1.) I dont get what exactly trim air does, can someone explain it?

 

2.) I was talking to a United 737 pilot who said they shut of the trim air while on the ramp, does this sound correct?

 

3.) do i need it off or on for engine start?

 

 

thanks

I dont get what exactly trim air does, can someone explain it?

My understanding is that the packs and trim air valves work together in order to achieve in and maintain the desired temperatures of each of the three aircraft zones (flight deck, forward cabin, and aft cabin). Trim air regulation is especially relevant when at fewest one of these three zones is set to a temperature different from those of the others.

 

The packs will cool all three zones to the temperature of the zone(s) set to the coolest temperature, and the trim air valves will add the amount of hot engine air required for each desired warmer zone to heat up to its target temperature. For example, if zones 1 and 3 are set to 25 °C and zone 2 is set to 20 °C, the packs would cool all three zones to 20 °C, and the trim air valves would increase their temperatures from 20 to 25 °C by allowing the appropriate amount of hot air to enter.

 

I'm not sure about question 2, as its answer probably varies from airline to airline. Because the effect of trim air regulation seems to be more of a passenger-comfort convenience, I would imagine that, under normal circumstances, the trim air setting on the ramp for some airlines would be as necessary or optional. Trim air should probably be on for engine start, though, as recommended in the FCOM preflight procedures.

Edited January 10, 201313 yr by zowen11

My understanding is that the packs and trim air valves work together in order to achieve and maintain the desired temperatures in each of the three aircraft zones (flight deck, forward cabin, and aft cabin). Trim air regulation is especially relevant when at fewest one of these three zones is set to a different temperature than those of the others.

 

The packs will cool all three zones to the temperature of the zone set to the coolest temperature, and the trim air valves will add the amount of hot engine air required for each warmer zone to heat up to its target temperature. For example, if zones 1 and 3 are set to 25 °C and zone 2 is set to 20 °C, the packs would cool all three zones to 20 °C, and the trim air valves would increase their temperatures from 20 to 25 °C by allowing the appropriate amount of hot air to enter.

 

 

so any idea when the valve would be shut off?

so any idea when the valve would be shut off?

When you were writing your reply, I was editing mine for greater accuracy and clarity. Please see the last paragraph.

This video might help you understand the bleed air.

 

Switching it off on the ground is a bit odd , at least in our operations. From memory though on some models you may switch them off on the ground as when you go to APU power, it will be load shed.

 

Normally switch is left on all the time. You switch the packs off for start so there is no air going into the trim air anyway.

To answer question #2 and 3, trim air is often turned off on the ground to prevent the PACK(s) from icing up. In hot and humid environments, and especially when performing max cooling on the ground (PACKs in HIGH) PACKs can freeze. Normally the L PACK to be exact. This is because the aft portion of the aircraft is most difficult to cool, so the zone temp selector should be the coldest temperature selected-- say for example 65 F. While the flight deck is demanding 70 F. With lackadaisical use of the fwd entry door and the flight deck door, it's going to be nearly impossible to reach 70 F in the flight deck (or CONT CAB). So that L PACK is going to work really really hard to cool down the flight deck. And here's the kicker-- that trim air switch that modulates bleed air to satisfy temperature demand, doesn't always work perfectly. So sometimes, since the CONT CAB (in this case) isn't the coldest zone temp control, the trim air valve for the CONT CAB has been known to erroneously open and close; effectively not modulating itself correctly, and therefore slows the cooling for the flight deck EVEN MORE, requiring MORE juice out of the PACK.

This was the drawn out, mostly laymen's language way of describing why we sometimes turn the trim air switch off. But the easy explanation is, our most current max cooling on the ground procedure tells us to do so. Granted, I'm a Navy guy and not governed by whatever checklists United pilots fly.

Lastly, it doesn't matter what position the switch is in for engine starts. Normally I'll leave it off for starts, and when I configure the bleed air control panel during my before taxi "flow", I'll turn the switch back on.

Hope this helps

5 hours ago, P8Pilot said:

To answer question #2 and 3, trim air is often turned off on the ground to prevent the PACK(s) from icing up. In hot and humid environments, and especially when performing max cooling on the ground (PACKs in HIGH) PACKs can freeze. Normally the L PACK to be exact. This is because the aft portion of the aircraft is most difficult to cool, so the zone temp selector should be the coldest temperature selected-- say for example 65 F. While the flight deck is demanding 70 F. With lackadaisical use of the fwd entry door and the flight deck door, it's going to be nearly impossible to reach 70 F in the flight deck (or CONT CAB). So that L PACK is going to work really really hard to cool down the flight deck. And here's the kicker-- that trim air switch that modulates bleed air to satisfy temperature demand, doesn't always work perfectly. So sometimes, since the CONT CAB (in this case) isn't the coldest zone temp control, the trim air valve for the CONT CAB has been known to erroneously open and close; effectively not modulating itself correctly, and therefore slows the cooling for the flight deck EVEN MORE, requiring MORE juice out of the PACK.

This was the drawn out, mostly laymen's language way of describing why we sometimes turn the trim air switch off. But the easy explanation is, our most current max cooling on the ground procedure tells us to do so. Granted, I'm a Navy guy and not governed by whatever checklists United pilots fly.

Lastly, it doesn't matter what position the switch is in for engine starts. Normally I'll leave it off for starts, and when I configure the bleed air control panel during my before taxi "flow", I'll turn the switch back on.

Hope this helps

Late but great :D professional insight is always very appreciated.. thank you! 

Just to throw confusion into the topic, at American the trim air is left on all the time. In nearly 14 years flying the plane I never had a problem doing it that way. I think United leaves it on all the time now too. 

3 hours ago, Ralgh said:

Just to throw confusion into the topic, at American the trim air is left on all the time.

I never understood why we used to turn it off on overnights, just to turn it on again first thing the next morning.

We only turn it off I directed by QRH during an abnormal. 

I have a Trim question for PMDG 737NGX

Before Takeoff I try to set the trim at 6.23 following FMC info. Well.... I move the wheel via keyboard (Home/End keys) and also placing the mouse point on top of the trim wheel. A tooltip shows the current value. So, while the trim wheel is in motion the tooltip number is changing accordingly but suddenly the showing number stops while the wheel still in motion. I have to stop moving the wheel and start again until the tooltip stops showing the number again and so on until I get the expected value. Is it the way on how the Trim value procedure works? The manual doesn't answer me.

21 minutes ago, trisho0 said:

I have a Trim question for PMDG 737NGX

Before Takeoff I try to set the trim at 6.23 following FMC info. Well.... I move the wheel via keyboard (Home/End keys) and also placing the mouse point on top of the trim wheel. A tooltip shows the current value. So, while the trim wheel is in motion the tooltip number is changing accordingly but suddenly the showing number stops while the wheel still in motion. I have to stop moving the wheel and start again until the tooltip stops showing the number again and so on until I get the expected value. Is it the way on how the Trim value procedure works? The manual doesn't answer me.

Trim Air is not Trim. Very different question when compared to the topic of this thread.

All the same: when you're in the real plane, you don't get a pop up when you place your hand on the trim wheel. Eyeball it on the trim index. That's good enough. Keep in mind that the trim value is calculated based off of the assumption all of the passengers weigh the same...let's be real here. The person in 1A could be 100 pounds different from the person in 40C.