Why Airlines Are Allowed to Fly With Broken Parts - And Why It’s Still Safe

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If you believe an aircraft must be 100% perfect to be allowed to fly, aviation will surprise you.

Every day, commercial airliners depart with inoperative systems, missing panels, or disabled components - and they do it legally, safely, and by design.

This is not negligence. It is called controlled unserviceability - a core principle of modern aviation safety.

To understand it, we need to look at the four documents that quietly keep global air transport moving:

  • MMEL

  • MEL

  • CDL

  • NEF

Together, they form the backbone of airline dispatch reliability.


What “Airworthy” Really Means

An aircraft is airworthy when:

  1. It conforms to its approved type design, and

  2. It is in a condition for safe operation

Normally, if something breaks, the aircraft is no longer airworthy.
But aviation doesn’t operate in a perfect world - systems fail, light bulbs burn out, sensors misbehave.

So regulators allow approved deviations from the type design -  but only under strict engineering control.

That’s where MMELs and MELs come in.

MMEL – The Master List from the Manufacturer

Every aircraft type has a Master Minimum Equipment List (MMEL) approved by regulators like the Federal Aviation Administration and EASA.

The MMEL is written by the aircraft manufacturer and answers one question:

Which systems can be inoperative while the aircraft still meets its certified level of safety?

If an item is on the MMEL, it means:

  • Engineers tested the failure

  • Pilots flew with it failed

  • Regulators agreed it is safe

But the MMEL is generic. It does not consider how a specific airline operates.

That is why airlines need a MEL.

MEL – The Airline’s Legal Permission

The Minimum Equipment List (MEL) is the airline-specific version of the MMEL.

It is approved by the aviation authority and lists:

  • Exactly which items may be inoperative

  • What maintenance actions are required

  • What operational limitations must be applied

If an airline has an approved MEL, the aircraft is legally airworthy even with broken systems - as long as all MEL conditions are met.

Without an MEL, even a minor failure can ground the aircraft.

Why Broken Items Cannot Be Ignored Forever

To stop airlines from flying for months with broken equipment, every MEL item has a repair time limit:

Category      Meaning
AMust be fixed within the specific time stated
BWithin 3 days
CWithin 10 days
DWithin 120 days (non-safety items)

The clock starts the day after the defect is recorded.
This ensures faults are controlled, not forgotten.

CDL – When Parts Are Missing From the Outside

Some aircraft fly with external parts missing - fairings, panels, or access doors.

That’s covered by the Configuration Deviation List (CDL).

The CDL does not deal with systems - it deals with aerodynamics.

A missing fairing increases drag, so the CDL imposes:

  • Fuel penalties

  • Performance penalties

  • Sometimes speed or altitude limits

If a missing part is not in the CDL, the aircraft is not airworthy.

NEF – Why a Broken Cup Holder Does Not Ground a Jet

The Non-Essential Equipment and Furnishings (NEF) program separates safety from comfort.

Things like:

  • Broken seat trim

  • Inoperative reading lights

  • Cracked plastic panels

These do not affect safety - but they must not:

  • Block emergency exits

  • Interfere with oxygen or fire equipment

NEF prevents airlines from grounding aircraft over cosmetic issues.


Why Performance and Fuel Are Affected

Every deferred system or missing part changes how the aircraft flies.

For example:

  • Engine anti-ice ON → higher fuel burn

  • Wing anti-ice inop → altitude restrictions

  • Missing fairing → more drag

These penalties are built into the MEL and CDL so flight crews and dispatchers can plan fuel, weight, and performance correctly.

Nothing is guessed.

Everything is engineered.

The Most Important Rule

MEL and CDL apply only until takeoff power is applied.

  • If a failure happens before takeoff → MEL rules apply

  • If it happens after takeoff → emergency procedures apply

That’s how dispatch and flight safety stay separated.


The Big Picture

MMEL, MEL, CDL, and NEF do not reduce safety. They protect it.

They allow airlines to keep aircraft flying without ever crossing below the certified safety level - while ensuring defects are repaired within controlled limits.

This system is why:

  • Flights stay on schedule

  • Aircraft remain safe

  • Global air transport keeps moving

It is not about flying broken airplanes. It is about flying engineered airplanes.

Follow Aviation2Day for more insights. ✈️

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