flight instrument check during taxi-out: EFIS vs. older directional gyro type
 

Can you experts out there please say whether the design of the modern flight instruments such as even the low end EFIS systems found on the ATR42 still makes it necessary to do the same instrument check during taxi-out for flight as it has always been with the older generation type (e.g. directional gyros and the turn and slip indicator) thus: "turning left - ball to the right, compass reading decreasing...turning right - ball to the left,compass reading increasing...etc". A little explanation would be appreciated.

Newer EFIS systems make such checks superfluous. Generally the left and right systems contain comparator circuitry and any difference in parameters between the systems will generate a warning on the screen. Some of the parameters and their triggering differences include Pitch – five deg. Roll – six deg. Heading – 12 deg. Localizer – half dot. Glideslope – three quarter dot. IAS – 5 knots and Altitude – 200 feet. Of course the balance ball may still be mechanical and require checking!

Im on the 737-800. The EFIS system on that plane is certified for Approach Navigation. Basically, the Map Display is as good/better than any conventional navigation display. If you in a 10 mile scale or less, and the airplane touches the magenta line, you're good.

For takeoff, we check airplane symbol touches the runway and headings are aligned with the takeoff runway, you're good to go.

During preflight, we do check other traditional stuff. but, those checks strike me as "Ancestor Worship".

I believe the JAR's (LPC) require a preflight instrument crosscheck. In Ryanair we check all piotot/static driven instruments plus Heading (via ADIRU's) before pushback. Also RVSM (maybe not in the ATR)requires certain altimetry tolerances which may be detected with a good instrument crosscheck. There is a good comparator system on all new Boeings that will produce alerts to the crew if instruments are in disagreement for longer than 5 seconds.

With us on the 737-700 an instrument crosscheck is carried out before pushback, and only the standby intruments checked on taxi-out.

Those worshipper of the magenta line should be wary of the phenomenon called "map shift"!

The FMC position is calculated using inputs from the IRS and from ground navaids. In some cases, ground navaids give erroneous distance/bearing information and can confuse the FMC thinking it is somewhere else because the FMC gives more "weight" to the navaid computed position.

Seen this happen a few times myself... with mapshift ranging from 1 Nm to 12 Nm. Smaller errors may be more insidous as they are more difficult to detect but can still put you between a rock and a hard place!

I was told the other day by a very reliable source that the largest navigational deviation so far by a 757 due to map shift is 60 miles!!! Back that EFIS info up with all forms of raw data boys.

Ah - more nonsense from the Nintendo generation! Why on earth not check instrument response during taxying? It doesn't cost anything - even a low cost couldn't complain at that!

Sorry BEagle, usually agree with most of your posts but not this time.

At JFK, at night, for example, narrow taxi ways, badly lit etc. on a 747-400 I would rather my F/O was watching where he was going than giving me the old 'needle left, ball right' etc.! The system copes very well with any discrepancies. Only required in the SIM for an IR by some authorities.

Regarding map shift before getting airborne, hitting the TOGA buttons on the take off roll should cure that on a -400, other FMCs have a Position Update feature I believe?

Not suggesting that you do a "Needles left, balls right, 8 decreasing, 3 erect etc" litany! Our SOP is merely to include a positive challenge of 'Flt Instruments' to which the response is 'Checked'. There are surely enough natural curves on a taxiway to verify that without the need for Chipmunk-style weaving!

[ 04 February 2002: Message edited by: BEagle ]</p>

You dont do that PPL cessna training for nothing, and i believe back to the basics is sometimes very rewarding. Take for example a triple IRS faiure. I want to know i can rely on those instruments before i have to use them, and im sure F/O would agree. Its good airmanship to stay one step ahead of the aircraft, for safety precautions predominantly, and if that triple IRS arises after that check on the ground, you are to rely on 'luck', which im sure you all would agree is not a first choice when flying. Certainly not my first!

G.Khan, i would like to back you up on what you said about JFK's taxiways, and that you would prefer your FO to keep his eyes out. I 'spose you have to gamble abit between what chance there is of using the instruments and weather to check them or not.

I wouldn't characterize myself as a worshipper of the "Magenta Line", and am fully aware of the mapshift phenomenon. However, in the 737-800, the cockpit design trend seems to moving away from reliance on ground based Nav-Aids for updating.

This aircraft has dual GPS updating, and the observed accuracy is head-and-shoulders above the 757 & 767. It's certified for Approach Navigation, unlike the 757/767. I've seen for myself the incredible accuracy of an LNAV/VNAV engaged approach down to 50 feet. Over the numbers every time! On a lucky day, the 757/767 might get within 300 feet of centerline.

So there is a distinction to be made between the various EFIS/NAV systems. Consider this....in some 3rd world carribien backwater like Haiti, where Nav-Aid inaccuracies is not surprising, all things being equal..."In Boeing/GPS I trust". That being said, the GIGO Principle is still alive-and-well on NG Boeings. If you observe conflicting information, climb and investigate further. My bet is the human screwed up machine, and not the other way around.

manene, on ATR's not only do you have the EFIS displays, but also clockwork cards, and these need to be checked, as it's quite possible for the related compass system to be working fine, the displays and SGU's to be hunky-dory, but the card to be jammed.

