Why Go-Arounds Go Wrong Alex Fisher GAPAN UKFSC July 2013 1 And - - PowerPoint PPT Presentation

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Why Go-Arounds Go Wrong

Alex Fisher GAPAN

UKFSC July 2013

Some

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And Finally.....

• Trim is not exactly the most complicated part of the aircraft,

and yet misunderstandings about it continue and have a significant effect on safety.

• How many other similar basic aerodynamic knowledge

failures are there, things we think everyone knows, but they don’t?

• e.g. How many people knew that stalling angle was a

f(Mach No) before AF447?

• By the way, it doesn’t help that the usual first demo of the

causes of lift are complete c**p

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Why are Go Arounds difficult?

• Low altitude, no room for error
• High thrust, typically maximum
• Underwing engines → pitch-up
• Low speed → low aerodynamic forces
• Thrust is (nearly) independent of

speed

• Elevator power is α(speed)2
• You need about twice the elevator at

140 kt you need at 200kt

• Result: large trim changes
• Conclusion: you had better know your

trim system!

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So Do You Understand your Trim?

• There are (at least) two basic different trim systems
• 'Conventional Trim', most basic trainers, C152,

PA28 etc, but also Lockheed TriStar, Concorde

• 'Tailplane Trim', all Boeings since the 707, all DCs

since DC8, all Airbus, but also Piper Cub and Mooneys

• These differences are rarely mentioned and are
• ften simply not understood.

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Conventional Trim 1

• 1. Initial Condition

Control Column Trim Wheel

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Conventional Trim 2

• 1. Initial Condition
• 2. Nose-down elevator

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Conventional Trim 3

1.Initial Condition 2.Nose-down elevator 3.Nose-down trim

• Column load is now zero
• Effect of trim is to

relieve load

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Tailplane Trim 1

• 1. Initial Condition

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Tailplane Trim 2

• 1. Initial Condition
• 2. Nose-down elevator

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Tailplane Trim 3

• 1. Initial Condition
• 2. Nose-down elevator
• 3. Start to trim

Both tailplane and elevator rotate, so to keep total tail load the same....

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Tailplane Trim 4

• 1. Initial Condition
• 2. Nose-down elevator
• 3. Start to trim

Both tailplane and elevator rotate, so to keep total tail load the same....

• 4. Start to relax column

back to neutral

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Tailplane Trim 5

• 1. Initial Condition
• 2. Nose-down elevator
• 3. Start to trim
• 4. Start to relax column

back to neutral

• 5. Trimming complete,

column back to neutral, tailplane and elevator in line

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Trim summary

Conventional Tailplane

Column is held in position during trimming Column has to be relaxed during trimming Trimmed column position reflects aircraft state: Forward high speed Back Low speed Column is always in neutral position when in trim Trim only relieves column load Trim does not itself relieve column load but does provide additional load path

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What type of trim do I have?

Some all moving tailplanes are trimmed conventionally – the Trident’s was commanded by both the column and the trim wheel; the ‘elevator’ was a geared tab Trimming tailplanes aren’t confined to the jets – this Piper Cub has one, as shown by the slot near the tailplane l.e.

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Resulting Errors 1

• Failure to realise that if elevator is inadequate,

additional power is available from the tailplane (if you think the trim is just there to relieve column load, you might not realise it actually provides more control power)

• A300 Taipei www.airdisaster.com/cgi-

bin/view_details.cgi?date=02161998&reg=B-1814&airline=China+Airlines)

• Pitch up incident B737 at Hurn

www.skybrary.aero/bookshelf/books/875.pdf

• 747 Take-off incident LHR

www.aaib.dft.gov.uk/sites/aaib/publications/formal_reports/1_1995_g_bnly.cfm

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Resulting Errors 2

• Failure to note tailplane position; if it is already fully

nose up (typically following autopilot disconnect at low speed), there is no way the elevator will contain the pitch up from full power from underwing engines

• B737 Amsterdam www.skybrary.aero/bookshelf/books/1175.pdf
• A320 Perpignan www.skybrary.aero/bookshelf/books/1343.pdf

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Resulting Errors 3

• Failure to realise that loss of control in pitch is due

to independent operation of the tailplane, typically due to CWS or envelope 'protection'

• A310 upset at Orly (initially then..?) flap speed

protection) http://www.bea.aero/docspa/1994/yr-a940924a/pdf/yr-

a940924a.pdf

• A300 accident Nagoya (inadvertent activation of go-

around switches) http://www.skybrary.aero/bookshelf/books/808.pdf

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Resulting Errors 4

• Failure to understand the trim process itself, with

resulting in severe out of phase control inputs.

• Personal Observation
• B757 at Oslo.www.rnf.is/media/skyrslur/2002/Flugatvik_TF-

FIO_vid_Gardermoenflugvoll_22._januar_2002._(Endurutgafa).pdf

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Conclusion

• Trimming which seems such a simple

process is actually full of pitfalls

• Most pilots are never taught that

differences exist between types, and many never figure it out for themselves

• Possibly because they always follow

up any elevator input with small trim inputs

• Go around, with its large trim changes
• ften shows this lack of knowledge

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