Ext Air & Packs

[johnnycrockett 43]

I've tried to dig up some information somewhere but haven't found anything concrete. When using Ext air, are the Packs switched off and only turned on when the Ext air is disconnected? Thanks!

[kevinh 409]

Before connecting ground conditioned air the packs should be switched OFF (and recirc fans OFF). FCOM SP2.2

If you mean external air for engine start then the packs could be on or off as required.

[downscc 1,327]

1 hour ago, johnnycrockett said:

I've tried to dig up some information somewhere but haven't found anything concrete. When using Ext air, are the Packs switched off and only turned on when the Ext air is disconnected? Thanks!

The 737/777 FCOMs are clearer on how ground conditioned air enters the system, the 747 document doesn't specifically treat the subject.  As Kevin mentioned, using a ground puffer (HP air source) you are essentially getting bleed air from the ground instead of the APU and it can be used to run the packs or start engines.  However, puffers tend to be very loud and expensive and their use is limited.  Ground conditioned air in the 737/777 is routed to an air mixing plenum downstream or after the packs and provides conditioned air to the cabin and cockpit, and equipment cooling.  The conditioned air in the 747 is similar I assume, there is no control over flow rate and temperature.  For this reason, one will often be going to APU to operate the packs as the aircraft is loading much earlier than required in the 737/777.  One might not start up the APU in the 777 until 5-10 min before pushback but in the 747 it might be 20-40 min depending on local airport rules.

[berts 97]

1 hour ago, johnnycrockett said:

I've tried to dig up some information somewhere but haven't found anything concrete. When using Ext air, are the Packs switched off and only turned on when the Ext air is disconnected? Thanks!

External air is normally only used when the APU is unserviceable and at certain airfileds where restrictions are placed on the running of the APU. In these situations external air will be used to condition the air inside the fuselage (cooling or heating) and to start at least one engine on the gate. The other two (or three) engines are then started after the pushback has been completed using the Crossbleed Start procedure.

When using external air for engine start (i.e a Ground Pneumatic Start) the Packs must always be switched OFF and it is normal to have two air carts available. Only one engine at a time should be started and it is important to check the duct pressure to ensure it is a minimum of 30psi at sea level, otherwise there is an increased risk of a hot or aborted start. If air conditioning is required during the subsequent pushback the Park Brake must not be released and the Packs should not be placed ON again until the duct pressure reads zero and the ground engineer has confirmed that the ground air and all other equipment has been safely removed.

Bertie G

[johnnycrockett 43]

Thank you all for the responses - sorry if I wasn't a bit more clear. Was referring to when parked at a gate, prior to boarding and during boarding, if using external air-condtioner what to do with the packs. I've always left them off and then turned them on when disconnecting air and using apu. 

6 hours ago, kevinh said:

Before connecting ground conditioned air the packs should be switched OFF (and recirc fans OFF). FCOM SP2.2

If you mean external air for engine start then the packs could be on or off as required.

I missed that bit in the FCOM, I've learnt Normal Procedures and also read up on the fuel system & engines but didn't see this bit.

5 hours ago, downscc said:

The 737/777 FCOMs are clearer on how ground conditioned air enters the system, the 747 document doesn't specifically treat the subject.  As Kevin mentioned, using a ground puffer (HP air source) you are essentially getting bleed air from the ground instead of the APU and it can be used to run the packs or start engines.  However, puffers tend to be very loud and expensive and their use is limited.  Ground conditioned air in the 737/777 is routed to an air mixing plenum downstream or after the packs and provides conditioned air to the cabin and cockpit, and equipment cooling.  The conditioned air in the 747 is similar I assume, there is no control over flow rate and temperature.  For this reason, one will often be going to APU to operate the packs as the aircraft is loading much earlier than required in the 737/777.  One might not start up the APU in the 777 until 5-10 min before pushback but in the 747 it might be 20-40 min depending on local airport rules.

