Failure of Airplane Flaps

Have you discovered failure of airplane flaps before take off? Or maybe mid flight. What are the probable causes and what’s the best course of action? Read on..

Introduction – Failure of Airplane Flaps

If you discover flap failure, you’ll need to troubleshoot. It’s important you know what the causes may be, as well as how to control the aircraft when it happens. So, when are you likely to discover the flaps have failed, what makes a flapless circuit different from a normal circuit and in what other situations might you choose to land flapless? Let’s unpack it here.

Aircraft flaps

Aircraft flaps are a secondary flight control which are used to reduce the normal airspeed at which aircraft take off and land, as well as to help reduce the pitch attitude on approach. The reduced airspeed on take-off or landing helps reduce the landing or take-off ground roll, letting aircraft fly into shorter strips, or conversely take off with higher weights or in more limiting conditions (i.e. higher elevation airstrips, or in higher temperatures.)

Failure of Airplane Flaps
Each unique type of flap has a different type of mechanical operation and affects airflow differently

For some aircraft, the use of take-off flaps can also help with obstacle clearance, and controllability during asymmetric conditions (engine failure on multi-engine aircraft)

There are different types of flaps, but most airplanes use;

  • Plain flaps
  • Fowler flaps
  • Split Flaps
  • Slotted flaps

Each unique type of flap has a different type of mechanical operation and affects airflow differently. This means it’s important not to overly generalise the use of flaps, and to ensure you’re following your pilot’s handbook.

When are you likely to discover that your flaps aren’t working?

Most likely when the flap deploys as part of the configuration checklist in an established approach, or downwind or base leg, or even on final approach when flap has been selected. 

In most cases, it shouldn’t be a major issue. Go around and orbit above the circuit and troubleshoot using your flap failure emergency actions – which often means matching flap selector to the observed flap position (i.e. if they don’t deploy, select flap to the up position).

It may simply be a circuit breaker has popped, or the selector switch needed caressing. However, if they are stuck up, or deploying assemetrically, then calculate a higher threshold speed, put the selector switch back to match the actual flap position, confirm that you have a nice long runway for a flapless approach, or consider diverting to a more suitable destination. 

This ATSB report outlines the actions of a C206 pilot with flap failure: https://www.atsb.gov.au/publications/occurrence-briefs/2018/aviation/ab-2018-032/

” At about 1045 Eastern Standard Time, as the aircraft was on approach to Dimbulah, with flaps selected to 20 degrees and at 80 knots, the flight crew heard a loud clunk. The flight crew thought that they may have struck a bird and discontinued the approach. They commenced climbing and found that they required significant right aileron to remain tracking straight. Once at a safe altitude, the crew raised the flaps in stages.

The crew diverted the aircraft to Mareeba to conduct a flapless straight-in approach. As the aircraft slowed during the landing roll, the flaps extended towards 20 degrees. The flight crew immediately conducted a go-around, which allowed them time to consider the implications of the technical failure and the opportunity to conduct a diversion to an airport where appropriate emergency response facilities were available if required.

The operator inspected the aircraft and found several issues, including:

* Failure of the synchronising rod at the rod-end

* Disconnection of the transmission worm drive between the actuating tube and the collar

* Damage to the preselect cable clamp

* Damage to the right flap track

* Failure of the right centre aft roller. “

ATSB Flap failure involving Cessna U206, Dimbulah, Qld, on 6 March 2018

If flap failures happen during your pre-flight or pre-takeoff checks, then you don’t want to get airborne with this issue. It’s likely to be a mechanical, electrical or possibly a hydraulic issue that warrants further investigation by an engineer.

Failure of Airplane Flaps
In most cases, flap failure shouldn’t be a major issue

If you’ve discovered a problem after takeoff (and they don’t come up), then those same concerns are likely to see you stay in the circuit, land safely and seek more engineering assistance. It’s unlikely that you will continue with your planned flight due to the increased fuel burn, reduced air speed and reduced climb rate, but if your airfield had been over run by enemy combatants then consider these limitations. It’s also likely that you have reduced G-limits when flap is lowered, but this is unlikely to be an issue in the circuit…

What makes a flapless circuit different from your normal circuit?  

