[] SLATS DRIVE

The Slats (as opposed to Flaps) are high lift, leading edge devices designed to increase the lift on a wing that’s designed for high speed / high altitude flight, not this low speed lifting 350 tons off the ground manoeuvre that we insist on performing at the beginning of every flight.

Sometimes they fail and one of the possible failures is a [] SLATS DRIVE message that leaves them stuck in the current position. At heavy weights, retraction of Flaps after a [] SLATS DRIVE on a heavy weight takeoff gets tricky …

Procedures & Techniques : Slats Drive Go-Around

Slats Drive go-around has been shown in the simulator to have unusual flight characteristics. Boeing have confirmed that the simulator is following the same control law as the aircraft, and as such crew should expect the aircraft to behave the same way.

Essentially during a go around after a Slats Drive failure, the AFDS increases pitch rate slowly to a target of about 8-10 while airspeed continues to accelerate through the flap limit speed. The solution is to either disconnect and fly manually, or potentially a reversion to FLCH will restore correct AFDS speed/pitch behaviour.

Engine out Slats Drive Go-Around has the same pitch problem, although the result is less marked owing to reduced performance. Note that the use of FLCH to recover will reduce the thrust limit setting to the CLB/CON limit prematurely, and may compromise go around acceleration/performance.

Procedures & Techniques : Slats Drive & Flap Retraction

When a [] SLATS DRIVE failure occurs during takeoff – particularly at heavy weight – crew need to carefully consider continued flap retraction as an increasing minimum manoeuvre/stick shaker can combine with the flap/slat limit speed to leave the crew with no room to manoeuvre. Either AIRSPEED LOW or Flap Limit speed exceedences are likely. In a clash, the AFDS will exceed the flap limit speed, rather than fly a speed below minimum manoeuvre speed.

Procedures & Techniques : Slats Drive – Do We Extend the Flaps?

There is occasionally some confusion on the flight deck associated with the [] SLATS DRIVE NNM. A Slats Drive failure leaves the Slats unavailable to the crew. While not necessarily mechanically correct – you can think of this failure as the detection of an asymmetric Slat extension and therefore the FSEU has shut down the affected system to preclude further asymmetric control surface deployment. This is one reason why Alternate Flaps – which overrides the FSEU for both Flaps and Slats – is specifically forbidden.

Having been told they must not use Alternate Flap extension, Crew are sometimes reticent to extend Flaps (not the Slats) through the normal Flap lever. However, a visual review on the PFD of the nominated QRH NNM approach speed (Vref 30+30) confirms the need for additional flap extension for landing. The checklist also says “Use Flaps 20 and … for landing.”

[] FLAPS DRIVE

The Flaps (as opposed to Slats) are high lift, trailing edge devices designed to increase the lift on a wing that’s designed for high speed / high altitude flight, not this low speed lifting 350 tons off the ground manoeuvre that we insist on performing at the beginning of every flight.

Sometimes they fail and one of the possible failures is a FLAPS DRIVE message that leaves them stuck in the current position. When you run the Non Normal (NNM) checklist – the question is asked of the crew; what position are they in? This should be a simple question …

Practices & Techniques : FLAPS DRIVE – Where are the Flaps?

In the event of a FLAPS DRIVE failure, the checklist asks the crew to confirm the position of the Flaps. This sounds simple, but can be made complex by the situation … and the crew …

Flaps Drive is the response of the FSEU to either a failure of the flap extension system – or the detection of flap asymmetry. Typically the FSEU doesn’t act to prevent the commencement of an asymmetry – it acts to contain the increase of a detected asymmetric condition. Asymmetry can be caused by either:

  • Failure of the flaps to respond to a command to move; or
  • Failure of the flaps to respond to a command to stop moving.

When asked to look at the position of the Flaps, crew have several things they could consider:

  • Expanded EICAS Flap Position indicator;
  • Flap Lever position (combined with where the lever was before the failure occurred);
  • PFD Flap Speed Limit indication.

This last one bears a little discussion. The speed limit indication markings on the PFD (red square dots at the top of the speed tape) indicate the flap limit speed for the current flap position (not current flap selection). The sensors for the left and right indication limit speed detections take their inputs from different flaps; hence it’s possible during a Flaps Drive failure to have two different speed limit indications. This the PFD speed limit can be another indication of Flap position.

Question : Which indicated speed limit should you respect?

Answer : Both of them, of course.

Scenario :  Takeoff at Flap 15, Failure occurs during retraction to Flap 5. Where are the Flaps?

A good look at the expanded Flap Indicator gives the information required.

Magenta “5” indicates the commanded flap position by the flap lever is Flap 5 – the magenta colour indicates that the flaps are NOT at 5. If the flaps were detected at commanded position, the indication would be green. Amber indication reflects the existence of the current Flaps Drive failure. The Flaps aren’t going anywhere.

During takeoff, the Flap Lever was at 15, and now it’s at 5. Therefore the flaps are somewhere between 15 and 5. Unless …

If the failure was uncommanded movement – that is, the flaps were retracted to 5 and one of the flaps failed to respond to the command to stop moving – where would that flap be now? Would this flap position at less than 5 be indicated on the expanded display? Hmmm.

Finally – in this particular scenario, one of the PFD speed limits reflects 245 knots – Flaps 5; the other reflects a higher speed – Flaps something less than 5.

Solution :  The checklist doesn’t offer you Flap 5. It says “5 or less” and this is the correct response. Speed selected is based on Flaps Up and you’ll fly your approach with the excess speed associated with a midrange slats only extension.

The (probably) incorrect choice of “between 5 and 20” has you on approach assuming Flaps 5, which may be slightly slower than the minimum recommended speed – but it in all likelihood well within the margin of safety.

VNAV Non-Precision Approach

VNAV is the Mode-of-Choice for Non-Precision approach. The highest level of automation; lowest flight deck workload, and a managed flight path to the runway.

Mostly.

Continuing in my process to convert 10 years of powerpoints into training videos – see the Infinidim VNAV NPA Training Video on YouTube – also on the Infinidim Training Videos page.

Ken.


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