Lately, I've been leaving the APU off until a few minutes before pushback. With the fuel policies I've been using they don't seem to account for APU use for 30-45 mins before departure and I end up getting to the runway below the Min T/O fuel.

5 hours ago, berts said:

External air is normally only used when the APU is unserviceable and at certain airfileds where restrictions are placed on the running of the APU. In these situations external air will be used to condition the air inside the fuselage (cooling or heating) and to start at least one engine on the gate. The other two (or three) engines are then started after the pushback has been completed using the Crossbleed Start procedure.

When using external air for engine start (i.e a Ground Pneumatic Start) the Packs must always be switched OFF and it is normal to have two air carts available. Only one engine at a time should be started and it is important to check the duct pressure to ensure it is a minimum of 30psi at sea level, otherwise there is an increased risk of a hot or aborted start. If air conditioning is required during the subsequent pushback the Park Brake must not be released and the Packs should not be placed ON again until the duct pressure reads zero and the ground engineer has confirmed that the ground air and all other equipment has been safely removed.

Bertie G

I haven't tried single engine start at the gate yet. I think there was a discussion somewhere about APU use at EGLL. The beauty of the 747 is that currently it only services so many airports so it is easier to keep on top of requirements but I'm only just starting to get to the noise restrictions. I think I read that APU could only be run XX min prior to pushback for power but not sure on cooling/heating use.

Presently I start at the gate with packs off and ext air (LP) connected. Shortly before pushback APU is started and LP air disconnected, packs turned on until ready for pushback. At pushback I leave 1 pack on for engine start then after start 3 packs again until takeoff. I read that 300,000KG (661,000lbs) TOW or more Boeing suggest packs off so I will go back to 1 pack again. Who knows, this could all be different airline to airline.

What flow/process does everyone else use?

[Qavion2 39]

3 hours ago, johnnycrockett said:

if using external air-condtioner what to do with the packs. I've always left them off and then turned them on when disconnecting air and using apu.  

The packs must be turned off prior to the connection of conditioned air (and left off). There is a one way (check) flapper valve on the conditioned air connection (see diagram).

Conditioned Air setup

If the pack output pressure and the conditioned air pressure are roughly equal, the valve may open and close rapidly. We believe this causes premature failure of the valve hinge. We have found the valve flapper detached and stuck downstream of the connection. This may cause air to be diverted into the  body fairing panel area and blow out the panels (The panels are of relatively thin composite material... and will not survive the pressurisation forces (which the packs are generating). Also, if the valve is not noticed to be missing, the pack may reverse pressure the ground conditioned air ducts and damage the ground air compressor.

The pilots must tell the engineers downstairs if they intend turning on the APU and packs (so the engineers can remove the conditioned air hoses).

Edited May 16, 2018 by Qavion2

[skelsey 765]

6 hours ago, johnnycrockett said:

I missed that bit in the FCOM, I've learnt Normal Procedures and also read up on the fuel system & engines but didn't see this bit.

A good reminder that the FCOM (etc) is not a "read cover to cover and memorise it" document - it is fundamentally a reference manual and nobody could be expected to commit every word of the thousands of pages to memory.

Whilst obviously the normal procedures/flows need to be memorised - everything else really, from a sim point of view, is stuff which is looked up as and when needed. ITRW it would be quite common to "read and do" many of the supplementary procedures from the manual rather than trying to remember something which one might only have done on a small number of occasions previously! The important thing is to have a very general appreciation of the content and structure of the various manuals. Generally speaking, if you have a question about how to operate the aircraft Boeing will almost always have thought about it first - you just need to find the right book and the right search term!

Not an admonishment by any means as posing questions like this in the forum leads to interesting discussions for all of us: just a general observation that many are scared off by the size of the manuals. Treat the manuals like a dictionary or encyclopaedia - look up the specific things you need but don't feel as though you need to know every word on every page! If you want to know more about a particular system or subject then by all means read those sections, but beyond the normal flows and QRH memory items you don't need to store it all in your head!

