Some Why Go-Arounds Go Wrong Alex Fisher GAPAN UKFSC July 2013 1
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 2 2
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! UKFSC July 2013 2 3
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 often simply not understood. UKFSC July 2013 4
Conventional Trim 1 Control Column Trim Wheel 1. Initial Condition UKFSC July 2013 5
Conventional Trim 2 1. Initial Condition 2. Nose-down elevator UKFSC July 2013 6
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 UKFSC July 2013 7
Tailplane Trim 1 1. Initial Condition UKFSC July 2013 8
Tailplane Trim 2 1. Initial Condition 2. Nose-down elevator UKFSC July 2013 9
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.... UKFSC July 2013 10
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 UKFSC July 2013 11
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 UKFSC July 2013 12
Trim summary Conventional Tailplane Column is held in position Column has to be relaxed during trimming during trimming Trimmed column position Column is always in reflects aircraft state: neutral position when in Forward high speed trim Back Low speed Trim does not itself relieve Trim only relieves column column load but does load provide additional load path 13
What type of trim do I have? Some all moving tailplanes are trimmed conventionally – the Trident’s was commanded Trimming tailplanes aren’t by both the column and the confined to the jets – this Piper trim wheel; the ‘elevator’ was Cub has one, as shown by the a geared tab slot near the tailplane l.e. 14
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®=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 UKFSC July 2013 15
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 UKFSC July 2013 16
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 UKFSC July 2013 17
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 UKFSC July 2013 18
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 often shows this lack of knowledge UKFSC July 2013 19
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