Likewise, you also need to check the standby instruments.

Software errors are, of course, always possible.

Anyone who says "You don't need to check modern stuff nowadays" is an accident waiting to happen. Don't trust - verify!

Likewise, even in a 747-400 @ JFK, it's quite possible (and prudent) for the pilot doing the taxying to look out and, in normal turns, the other pilot to run through a quick check of the instruments.

I'm with Ghengis. Yes, the check should be done if possible, and my company requires them on the 744 (a throwback from the Classic?) but somewhere like NWK/JFK/SFO/LAX/BOS(or any USA airfield with inadequate lighting/complex taxy routes/narrow taxyways for that matter) where taxying is about the most perilous part of the trip, it should be optional IMHO. The diagnostics should and will highlight any discrepancy.

Map shift is largely a thing of the past with GPS updating. The FMC will take the best position, and this always seems to be the GPS. TOGA updating on the take off roll may not be the best postion - does that still happen with newer systems with GPS update?. in my experience the aircraft database is not the most accurate info. Nearly always, the GPS pos and Jepp plate agree on IRS align, the aircraft database is often slightly out.

Computers are far superior at monitoring parameters than humans. A five second check by a crewmember during taxi is next to useless if you consider that the computer monitors the system from the moment that gyros erect until shutdown. Computed comparisons also check minute discrepancies that may go unnoticed in poor light or during the many distractions enroute to the holding point.

EFIS systems also come with a test button that self-tests some fifteen or so parameters, much like the POST conducted when you switch on your desktop computer! The purpose of such systems is to permit crews to spend more time on other vital actions. Besides if you folk consider the multitude of components that make up an EFIS generated display, including inputs from air data computers, navigation aids, transducers, AC and DC power sources, internal electrical components and circuits, you could spend an hour checking the authenticity of the indications and still not arrive at a positive conclusion.

A check of the standby/emergency system is important and this may be done by visually comparing the information presented to that displayed by the primary system.

I know that it is hard to change from old habits but such a test as turn right, skid left, wings level, numbers increasing does not scratch the surface of EFIS generated information.

Whats lacking is airmanship which the push-button computer generation are either not taught or shrug off as old outdated crap (until they screw up big time but by then there dead alreadey.) But until the Quake generation gets into power and changes everything, all flight instruments are checked for IFR tolerances before TO. Full stop no argument.

A few things our Nintendo set has done in the past on our EFIS 737-300s:

* Take off into IMC with standby ADI u/s ( X flag visible since engine start)

* Take off with FOs EHSI heading 75 deg out since IRS selected NAV

* Take Off with standby ADI upside down! They didnt bother to erect or even look at the instrument!

* Take off with both VORs u/s. Flags visible. A/c was MEL'd with FOs VOR/ILS u/s. Capts VOR failed just after APU shutdown after start. A/C diverted from bad wx destination requiring VOR approach (no NDB. DME only).

The Atari jet-set kids should read up accidents on modern EFIS aircraft and realise the fancey toys can let you down in more ways than one.

[ 10 February 2002: Message edited by: Slasher ]</p>

Slasher - everything you mention should have been picked up before brake-release.. .Mount'in Man/Dan Winterland say it all regarding taxy checks IMHO.

Dan...

Our 747-400 TOGA updating is inhibited with GPS active (FMC logic). I don't know if this applies to your/ all airplane types however.

Rgds.. .Q.

Folks,. .Perhaps I should register a concern here, at what could be regarded as wooly thinking.

Whether it is old vacuum gyros, or whatever through to our latest and wonderfully accurate integrated IRS/FMCS/Autopilot systems, what is it we are trying to do??

Check System Serviceability?. .Check System Accuracy??. .Check Crew Serviceable, and awake??. .All of the above??

SOPs go seriously wrong when individual operators try to produce SOPs that attempt to have common procedures across fleets that do not have a common design era, let alone a common operating philosophy.

The answer: Horses for courses, with particular regards for the manufacturer's recommended practices and procedures.

All applied in a manner dictated by common sense and professional expertise and experience.

The quite amazing reliability of modern equipment rapidly leads to complacency, complacency and lack of professional discipline leads to most of the errors posted here.Ahhh!!.. the Human Factor.

And never let your hand flying capability/ real instrument scan degrade, you only have to look at the incident/accident record to understand why.

Don't forget Murphie's Law is totally unforgiving.

Tootle pip!!

PS The B757/767 was certified to FAA AC 90-45A . .(Area Nav, including approach) and were Cat 111 capable from day one. So what are the new tricks that the new toys can do.

Not much, actually, there has been little substantive change in these certification standards for a long time.The MTBFs have improved greatly, but that's economics, not certification.

And don't anybody forget that there is not much that the current generation do,avionics/autopilot wise, that the L1011 didn't do a long time ago, including a moving map.

And today's question is: What was the first aircraft in day to day airline service with a British airline, that was certified for auto coupled approach/auto land Cat.111.

LeadSled I think it was the Trident in the 60s with BOAC.