6 hours ago, johnnycrockett said:

With the fuel policies I've been using they don't seem to account for APU use for 30-45 mins before departure and I end up getting to the runway below the Min T/O fuel.

APU fuel should be accounted for as part of the taxi fuel (fuel for start, taxi and APU usage if significant). Remember that whatever the "fuel policy" - ultimately as the Commander you are responsible for loading the fuel that you want. Of course, that doesn't mean you have to go nuts - but if the computer hasn't accounted for APU usage then you should do yourself! As a guide, the 747-400 APU burns about 300kg/hr.

6 hours ago, johnnycrockett said:

I read that 300,000KG (661,000lbs) TOW or more Boeing suggest packs off so I will go back to 1 pack again. Who knows, this could all be different airline to airline.

This (as with pack usage on startup) s very much airline specific (and environmentally specific as well to an extent). BA turn packs off above 300 tonnes as this reduces EGT. They also turn all the packs off for start, but other airlines may leave one on (provided you have sufficient duct pressure, which I would suggest may not be the case with an external start cart).

[johnnycrockett 43]

4 hours ago, Qavion2 said:

The packs must be turned off prior to the connection of conditioned air (and left off). There is a one way (check) flapper valve on the conditioned air connection (see diagram).

Conditioned Air setup

If the pack output pressure and the conditioned air pressure are roughly equal, the valve may open and close rapidly. We believe this causes premature failure of the valve hinge. We have found the valve flapper detached and stuck downstream of the connection. This may cause air to be diverted into the  body fairing panel area and blow out the panels (The panels are of relatively thin composite material... and will not survive the pressurisation forces (which the packs are generating). Also, if the valve is not noticed to be missing, the pack may reverse pressure the ground conditioned air ducts and damage the ground air compressor.

The pilots must tell the engineers downstairs if they intend turning on the APU and packs (so the engineers can remove the conditioned air hoses).

That would no doubt lead to an interesting failure! Has this ever happened that you know of?

52 minutes ago, skelsey said:

A good reminder that the FCOM (etc) is not a "read cover to cover and memorise it" document - it is fundamentally a reference manual and nobody could be expected to commit every word of the thousands of pages to memory.

Whilst obviously the normal procedures/flows need to be memorised - everything else really, from a sim point of view, is stuff which is looked up as and when needed. ITRW it would be quite common to "read and do" many of the supplementary procedures from the manual rather than trying to remember something which one might only have done on a small number of occasions previously! The important thing is to have a very general appreciation of the content and structure of the various manuals. Generally speaking, if you have a question about how to operate the aircraft Boeing will almost always have thought about it first - you just need to find the right book and the right search term!

Not an admonishment by any means as posing questions like this in the forum leads to interesting discussions for all of us: just a general observation that many are scared off by the size of the manuals. Treat the manuals like a dictionary or encyclopaedia - look up the specific things you need but don't feel as though you need to know every word on every page! If you want to know more about a particular system or subject then by all means read those sections, but beyond the normal flows and QRH memory items you don't need to store it all in your head!

APU fuel should be accounted for as part of the taxi fuel (fuel for start, taxi and APU usage if significant). Remember that whatever the "fuel policy" - ultimately as the Commander you are responsible for loading the fuel that you want. Of course, that doesn't mean you have to go nuts - but if the computer hasn't accounted for APU usage then you should do yourself! As a guide, the 747-400 APU burns about 300kg/hr.

This (as with pack usage on startup) s very much airline specific (and environmentally specific as well to an extent). BA turn packs off above 300 tonnes as this reduces EGT. They also turn all the packs off for start, but other airlines may leave one on (provided you have sufficient duct pressure, which I would suggest may not be the case with an external start cart).

Some brilliant pointers here. I've enjoyed using the FCOM to learn procedures in more detail but without first hand airline knowledge I didn't know how to effectively use it - like you say it's more a tool than a book and is a great guide to anything you may come across while operating the 747. Thanks for the info.

