Jump to content

Sign in to follow this  
Guest Nightflight744

Descent using FLCh

Recommended Posts

Guest Nightflight744

Hi guys,when descending using the FLCh mode thrust should be automatically set to idle and the speed is regulated by pitch. Unless the speed is set manually so high that more thrust is needed it should remain at idle. That

Share this post


Link to post
Guest s_L_Y_F_a

at high altitudes 40% idle should be normal

Share this post


Link to post

Easy check.....do the same thing. After it has stabilised in the descent and the N1 is holding whatever value, turn off the ATS and press N1. If it remains constant or nearly constant you know that the % is correct. As the matter of fact 41% isn't even that hign. I've seen higher. If I remember the higher you are, the higher the base N1% will be. As you descend it will slowly decrease to around 30%.Xander


Xander Koote

All round aviation geek

1st Officer Boeing 777

Share this post


Link to post
Guest Nightflight744

Hi Ben,Anti-ice was off as far as I recall. Does anti ice increase N1?Fabian

Share this post


Link to post
Guest Nightflight744

Hi Xander,thanks for your suggestion! What stands ATS for?? Autothrust?Fabian

Share this post


Link to post
Guest Clansman

>Hi Ben,>>Anti-ice was off as far as I recall. Does anti ice increase>N1?>>FabianI'd say yes, since anti-ice demands bleed air... does anyone have a quantitative answer?Cheers,Pedro Venda.

Share this post


Link to post

You can always test it in PMDG. Already went to page 58 in the manual. There's no specific mention of raising the pressure, hence N1. For Nacelle Anti Ice I doubt a problem exists since it probably takes air directly from the HP port. The whole system is contained inside the engine. There is a limitation however of selecting N1 higher than 50% if in icing condition for more than 10 minutes on the ground.I don't know how the 747's WAI system is. Probably takes bleed from a common manifold. On the MD-80 we had a minimum anti-ice pressure limitation, and anti-icing temp and press caution lights. Since I don't think the 400 has these cautions it will either have more than enough bleed press and temp from those high bypass fans, or there's a system to automatically increase N1 to maintain proper pressure.I lean towards the latter since the FMC wants to know if you're using anti ice during descent or not. Probably means that with AI ON more power is pallied to idle N1, hence a quicker TOD.Xander


Xander Koote

All round aviation geek

1st Officer Boeing 777

Share this post


Link to post

Yes the use of anti ice requires an increase in N1. This will affect the descent profile. If you go to the FMC and select VNAV descent page you will see a prompt descent forcast. If you select this it will take you to a page where you can enter forcast descent winds as well as an altitude that you anticipate having to use anti ice. The FMC will then calculate a descent path based on this. I find it is better to actually enter the band of anti ice as an altitude rather than the intial commencement altitude (unless you expect icing all the way to the ground). For example if icing conditions were expected to start at 25,000 ft and end at 10,000 I would enter 15,000 instead of 25,000 so that the descent path was only corrected for the period of anti ice use within that band width.CheersSteve


Cheers

Steve Hall

Share this post


Link to post

During IDLE descent (FLCH or VNAV), N2 becomes the primary parameter (N3 on an RB211). N2 is ramped up to "Approach Idle" during Nacelle Anti-Ice ops. As a consequence, you would probably see an increase in N1.Approach Idle (can be called other things) is when the idle speed is increased (with flap extension) for safety. It allows a quicker spool up time for Go Around.Althought the N2 rise is visible in this case, note that, in the real world, some engine management processes are hidden from the pilot to avoid confusion. Not everything is transparent in the cockpit. Some of your engine parameters may be lying to you. E.g. in the case of EGT on some engines, the value you see maybe a lot less than actual values. Pilots don't like to see 4-digit EGT values, so a "ballast" resistor is placed in the wiring to reduce the cockpit display value by a certain amount (a few hundred degrees). Cheers.Q>

