B737 Engine handling - Flight in Moderate to Heavy rain

[Tee Emm]

B737-300 FCOM (Supplementary Procedures) Page SP.16.18 (date December 2006) under the heading "Moderate to Heavy Rain or Sleet" mentions that flight should be conducted to avoid thunderstorm or hail activity by overflight or circumnavigation and that where possible moderate to heavy rain, hail or sleet should also be avoided.

It states if moderate to heavy rain, hail or sleet is encountered the engine start switches should be selected to CONT, the AT disengaged, and thrust levers should be adjusted slowly. It also says "maintain an increased minimum thrust setting"

The latter advice has me tossed. Request opinions (facts?) on this statement. Is there a figure in the FCOM that defines "increased minimum thrust setting" in relation to moderate to heavy rain, hail, or sleet?

[ClimbSequence]

Back in 1988 a Taca's 733 Experienced a dual engine flameout while descending in heavy precipitation with both thrust levers close and A/T on. An AD was issued after this incident labeled AD 6-14-88. It restrict the use of autothrust and recommends a minimun of 45% N1.
Hope this helps!

[lomapaseo]

I'm not sure just what values are intended but I think the rationale behind them is to :

1) Raise the fan RPM to incease the rain/sleet defelection cabilities of the spinner shape and the fan blade roots. so as to spit most of the stuff out the fan discharge

2) Raise the operating line so that it will make it less likley that the N2 will roll off while processing the rain/hail for the burner

3) should the combinations above not work (for the level of threat encountered) the igntion may recover the engines before they spool down too far

You really don't want to let some aircraft black box spool the engines down while they are eating their full of rain/sleet. At the same time you don't want to be commanding bodie accel/decels that might squeeze you against a depressed surge line while in rain/sleet.

[stator vane]

old system---

used to have a placard, to maintain minimum 45% N1 in icing conditions to prevent the ice from going into the core--newer engine mods automatically raise the idle when anti-ice and ignition switches are on, in addition to the redesign of the cone on the fan. we called the different engine cones--Dolly Partons (rounded) or Madonnas (pointed)

whichever mod your aircraft has, should either be placarded to telll you to keep 45 N1 or not.

so must have been the same idea for the heavy rain--to propell the heavier rain drops outward away from the core.

[cwatters]

It takes a lot of energy to boil water - perhaps it's as simple as not enough heat being produced at idle to stop the flame being put out.

[Tee Emm]

--newer engine mods automatically raise the idle when anti-ice and ignition switches are on, in addition to the redesign of the

My understanding is that the minimum of 45%N1 was the quick fix until the modification to flight idle came into vogue. This ensured ground idle was around 22% N1 and in flight idle became around 32% N1. Once the new flight idle was introduced, the automatic increase in N1 associated with the start switches or engine anti-icing switches on, was no longer needed. Nor was 45% N1.

I assume therefore that the current reference to minimum N1 in moderate to heavy rain etc is the manufacturer covering the case where the old 45% N1 advice is still valid. But I cannot be certain of that - hence the initial query.

[stator vane]

newer engine mods

would include the change of the spinner dome shape as well as the auto increase of the N1's.

only passing on what i find in the various books i have and from a faulty memory of too many hours on the 737.

[None]

"Tee Emm wrote: It also says "maintain an increased minimum thrust setting"
The latter advice has me tossed. Request opinions (facts?) on this statement."


My recollection on how this was explained to me concerns the amount of air an engine can process at given thrust settings. This explanation was simplified so that I can understand it...others will be able to explain it better.


At high thrust settings, the engine can process a lot of air without inhibiting much or any from entering the intake. At low thrust settings (and higher speeds), some or a lot of air is forced to bypass the inlet because it is "full."
Now add in rainwater. At low thrust settings, you would want some or a lot of the rainwater to bypass the inlet, just like the excess air. But it does not behave like air (it can't make the turn like air can) and most of the rainwater will enter the engine.


At higher thrust settings, I picture the engine being better able to handle the increased air and the rainwater. At low thrust settings, I picture the engine as being closer to a possible flameout due to the rainwater entering the intake. The low thrust setting and high airspeed would represent the least favorable scenario, so as a technique (or procedure for some types) I prefer a higher thrust setting along with continuous ignition.

[lomapaseo]

None

Ah spillage effects or not...... makes sense

Thanks