September 29, 2014

How You Can Still Fly Safely With Broken Plane Parts: Part 1

Editor’s note: This is the first installment of a two-part series. Read part two here.

Relax. Everything will be just fine.

Relax. Everything will be just fine.

As you relax sipping ginger ale on your next flight, here’s something to ponder: the airplane you are riding in might have stuff broken. It might even have parts missing! Starting to lose that warm and fuzzy feeling? Cruising through the sky at close to the speed of sound and the airplane has maintenance problems? Don’t worry; we fly with stuff broken or missing all the time. Here’s how we do it while keeping you perfectly safe.

The Logbook

The most important document in an airplane is its maintenance logbook. The logbook is a record of every mechanical problem (discrepancy) and maintenance function performed on an aircraft. Everything goes in the logbook. If a light bulb burns out, we write it in the logbook. This is called an “open” discrepancy. When a maintenance technician fixes the light bulb, the repair is entered in the logbook and the discrepancy is now “closed.” Oil changes, tire changes, engine changes, loose fasteners, even a broken toilet – they all get logged in the logbook.

When Something Is Broken At The Gate

Commandment number one for airlines is “Thou shalt not depart with an open discrepancy in the logbook.” This is a really good rule. No one wants to ride in a jet until a mechanic gives it the okay.

Problems discovered during preflight checks vary in complexity. The following are three situations I’ve run into.

Situation #1: A Missing Frequency Selector Knob

VHF Radio Frequency Selector Knob

VHF Radio Frequency Selector Knob

Sitting at the gate, we notice a VHF radio frequency selector knob has fallen off a radio. We have to log the problem in the logbook and we can’t depart until we do something about it. The next step is to call maintenance. The mechanic promptly arrives with a new selector knob and fixes our radio. He or she then updates the logbook and “closes” our open discrepancy. Perfect! Time for an on-time departure.

Uh-Oh. Better Grab The MEL

What happens if the mechanic doesn’t have a spare selector knob? Are the passengers out of luck?

When maintenance can’t fix the problem, we reach for a really fat book called the Minimum Equipment List or MEL. Airliners have a lot of redundancy. When something can’t immediately be

fixed, there is a very good chance we still have enough back up systems to safely depart. The MEL tells us what stuff we are allowed to have broken or missing before we depart.

Sure enough, the MEL has an entry for a VHF Frequency Selector Knob. This particular aircraft has three VHF radios with two selector knobs on each radio. We have a total of six frequency selector knobs (how’s that for redundancy?). The MEL says only two knobs are required for a safe flight, so we are in luck!

Example of an aircraft Minimum Equipment List -- VHF Frequency Selector Knob

Example of an aircraft Minimum Equipment List — VHF Frequency Selector Knob

Maintenance will put an entry in the logbook saying that the broken selector knob is covered by the MEL and the aircraft is healthy and ready to depart. Does the broken knob ever need to be fixed? Yes! The MEL example above says: Repair Int (interval) Category: C. This means that the problem must be repaired within ten days or the aircraft is grounded. This prevents airlines from letting airplanes slowly fall apart without repairing them.

Some Issues Aren’t Quite As Easy

Our frequency selector knob problem was simple. Occasionally, we’ll come across MEL items that have restrictions associated with them. We can go with certain parts broken or missing as long as we don’t need them due to weather or other flight conditions.

Situation #2: A Broken Windshield Wiper

Here’s a problem I had a few years ago. During our preflight inspection, we noticed the rubber strip had started to peel off of a windshield wiper. Our flight was heading to Atlanta where the forecast was calling for a chance of rain. Take a look at the windshield wiper MEL entry:

Example of an aircraft Minimum Equipment List - Windshield Wipers

Example of an aircraft Minimum Equipment List – Windshield Wipers

Don't need wipers (unless it's raining).

Don’t need wipers (unless it’s raining).

The MEL says we aren’t required to have windshield wipers unless there will be precipitation (rain, snow, mist, etc) within 5 miles of the departure or destination airport. Atlanta was expecting rain, so the MEL restricted us from going unless the wiper blade was repaired.