[kevinh 409]

17 hours ago, johnnycrockett said:

I missed that bit in the FCOM, I've learnt Normal Procedures and also read up on the fuel system & engines but didn't see this bit.

In addition to what Simon said, the manuals are searchable so it's easy to find specific information you may need.

[berts 97]

20 hours ago, johnnycrockett said:

Presently I start at the gate with packs off and ext air (LP) connected. Shortly before pushback APU is started and LP air disconnected, packs turned on until ready for pushback. At pushback I leave 1 pack on for engine start then after start 3 packs again until takeoff. I read that 300,000KG (661,000lbs) TOW or more Boeing suggest packs off so I will go back to 1 pack again. Who knows, this could all be different airline to airline.
 

What flow/process does everyone else use?

The normal procedure is to use the APU electrics and air for engine start.  It will also provide conditioned air on the ground, so ground units are not usually required.  All 3 packs are switched off prior to engine start so that the APU will provide sufficient air to autostart two engines at a time during the pushback (i.e 3 & 4 followed by 1 & 2). Above 2000ft AMSL or an OAT of 36C+ you should only autostart one engine at a time. Above 6,000ft AMSL a manual start is required.

Airline performance procedures do vary, but in general terms it is normal to assume all Air Conditioning Packs are ON for takeoff.  However, the Takeoff weight can be increased if the Packs are OFF or if the APU to Centre Pack configuration is used at weights above 300,000Kgs.  Individual runway performance pages are provided to calculate the actual takeoff weight and speeds using the Assumed Temperature method, which has the advantage of deriving a reduced thrust (EPR) setting for takeoff in the FMC.

Bertie   

Edited May 16, 2018 by berts
typo correction

[Qavion2 39]

14 hours ago, johnnycrockett said:

That would no doubt lead to an interesting failure! Has this ever happened that you know of? 

Indeed it has... and it has been messy.... the panels literally explode and the shrapnel can go everywhere. Fortunately, the pieces are lightweight and the damage is usually limited to scratches and small dents and perhaps some wiring damage. I have heard it is audible in the cabin (perhaps not the explosion, but the sudden change in pack behaviour when the affected pack air exits the aircraft instead of going to the cabin).

Edited May 17, 2018 by Qavion2

[aeromaks 1]

10 hours ago, berts said:

Above 2000ft AMSL or an OAT of 36C+ you should only autostart one engine at a time. Above 6,000ft AMSL a manual start is required.

Hi Bertie,

Am I correct that this is because of low bleed air flow due to less dense air in such conditions? Could you please point to the source of this limitations? I did'n find any in FCOM. Also I don't know if those limitations are modelled in 747 QOTSII but if I remember correctly I was able to autostart two engines at a time in Quito Ecuador about 9200ft AMSL (in sim of course) . I did this habitually knowing nothing about those limitations.

[kevinh 409]

On 5/16/2018 at 11:01 AM, skelsey said:

They also turn all the packs off for start, but other airlines may leave one on (provided you have sufficient duct pressure, which I would suggest may not be the case with an external start cart).

Not the way PMDG have modelled it. The 747v3 start cart gives you 57 psi. You could start all four together with that.

[berts 97]

On 5/17/2018 at 11:51 AM, aeromaks said:

Hi Bertie,

Am I correct that this is because of low bleed air flow due to less dense air in such conditions? Could you please point to the source of this limitations? I did'n find any in FCOM. Also I don't know if those limitations are modelled in 747 QOTSII but if I remember correctly I was able to autostart two engines at a time in Quito Ecuador about 9200ft AMSL (in sim of course) . I did this habitually knowing nothing about those limitations.

Correct. The the altitudes I mentioned are not Aircraft Limitations as such, only recommendations mainly intended for use with the RR engine autostart procedures and are contained in the actual B744 Flying Manual. The minimum recommended duct pressure for a Crossbleed Start is 40psi with all three Packs OFF, so if you are getting 57psi from the ground air start carts you are on to a real winner!

Bertie