Packs Quiet on Spool up

[Ben Cap 89]

Everytime I fly in a real airliner in real life I have always noticed that when they pilots pull the plane onto the runway and spool up the engines the packs are completely silent. It is that they turn off the packs during the t/o? or do the engines just "hog" all the noise in the cabin during the t/o roll?If they aren't turned off and this is just an illusion to me, is this modeled properly in the NGX?- What I am trying to say is when the engines are ready for t/o and you spool em up for t/o and engage TO/GA thrust is the engine sound dominant during the takeoff roll?

[lovetofly28 0]

Everytime I fly in a real airliner in real life I have always noticed that when they pilots pull the plane onto the runway and spool up the engines the packs are completely silent. It is that they turn off the packs during the t/o? or do the engines just "hog" all the noise in the cabin during the t/o roll?If they aren't turned off and this is just an illusion to me, is this modeled properly in the NGX?- What I am trying to say is when the engines are ready for t/o and you spool em up for t/o and engage TO/GA thrust is the engine sound dominant during the takeoff roll?

Correct me if I'm wrong, but the pilot has a choice if the packs are on or off during t/o in the FMS.-Spencer Hayes

[markadeane 274]

Ben,Packs are often switched off during takeoff. Not on all aircraft mind you, but for example PMDG's own MD11 has this feature.

[Ben Cap 89]

Ben,Packs are often switched off during takeoff. Not on all aircraft mind you, but for example PMDG's own MD11 has this feature.

Thanks so much for the speedy reply. Anyways, is there a reason that they switch off the packs for t/o? What difference does it make?

[StevenHPerry 20]

And the technical reason is... ?My edumacated guess is that turning the packs off preserves the power for thrust rather than creature comfort.Is this the same as a "no bleed" or "bleeds off" departure?

[Cras 6]

If the plane has got plenty of runway, then the packs will most likely stay on. Actually, when the engines are spooled up and the packs are on, isn't there a bit of an increase in the pack air?At the end of the day, it is a performance issue.

[SlickNick 0]

It is a performance issue. Bleed air normally reserved for air conditioning can instead be used if the engines need just a little more power. If the runway is short, or it is hot and or humid out, the packs may be switched off in an attempt to provide just a tad more thrust on the engines should one fail.

[andysim 0]

its like Air con on your car. turning off the AC gives you more oompphh cause your engine is not powering the unit.

[teropa 4]

And in some cases, read Airbus A330 and A340, the packs should always be turned OFF prior to departure. Because often times same pilots are piloting both the wide bodies and the smaller A320 series, the procedure is to turn packs off in the A320S as well, to keep the amount of "uncommon operational items" to a minimum. This helps the pilots to fly both types, as they don't have to remember two sets of procedures.Tero

[rsrandazzo 6,353]

Ben-Here is your technical answer!The air conditioning packs use high pressure air from bleed ducts that remove the air from either the 5th compression stage or the 9th compression stage of the engines (depending on whether you are at high or low power on the engine.)Taking this high pressure air has the effect of removing some of the air mass flow through the engine core, which causes the temperature of the air mass flow to increase.Sine all gas turbine engines have temperature limits that must be observed, anything that increases the temperature of the air mass moving through the engine will have the effect of lowering the amount of thrust you can obtain from the engine. To put it another way, you are taking some of the airflow out of the engine which causes the exhaust gas temperature to be higher than it would otherwise be... So you can't get the maximum amount of thrust from the engine as long as you are changing the combustion mixture inside that engine...For this reason, many aircraft types require the bleeds to be closed (and thus the packs turned off) during takeoff in order to preserve every last bit of thrust for the takeoff process.Some airplanes have extremely powerful engines and do not need the extra thrust so they can leave the bleeds/packs running during all by the most extreme high altitude airport, hot day, heavy weight takeoffs...On the NG, you can actually run into a different type of problem during a packs off takeoff: Since the packs are not consuming air through the bleed ducts, you might actually over pressurize those ducts during takeoff with the bleeds open and the packs off- triggering a fault on the overhead panel...It is a simple thing to resolve... just turn on the cowl anti ice after you have the gear up and it will consume enough bleed pressure to get you back into the normal range.

