inhibits on takeoff roll Boeing 737 NG

[salamanderpress]

Some cautions and warnings are inhibit during takeoff roll possibly to avoid spooking pilots into an unnecessary high speed abort.

Quoting FCOM II, Warnings chapter
"During takeoff and landing, new predictive windshear caution alerts are inhibited between 80 knots and 400 feet RA, and new warning alerts between 100 knots and 50 feet RA."

I know from experience in the sim that Vibration exceedance is also inhibit. Any info on what else is similarly inhibit. Seems to me that any and every warning of failure NOT warranting a high speed reject should be inhibit between 80 knots and 400 ft when you shouldn't be doing anything about it anyway? But I haven't been able to access any authentic Boeing documentation except for the PWS alets quoted above.

In a larger vein, every design feature built into the aircraft is the result of deliberate design effort and a lot of dollars in procurement and maintenance. If we could access the source of the requirement, ie Operational Specifications, it would be much easier to understand and intelligently discuss why some things happen the way they do or do not.

Would love any discussion and information on this. Is there a "mother-lode" of Boeing documentation somewhere which tells us why each feature was designed and what improvements were made in its operation between the -200 to the NG.?

[parkfell]

AP engagement

In a similar light, AP engagement on the NG is permitted on departure
when 400ft+. Earlier models had a higher figure, eg. -300 when 1,000ft+.

I believe that prior to this series, after departure the flaps had to be up prior to AP engagement. Was this a trimming issue?

Perhaps an "old hand" would like to amplify.........??

[Denti]

At least in the classic (300/500 i know first hand) flaps up was not a requirement. Dunno about the jurassic ones though (-100/-200).

[BOAC]

Don't think so - it was just a 'point' at which A/P use was considered to be 'OK' ie when clean.

[salamanderpress]

As far as I can figure, AP engagement is NOT inhibited, just recommended to be used only above Minimum Use Height. If you engage it on the ground it will probably engage and move the controls about as you roll for takeoff.

If anyone has info on the INHIBIT feature, I'd really like to have it.

Thanks.

[CaptainSandL]

There is no “INHIBIT” feature but many warnings & indications are inhibited at various times, usually near the ground. Here is a non-exclusive list:

• PSEU indications are inhibited from when the thrust levers are stood up for T/O until 30secs after landing.
• Icing advisory lights are inhibited on the ground.
• The ELEC light is inhibited in-flight
• Max cont EGT limit indication turning amber is inhibited during t/o & go-around for 5/10 minutes.
• Blinking oil filter bypass ind is inhibited on takeoff from 80kts to 400ft/30secs and also on landing.
• Lots of speed tape stuff eg min manoeuvre speed, approach AoA, stick shake speed all inhibited on T/O.
• Fuel IMBAL warning inhibited on ground
• Descend RAs below 1100ft / any RAs below 1000ft
• TCAS voice alerts below 500ft

Re “Is there a "mother-lode" of Boeing documentation somewhere which tells us why each feature was designed and what improvements were made in its operation between the -200 to the NG.?”. You wont get this from Boeing, however the genealogy and development of the 737 is a speciality of mine, you can find my work on the net and in print.

[BOAC]

Since the Captain is reluctant, may I do it for him?

The Boeing 737 Technical Site

Well worth a visit.

[salamanderpress]

many thanks

Thanks, CapnSandl and BOAC.

I did look up FCOM using "inhibited" and found those references. However, there's no reference to Vibration display being inhibited. I do prowl the website often. And I have the book. It's really useful.

Just thought I'd look for some Operational Requirements which would give us an insight into what the intention behind each feature is.

Thanks again.

[BOAC]

Vibration - not sure if Boeing have recently changed, but they always used to be 'AVM's ie Airborne VMs and were not 'calibrated' for ground use. There were other 'limitations' placed on use of their readings I recall. Have you checked the link?

[ampclamp]

A/P not inhibited per se'
During maintenance A/P engagement is accomplished regularly.There are interlocks of course on gnd or otherewise.

[salamanderpress]

Dear BOAC,

Thanks for the input.

Checked the link. Also checked FCOM. The term used now is Engine Vibration, though I do recall hearing the term AVM too somewhere. No mention of any inhibit conditions, unlike other inhibits which are mentioned in every chapter. (Found it useful to search for specific words in the FCOM using Adobe, so you can examine every single occurence.)

Re autopilot engagement, there are no cutouts on ground, otherwise you would not have to disengage AP after landing off a dual autopilot approach. You can (probably) check Autopilot operation during taxy by going Heading Select and watch the controls try to put on bank as you turn off the heading. Haven't done this myself, but will check it out. Should work as it would on every other aircraft fitted with a similar autopilot function.?

[BOAC]

Indeed not 'inhibited', but from the link
Airborne Vibration Monitors (AVM)

All series of 737 have the facility for AVM although not all 737-200's have them fitted. The early 737-1/200's had two vibration pickup points; One at the turbine section and one at the engine inlet there was a selector switch so that the crew could choose which to monitor. Some even had a high and low frequency filter selection switch.
From Boeing Flt Ops Review, Feb 2003: "On airplanes with AVM procedures, flight crews should also be made aware that AVM indications are not valid while at takeoff power settings, during power changes, or until after engine thermal stabilization. High AVM indications can also be observed during operations in icing conditions."

[salamanderpress]

Thanks BOAC, that was interesting reading.

Would you say that Vib Monitoring was a requirement of the engine manufacturer as a sort of preventive maintenance aid? Perhaps the new engines run rather smoothly when okay but could really go to pieces if something goes wrong, since they're operating rather closer to surge limits than the older JT -8s.?

Look forward to sharing your thoughts on this and similar themes.

[BOAC]

I'm a little out of my depth here, but I would think that AVM values and surges are quite disconnected?

The WHOLE of my link is 'interesting reading'!

[salamanderpress]

True, vibration and surge are disconnected. But a modern engine (operating close to its surge limit for higher efficiency) would need to be more carefully monitored than an older engine (operating significantly below its surge margin).

And the link IS 100 % interesting all through.

Thanks.

[TopBunk]

My (very) old manual for the -200 and -400 suggests that the use of autopilot below 500ft agl after take off is not allowed.