For starters, your ‘clean’ stall speed is higher than your fully configured stall speed (with extended flap), therefore your threshold speed and approach speeds will also need to be higher.  If you haven’t memorized those speeds then start searching your Flight Manual, and for more complex aircraft refer to your SOP which may mean using TOLD cards (take off and landing cards based on aircraft weight and configuration) or the use of a Flight Computer to calculate speeds.

Typically, Flapless approaches are faster, and this increased threshold speed also means that you will need a longer runway length to stop – around a 40% higher landing distance.  The lack of drag means that your airspeed is harder to control, your approach will be flatter (with less obstacle clearance), your nose attitude will be higher (which could make it hard to see) there is a less pronounced flare and a more pronounced roll.

To help set up for this approach, space your downwind leg a little wider than usual – to allow for your increased turn radius at this higher speed.  In fact, extend downwind slightly further than usual, in order to allow for the shallower final leg.  

The lost art of side slipping may need to be utilized (as is used in many aerobatic aircraft which don’t even have flaps to begin with!). Lower one wing with aileron (usually lower the wing into any crosswind if there is any) and press opposite rudder for directional control to remain on centreline – but beware, you now have pro spin flight controls with rudder and opposite aileron, so you must carefully manage your descent attitude, pitch control (elevator control position) and watch your airspeed (which may now be in error due to the sidelip) – always be aware of the stall stick position as you can still stall an aircraft on descent whilst sideslipping.

The increased drag by presenting the fuselage to the slipstream will increase your rate of descent. Ask any Cub or Extra pilot for tips on how to do this, or do some tail wheel training in a similar type with a qualified flying instructor.  

Failure of Airplane Flaps
As a pilot, your priority is keeping the aircraft in an airworthy condition and make educated decisions to be able to land safely

 When else might you choose to land flapless?

Maybe you’ve had a structural issue after hitting something (like a tree on takeoff, or another aeroplane wing during formation, or even a major bird strike). A friend of mine hit a large bird, and upon lowering his flaps discovered that only one lowered, and the other remained up. The resultant aerobatics would have proved fatal if he was lower.  He wisely undid his flap selection and raised them, simply landing flapless. The bottom line is that you shouldn’t risk reducing the structural integrity of the wing following any impact. Your priority is to keep the aircraft in an airworthy condition, and make educated corrections and decisions in order to land the plane safely, or as safely as possible. One other issue with wing or tail damage is the likelihood that the aircraft won’t fly as efficiently as you are used to, and you may even need to add a few more knots to retain flight control. 

Assymetric flap deployment

A potentially more dangerous flap failure is when one flap deploys whilst the other flap stays up, or they deploy in a split flap condition (for example, one at take-off position and one in the landing position).

This will be fairly self-evident due to controllability issues and the roll encountered, it may take substantial opposite rudder or aileron input to maintain directional control; this is why the boldface is often to select the flap situation back up (or to match to flap position).

How do flaps work?

 You don’t remember how flaps work? Take a look at this article: https://www.boldmethod.com/learn-to-fly/aircraft-systems/how-the-four-types-of-aircraft-flaps-work/

Summary – Failure of Airplane Flaps

 Before a real emergency affects you, be sure to get out there and practise flapless circuits. Check your Flight Manual for the stall speeds and suggested approach speeds, and head out and refresh your flapless circuits. If you’re not confident, then grab an instructor for some dual refresher lessons. 

Failure of Airplane Flaps
Practice practice practice! Practice flapless circuits – ask for help if you need it!
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Jorgo

Michael Jorgensen is a specialist formation instructor and Australia's premier air to air formation action photographer, based in Sydney, Australia. Jorgo has a wealth of experience, stemming from his career as a military fast jet pilot, and heavy air-to-air refuelling tanker pilot flying for both the New Zealand and British Air Forces. Find out more

Jorgo has 22 posts and counting. See all posts by Jorgo

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