Share this post


Link to post
Guest D17S

Something to keep in mind about FLCH SPD is that its thrust partner is "THR". A FLCH SPD level change does not necessarily just retard thrust to idle for a descent . . . or advance power to EICAS thrust limit for a climb. Thrust's "THR" mode will adjust power on the basis of how much descent (or climb) you have selected. The non-idle N1 certainly could have been because of wing or nose cowl anti-ice being on, but only if the descent chosen was greater than ~8000 ft. If it was less than that, the thrust level above idle was because "THR" was targeting a reduced rate of descent. Here's how it works. This mode (either for a climb or a descent) uses a kinda modified speed on pitch scheme. Thrust always initiates this maunver then FLCH SPD works to maintain the MCP's airspeed with pitch (at least it's easier to think about it this way). Here we go. If the descent is less than 2000 ft, thrust will back off from its current power seting. Just a little bit. FLCH will notice the airplane slowing down and pitch over to maintain airspeed. This is the speed on pitch part. The AFS's (autoflight system) "THR" mode will watch the resulting rate of descent it just "provoked" from FLCH. At a 2000 foot descent selection, "THR" is looking for a 1200 fpm rate of descent (ROD). If THR isn't quite getting the rate it wants, it will retard thrust a bit more and provoke a little more pitch down from FLCH. As FLCH pitches over just a smidgeon more to maintain speed, THR will get it's devious way and a greater ROD will occur. Very sneaky that "THR", but effective. There is a real "dance" going on between THR and FLCH SPD. THR leads and FLCH follows. Here's your test flight to check it out: Get up high in a SPD/ALT cruise. Initially dial in a 2000 foot descent and select FLCH. Notice right FMA annunciates FLCH SPD and the left FMA, "THR." Notice thrust retards, but not much. Notice a 1200 fpm ROD occurs. Now, while you are still descending, try dialing in a lower altitude into the MCP. Watch how thrust backs off to cause FLCH to lower the nose even further. THR is looking for a greater ROD. Now, while still in the same FLCH descent, dial the MCP back up toward your current altitude. Watch thrust come back in to slow down the ROD. Now back down. You can directly control thrust with the MCP's altitude window. Play with it. It's really fun to watch. At about an 8000 foot descent selection, THR will finally retard thrust all the way back to IDLE. Finally you will get a maximum rate, speed on pitch descent. But you won't get idle until you have selected over an 8000 foot descent. FLCH works exactly the same for a climb. Except "THR" is working its way up to the EICAS thrust limit, not down to IDLE. Try FLCH to climb back up. (Set EICAS thrust limit to "CON", just to give yourself some room.) Now play. An 8000 ft climb selection will finally get you full EICAS thrust. Anything between 8000 and 2000 will get you various thrust levels. Once you see what's happening, you can directly control thrust with the with the MCP's altitude knob. Great fun.

Share this post


Link to post

Basically, FLCH in PMDG's 744 was designed to reach the target altitude within 125 seconds (within the limits of current thrust limit or idle). This, BTW, agrees with my 744 training notes.Therefore, airspeed, drag, etc, will determine whether or not the thrust will go to idle or the thrust limit. Large changes in altitude (descending) will inevitably see the levers at idle.Cheers.Q>

Share this post


Link to post
Guest D17S

Try the test flight. Might have a tid-bit for the next update. Anything below a 2000 foot climb/descent request, "THR" simply provokes FLCH into a pitch that causes a 1200 fpm rate. Then, from a 2000 foot to an 8000 foot selection, thrust is progressively increased or decreased. A rate is the way anything gets from point A to point B in Y time. If I say that I'm seeing 60 MPH on a speedometer, one could argue this activity is not about speed (i.e., rate). The actual activity is about traveling 60 miles in 1 hour. That's true 'nuff . . . too. Regardless, the 125 seconds is interesting background though. That means that THR is even smarter than I initially considered. It will compensate for the 'hills' too. The bumps and headwinds will be factors. Thrust will adjust to provide an increasing rate of climb/descent as the MCP is adjusted further away from the current altitude. As its destination gets further away, the ol

Share this post


Link to post

Listen to Qavion and Sam guys - they're right, FL CH in the 744 (also DES NOW) is a "modulated" mode that varies thrust and descent rate based upon rules the FMC/AFDS is following...


Ryan Maziarz
devteam.jpg

For fastest support, please submit a ticket at http://support.precisionmanuals.com

Share this post


Link to post

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  
  • Tom Allensworth,
    Founder of AVSIM Online


  • Flight Simulation's Premier Resource!

    AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!

    Click here for more information and to see all donations year to date.
  • Donation Goals

    AVSIM's 2020 Fundraising Goal

    Donate to our annual general fundraising goal. This donation keeps our doors open and providing you service 24 x 7 x 365. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. We reset this goal every new year for the following year's goal.


    2%
    $640.00 of $25,000.00 Donate Now
×
×
  • Create New...