Bleeds off takeoff

[tcas_climb 45]

Anyone else notice that when configuring for a bleeds off takeoff, and you let packs run off the APU, the engine N1 rating doesn't change even if the engine bleeds are off, but as soon as you turn the packs OFF the N1 rating increases, am i missing something or is the N1 suppose to increase when the engine bleed switches are off regardless of whether or not the packs are running?

[Capt. Farhadi 5]

When you turn eng bleeds off,engines power and thrust increases due to letting air go off the bleed valve in the engines,so,you have more thrust in t/o

But suggestion to turn packs off during takeoff in airbus(as i know) isn't cause of that,its because in takeoff,you'll increase preassuer on packs and you'll probably let packs die befor their time!

[FLEX1978 373]

When you turn eng bleeds off,engines power and thrust increases due to letting air go off the bleed valve in the engines,so,you have more thrust in t/o

But suggestion to turn packs off during takeoff in airbus(as i know) isn't cause of that,its because in takeoff,you'll increase preassuer on packs and you'll probably let packs die befor their time!

That's incorrect regarding the bus, engines are EGT limited on take off, packs off lowers the EGT. IN fact 'some' operators suggest packs on take off since the cycling of the packs eventually causes more damage.

[tcas_climb 45]

So Rob dont you think that the engine is suppose to produce more thrust as long as the engine isn't supplying bleed air, whether or not the packs are running?

[scandinavian13 5,192]

So Rob dont you think that the engine is suppose to produce more thrust as long as the engine isn't supplying bleed air, whether or not the packs are running?

 

Based on your post, it seems like you're not understanding the mechanics of what's going on.  I could be misreading, however.

 

Yes, the engine is throwing more thrust out the back of the engine when the bleeds are off, but N1 is not a direct indicator of thrust - it's fan speed.

 

The difference is actually that your EGT is lower with the bleeds off.  What you're seeing is that with the bleeds on, the EGT is limited, which means the engine speed is being restricted through N1.  When the bleeds get turned off, that EGT limit is higher, and the N1 gets bumped up.

[tcas_climb 45]

Okay here is what is happening in the NGX,

 

Lets say in a given condition, having bleeds ON and packs ON would give you a takeoff N1 of say 88%

 

Configure the aircraft for an UNPRESSURIZED takeoff, i.e. engine bleeds and packs OFF would give you an N1 of say 90%

 

Configure the aircraft for a BLEEDS OFF takeoff i.e. Engine bleeds OFF, APU Running and supplying bleed air to the packs, still gives you an N1 of 88%, where im thinking it should be 90% as the engine bleeds are OFF.

 

Interestingly enough, if you close the Isolation valve in ^^^^ configuration, the pack on the side that the APU is supplying bleed air to, that engine would give an N1 of 88% while engine on the side not receiving bleed air would read 90%.

 

Hopefully everyone understands the problem now.

 

 

Edit: hmmmm so Kyle what you are saying is that even though the N1 doesn't change, with the bleeds off the engines are throwing out more thrust and i can see why this is so. So then why would the N1 increase when the packs are off in the same conditions?

[Capt. Farhadi 5]

Hey flex1978

As a real pilot said,airbus strongly recommand to turn packs off for t/o,also some airlines policys are saying so,for example easy jet,air france,iran air,alitalia,lufthansa and etc.

But i agree turning packs off,decreases egt,and egt is "one of"reasons to turn packs to off

You told cycling them makes more damage?!no,not at all,at least for a 20 years old bus,having a lot of hp air damages them more of turning them off during start and t/o

[Capt. Farhadi 5]

Tcas

As scandinavian said,n1 is not a scale to measure the thrust,so ther's no relation between those.the only things which have a linear relation with thrust are diff preassure -temp-speed of intake and outgoing air(parameters in thermodynamical normal gas law)

[FLEX1978 373]

Hey Capt,

 

Keeping packs on to save life on cycles is not a SOP I have come across personally, however I have heard it from guy's that have come from previous operators. As I am sure you already know the packs are getting cycled on every engine start anyway.. At the end of the day many strange non Boeing/Airbus SOPS are applied by airlines around the globe

 

Regards

[Capt. Farhadi 5]

Yeah flex,youre right.airlines decide what to do by their own capabilities in servicing aircrafts

I said if there is a massive damage in each turning packs on/off during start,packs should die between 5 and 7 years,but they wont

Regards

[Airbusman330 300]

Some airlines run the APU to feed the packs so they can stay on while the bleeds are turned off, common for very hot and humid areas where having the packs off even for a little bit can cause problems. 

[tcas_climb 45]

Tcas

As scandinavian said,n1 is not a scale to measure the thrust,so ther's no relation between those.the only things which have a linear relation with thrust are diff preassure -temp-speed of intake and outgoing air(parameters in thermodynamical normal gas law)

 

Yeah i understand now, however what makes the N1 limit increase when the packs and engine bleeds are turned off and nothing else (aircraft configuration, assumed temperature and OAT) changes?

[kevinh 409]

Anyone else notice that when configuring for a bleeds off takeoff, and you let packs run off the APU, the engine N1 rating doesn't change even if the engine bleeds are off, but as soon as you turn the packs OFF the N1 rating increases, am i missing something or is the N1 suppose to increase when the engine bleed switches are off regardless of whether or not the packs are running?

You aren't missing anything and I think PMDG have modelled this incorrectly.  In the real aircraft, thrust limits (as shown by the N1 command) have bleed correction based purely on switch positions.  They should also reflect pack high and low flow positions too.

 

I just looked at this with engines off and the APU running.  With both packs running both N1 command values are reduced by about 0.9%.  Simply by switching off APU bleed (and therefore shutting off the air to the packs) the engine N1 limit increases.  With APU bleed on and the isolation valve closed the left pack runs and the right pack doesnt' and this is reflected in the N1 limits for both engines.  PMDG seem to be checking if the pack is running and not the bleed switch configuration.  This is not right.  Engine N1 limit calculations should look at the pack switches, bleed switches and anti-ice switches.

 

Regarding the other discussion, N1 is not directly related to thrust, but it's a pretty darn close relationship.  For a given condition if you know N1 you can compute thrust.  The N1 TO rating limit is computed such that if you set N1 to that value the engine produces TO rated thrust, similarly for the other ratings.  Also, if the EGT really was limiting thrust you wouldn't be able to achieve the rated N1 value, either that or you would see the N1 limit reduce as EGT approached it's limit.

[tcas_climb 45]

Finally someone gets what I've been trying to say thanks Kevin.

[kevinh 409]

Finally someone gets what I've been trying to say thanks Kevin.

I've opened a ticket with PMDG about this.