.

Additionally, required flight visibility must be met — which may differ from reported ground visibility.

Lost Communications — Timing Matters

If cleared to a fix with an “expect further clearance” (EFC) time, you use that time in determining when to begin descent.

Lost communications procedures are structured to keep you predictable within the ATC system.

Predictability equals safety.

Why IFR Rules Are Structured This Way

When flying IFR in small aircraft:

• You may be in cloud.

• You may not see other traffic.

• You are relying on separation services.

The regulations ensure:

• Standardization,

• Predictability,

• Fuel planning margins,

• Clear decision points at minimums.

They remove ambiguity when visibility is limited.

Looking Ahead

In Article 8 — the final installment of our Part 91 series — we’ll cover:

• Maintenance responsibilities

• Required inspections (annual, 100-hour)

• Pitot-static and transponder checks

• Operation after maintenance

Because even the most precise IFR procedure means nothing if the aircraft itself isn’t legally maintained.

Until then:

File correctly.
Plan alternates carefully.
Respect minimums.
And remember — in IFR flight, structure is your safety net. 🛩️