Max T/O power, no engine bleeds T/O, MLW

[Iain]

Hi Jan

Just imagine, there are some crazy simmers out there, pushing your 737 to the limit! I have some various questions that would help me improve my knowledge of the aircraft.

1) Does the engine bleed off takeoff work?  It works in the -800, but in IXEG, on the N1 limit page, when I complete the procedure, the 20k rating actually lowers by 1.0?  It seems that putting the packs from On to Auto reduces the N1 and the bleeds on or off have no change?  What is the best way to get max power with the AT engaged?

2) So if bleeds is not simulated or doesn't work for whatever reason, what is the correct way to get a higher N1 for field length limited T/O even with AT disengaged.  I have tried this and I just pull the manual N1 knobs, rotate to the desired N1 and use TOGA but without AT engaged, and it works.  But what limits should be used?  Up to the yellow arc is safe?  I see there is the red light which I guess EGT over limit limit?  Can this be on for a set period of time if really needed?

3) Regarding weights, I know there are many -300 configurations, but if we use a typical value of  33.3 empty ZFW, and we use a Max Landing weight of 51.7 to 52.5 (I found online), this means that it's fairly easy for a fully loaded aircraft (149 pax at 104kg per pax) to land overweight.  33.3+15.5 of pax making a ZFW of 48.8, means you can't land with more than 2.9 tonnes of fuel.  This can be limiting when flying long legs (Edinburgh - Tenerife, with full load) as you often want a bit more diversions full for remote areas. 

Are my calculations roughly correct? 

This also means the load table that comes with the aircraft is pretty much useless as you can load the aircraft to crazy weights that would never be allowed (I have never used it, because I think my numbers are correct?). 

Thanks for your time

Cheers

Iain
 

[Litjan]

Jet engine performance is complex.

 A modern jet engine is limited by 3 main parameters:

1. Maximum rotational speeds (N1, N2)
2. Maximum mechanical pressure (maximum thrust)
3. ITT or EGT temperature (metallurgical limit).

You are usually limited by the one you reach first (although two simultaneously is possible).

If you operate in a high density cool air, you will reach your maximum thrust with a low N1, long before rpm or temp becomes a problem.

If you operate in hot temperatures, ITT will become the limit, long before rpm or thrust are maximum.

If you operate in very thin air, maximum rpm will be reached before you ever get to max thrust or ITT.

I will have to look at the bleed logic again, normally the FMC will sense the bleed extraction logic and allow for the hotter EGT - but it is entirely possible that we have a bug in that logic :-)

Unless you enter a TASS, you will always get the maximum rated power for the prevailing temperature, there is no need to do anything "manual". The engine is "flat rated" at ISA+15C, which means that at a temperature below +30C at sea level you could theoretically run it faster and not exceed EGT, but you are reaching "maximum thrust" already.

As for fuel - if you land with 2.4 tons of fuel remaining, you are probably doing fine if not even having a bit "too much". The required fuel at touchdown is alternate fuel (ca. 800kgs, depending on distance to alternate) plus 1200kgs (to fly for half an hour). If you go to a situation where you need MORE fuel to get to the alternate, you need to limit your payload, this is completely normal and sometimes done (for example when we fly to BIKF and the weather in the general area is lousy).

[Iain]

Hi Jan

Thanks for the clear and concise answers.  Please check the bleed logic and let me know what you find, it's not so important but would be good to know.

So what is the purpose of the "pull to set N1" knobs under the engine gauges?  Are these left over from older versions of the aircraft, or some sort of over ride?

Thanks

Iain

[Litjan]

43 minutes ago, Iain said:

So what is the purpose of the "pull to set N1" knobs under the engine gauges?

It is perfectly legal to fly the 737 Classic with the FMC inoperative - in that case you would consult the books to find the correct N1 value, set the carots in manual mode and use the autothrottle or manual throttle input to set the power accordingly.

[Iain]

Great info Jan, I never thought about that reason.

 

Cheers

 

[Litjan]

@Ian

Hi, I have checked the behavior on the XP12 version I am testflying right now, but I am sure that the logic for this was not touched since the XP11 version:

When moving the pack switches to OFF (moving between auto and high should have no effect) the N1 indicated on the N1 LIMIT page increases by 1.0 percent N1 - this is correct and according "to the book". It makes sense, too - without the bleed air being extracted to drive the packs you can create more thrust before you hit the EGT limit.

What is probably missing is getting the same effect when turning OFF the engine bleeds - as you would in a takeoff where the APU powers the packs (as opposed to having packs off for an unpressurized takeoff).

I will add an issue to our internal list to also add this dependency.

Cheers, Jan

 

[AngelOfAttack]

104kg per pax sounds too much even including bags. We use 75kg including bag. Well, depending on which country you are in it might be different...

As for landing fuel, we also have 5% or 15min consistency fuel and 13min maneuvering fuel. With some extra, over 3.5t would be very normal.

[Iain]

Thanks for the info AoA, I watch a lot of YT video's, a few on the -300, many -400 (as they are still flying) and many NG versions.  Looking at there routes, weather, alternates etc and see that landing with 3.5T or more is very common.  But it works very similar in XP11, nice weather, close alternates etc, can land with 2500kg most of the time.  But I have a very adventurous side with this aircraft, Oslo - Svalbard is one of my favourite routes, that's 16'000kg of fuel every time, and from USA WA regions up to Yellowknife is also a favourite.  I also love flying in and out of 4000-5500ft runways (currently based in KAWO Arlington) and flying to remote Canadian locations, so fuel tankering is a factor along with bad weather and very hard mountain tops!

[GusRodrigues]

On 5/14/2023 at 9:23 PM, AngelOfAttack said:

104kg per pax sounds too much even including bags.

It isn't. Delta, for example, uses 230 lbs per adult on board. This provide weight computation on pre plan to match expected weight at departure time even with gate checked baggage due to full bins.

[AngelOfAttack]

11 hours ago, GusRodrigues said:

It isn't. Delta, for example, uses 230 lbs per adult on board. This provide weight computation on pre plan to match expected weight at departure time even with gate checked baggage due to full bins.

Well, that's impressive difference, we only use 77kg for long international flight, others all on75kg. adults including bag.

[GusRodrigues]

10 hours ago, AngelOfAttack said:

Well, that's impressive difference, we only use 77kg for long international flight, others all on75kg. adults including bag.

US operators have to provide to FAA in their Op Spec how they are computing weight. So, usually is the US national average weight (180 lbs) + the max allowed carry on weight (50 lbs). I know it could sound much, but this provide enough cushion for take-off performance, lower performance degradation, and a simplified computation for gate check baggage (that could easily miss handled and may hurt operations).

 

They have internal weight tracker to see how the assumed weight system is deviating from actual performance. Last talk with some people from this department, they are pretty closer to the OEM performance numbers with minimal degradation.