It was close to departure time. If maintenance attempted to replace the wiper blade, we would likely depart late. Our dispatcher came up with a clever solution. We switched airplanes with the Phoenix flight. In airline lingo, we call this a “tail swap.” There was no rain forecast for Phoenix, so the airplane could legally fly there with a broken windshield wiper. We flew the plane with the good wipers to Atlanta where it rained cats and dogs.

Situation #3: Who needs air conditioning?

On airliners, the air conditioning system not only controls the temperature of the cabin, but also pressurizes the aircraft. Here is an example of how we are allowed to fly with a major system inoperative as long as we follow the restrictions. Our example airplane has two air conditioning packages (or packs).


Example of an aircraft Minimum Equipment List - Air Conditioning Packs

Example of an aircraft Minimum Equipment List – Air Conditioning Packs

There are two important restrictions for flying with an inoperative air conditioning pack. The first is that we must remain at or below an altitude of 31,000 feet (FL310). The packs pump air into the cabin to keep the interior pressure at a safe, comfortable level for the passengers and crew. One pack can provide enough pressure to keep us happy as we cruise up to 31,000 feet. Above that, we need both packs to keep the cabin properly pressurized.

31,000 feet is plenty of altitude for an airliner to fly safely, but a higher altitude would be more fuel efficient. Higher altitudes also give us more flexibility when flying near bad weather.

The second MEL restriction states that we must remain within 60 minutes of a suitable airport at all times during our flight. Here’s why: we only have one pack to keep us pressurized. If the remaining pack should fail, the cabin will slowly depressurize. We’ll need to descend to 10,000 feet where the atmospheric pressure is high enough to provide us with adequate oxygen. Jet engines burn large amounts of fuel at 10,000 feet so we probably won’t have enough gas to make it to our destination. We’ll need to find a suitable airport nearby to land.

Descending to 10,000' over mountains could be problematic.

Descending to 10,000′ over mountains could be problematic.

There’s another problem to consider if the working pack fails. What if we’re over the Rocky Mountains that have peaks as high as 14,000 feet? We might not be able to safely descend to 10,000 feet. Because of this situation, it’s unlikely this aircraft would be dispatched to fly over high terrain. Atlanta to Chicago on a nice afternoon, no problem. Atlanta to Los Angeles? No thanks.

What if a broken item isn’t listed in the MEL?

If the broken or missing item isn’t in the MEL, then maintenance must fix the problem or the airplane can’t leave. When airplanes are at major hubs, parts and equipment are readily available and most repairs can be made with little or no delay. Sometimes the problem can’t be fixed before the scheduled departure time. The airline may be able to switch the aircraft with one being used on a later flight to buy some time. If the problem is serious enough, it may cause a delay or even a cancellation. No one likes to have their flight cancelled (airlines, pilots, flight attendants, and mechanics hate doing it), but that’s part of having a safe air transportation system.

What If Something Breaks In Flight?

When stuff breaks in flight, we follow different procedures, and we may have some tough decisions to make. Read part two at 


[The MEL examples used here, although based on actual FAA MMEL entries, were created for this article as general examples and are not FAA approved.]

Ken Hoke has been flying for over 30 years. He’s currently a Boeing 767 captain flying international routes for a package express airline. In his spare time, he writes the AeroSavvy blog. Follow Ken on TwitterFacebook, Tumblr or Google+.


About the Author

Ken Hoke
Ken Hoke has been flying for over 30 years. He’s currently a Boeing 757 & 767 captain flying international routes for a package express airline. In his spare time, he writes AeroSavvy



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  • TJ

    As a passenger I’ll happily wait as long as it takes to make sure the plane is 100% ready to fly.
    Miss the flight, big deal, I’ll get another. Anyone who is not happy about that
    should take a bus. Pretty darn hard to have mechanics get outside at altitude
    and make a repair or replace a part.

    Another really interesting article.