[Ingrimmsch 0]

Hi guys,A few months ago i heard a lecture of a Lufthansa Cargo captain about flying into high located airports. In this case he was referring to Quito in Ecuador.It was really interesting.He mentioned that when you are taking off in Quito (it is at 9000ft) with packs of you have to be careful becuase if you wait to long to turn them on again you'll get a cabin altitude alert and the oxygen mask will drop out of their panel. In this case it isn't that important because they are only flying with cargo ;-) So this is another thing you have to keep in mind when you're turning the packs off.CheersPaul Benke

[VIPERGTSR01 22]

On the NG, you can actually run into a different type of problem during a packs off takeoff: Since the packs are not consuming air through the bleed ducts, you might actually over pressurize those ducts during takeoff with the bleeds open and the packs off- triggering a fault on the overhead panel...It is a simple thing to resolve... just turn on the cowl anti ice after you have the gear up and it will consume enough bleed pressure to get you back into the normal range.

Haha*Adds to list of things to try*

[ExNusquam 11]

On the NG, you can actually run into a different type of problem during a packs off takeoff: Since the packs are not consuming air through the bleed ducts, you might actually over pressurize those ducts during takeoff with the bleeds open and the packs off- triggering a fault on the overhead panel...It is a simple thing to resolve... just turn on the cowl anti ice after you have the gear up and it will consume enough bleed pressure to get you back into the normal range.

Could you clarify that Robert? If memory serves me correctly, isn't the proper procedure on a 737 to kill the engine bleeds and power the packs via the APU? In theory the packs should stay on, and the bleed ducts won't over pressurize. Also, wouldn't turning EAI on kill any benefit you gained from turning the packs off? IIRC EAI increases EGT and fuel flow due to running warmer air through the engine.

[DylanC 181]

Ben-Here is your technical answer!The air conditioning packs use high pressure air from bleed ducts that remove the air from either the 5th compression stage or the 9th compression stage of the engines (depending on whether you are at high or low power on the engine.)Taking this high pressure air has the effect of removing some of the air mass flow through the engine core, which causes the temperature of the air mass flow to increase.Sine all gas turbine engines have temperature limits that must be observed, anything that increases the temperature of the air mass moving through the engine will have the effect of lowering the amount of thrust you can obtain from the engine. To put it another way, you are taking some of the airflow out of the engine which causes the exhaust gas temperature to be higher than it would otherwise be... So you can't get the maximum amount of thrust from the engine as long as you are changing the combustion mixture inside that engine...For this reason, many aircraft types require the bleeds to be closed (and thus the packs turned off) during takeoff in order to preserve every last bit of thrust for the takeoff process.Some airplanes have extremely powerful engines and do not need the extra thrust so they can leave the bleeds/packs running during all by the most extreme high altitude airport, hot day, heavy weight takeoffs...On the NG, you can actually run into a different type of problem during a packs off takeoff: Since the packs are not consuming air through the bleed ducts, you might actually over pressurize those ducts during takeoff with the bleeds open and the packs off- triggering a fault on the overhead panel...It is a simple thing to resolve... just turn on the cowl anti ice after you have the gear up and it will consume enough bleed pressure to get you back into the normal range.

One of the little nuances we may have missed when we take this bird for delivery.

[Jack_C 1]

Could you clarify that Robert? If memory serves me correctly, isn't the proper procedure on a 737 to kill the engine bleeds and power the packs via the APU? In theory the packs should stay on, and the bleed ducts won't over pressurize. Also, wouldn't turning EAI on kill any benefit you gained from turning the packs off? IIRC EAI increases EGT and fuel flow due to running warmer air through the engine.

The bleed ducts arent measured at the packs if my memory serves me correct. I think it is just downline from the engine. JackColwill