747V3 reverse thrust is wrong

[phil747fan 14]

A few things here:

 

We don't ban people for disagreeing. That accusation, however, says a lot about the level at which we're being engaged. The echo chamber comment does as well (upvotes "enforce" nothing - if life were based on upvotes, one would imagine that laws and recent political outcomes would be vastly different). Echo chambers do nothing but weaken the arguments of those who will do everything to remain in them. They're entirely counter productive.

 

Similarly, the idea that all comments deserve to be treated with velvety gloves is equally counter productive (the idea that "everyone's opinion is valid and deserves to be heard and respected" is to construct an echo chamber if you think about it). If you opt to offer up criticism, one should be prepared to have that criticism addressed in a manner equivalent to its offering. I challenge any of you to try this "well at least show some respect to a fellow human being's thoughts" in any other setting. Walk up to a chef and tell them they did some dish incorrectly, and you know because you watched the Tastemade channel on YouTube. Walk up to the photographer at an event and start telling them that their technique is wrong based on some YouTube videos you watched. Walk up to a cop directing traffic and explain to them how they're directing traffic wrong, based on the same. Walk up to a pilot after a flight and tell the pilot that some technique used during the flight was flatly wrong, based on some video you watched.

 

Criticism is fine, but the approach also matters. If you approach bluntly with a categorical statement, you will likely get a similar return.

 

 

 

Something you all may be missing, however, is the fact that I've already somewhat addressed this point in another thread. The same person who posted this thread had previously asked if we were going to model the short field package that KLM had (based on this exact video if I recall correctly). This doesn't exist. In that thread I noted that the EEC will determine the appropriate max for the situation at hand. As an example, warmer weather will drive the max up. You can see that the temp in the video of the real landing is around 30 degrees, and it seems that they've really pulled the levers up. Most of our testers have been using real world weather. Most of the areas in which they've flown have been in the northern hemisphere, where it is now winter. Colder temps mean lower max. Additionally, it isn't really necessary to hammer on the engines if you don't need to (and in my case, my hardware throttles will keep attempting to set throttle to zero while I'm holding the F2 key because I'm too lazy to set up my null zones), so most aren't using full reverse (which you can't see because none of us are showing camera views of the throttles while we're actively engaged in stopping tonnes of aircraft).

 

Here is an issue where someone saw one video, of one aircraft, doing one landing, at one field, in one weather condition, with one crew and assumed the entire fleet of 747s will always also do that. This is why operational experience vastly outweighs internet sleuthing, and why it's a little frustrating to see one person see a tiny aspect of a HUGE picture and have them tell you that it's categorically wrong. In a similar vein, if you look at only one pixel of an image on your monitor, it will look red, blue, or green. The whole picture, however...well, as you can see by reading this, it isn't red, blue, or green.

 

When I moved back from Phoenix, I managed to get 40 MPG between fueling up in Thoreau and getting to Albuquerque (where you descend about 2500' continually for about 90 miles). My specific car, doing one specific drive, in one area of the world, in those weather conditions, with me driving, accomplished that. That doesn't mean that it will behave that way everywhere (in fact, the car was geared to be 'peppy' on accel, so the highway mileage was pretty garbage). If some devs were going to be designing a game that included the car, I wouldn't have asserted my thought that they got fuel burn wrong because it didn't always behave like that one specific scenario. I had the operational experience in the car to know that the one specific scenario was just that.

 

 

 

By all means: share your thoughts. All the same, if your thought is "this is wrong because internet," be prepared for a relatively blunt response. Again, I'll go back to the earlier examples. Go tell some random pilot their landing was awful based on landing techniques you read online. Let me know if they took you seriously.

 

 

Because it's a forum meant for interaction. All the same, if you're going to bluntly walk up to anyone and say categorically "this is wrong" anywhere, you should be prepared with facts, evidence, and better yet, credentials. For anyone who is a pilot here, who has sat next to the newly minted PPL holder on a commercial flight telling someone else that the hard landing was garbage despite the gusty conditions, short runway, and recent rain that passed through? Said PPL holder is welcome to believe that. Everyone may have an opinion. That doesn't mean that the opinion is beyond reproach.

 

If someone doesn't rebuke the unfounded opinion of said PPL holder, that poor passenger now thinks that this pilot is bad, despite the pilot doing everything by the book for the situation. This is neither fair for the pilot, nor the passenger. And sure, it might seem a bit 'mean' to rebuke the new PPL holder here, but I'd bet you that said PPL holder won't be making that unfounded comment in the future, having learned something.

Kyle,

 

on another note not all 747 rated people (including pilot, tech, dispatcher etc...) are part of PMDG ....

 

enjoy your Christmas day.

[thibodba57 462]

I'd just like to point out that this is not true in all cases. Carbon brakes don't wear well if they don't get any heat into them during braking. Adding lots of reverse on a long dry runway may actually wear out the brakes prematurely.

 

https://airlinesafety.wordpress.com/2014/04/03/how-to-get-the-most-out-of-carbon-brakes/

 

Also, during rejected takeoffs, the deceleration rate is not fixed. 3000psi is applied to the brakes with only the antiskid and torque limiting system to stop the tyres shredding or the gear self-destructing. On a wet or slippery runway, the antiskid may cut in frequently to help stop skidding/aquaplaning. Reversers will most definitely help decrease the stopping distance in these situations.

 

Noise abatement is one of the primary reasons for not using reverse or using only idle reverse.

 

Cheers

John H Watson

 

RTO might not have a programmed deceleration rate.  However, Auto 1-MAX do.  The computer uses the IRS accelerometers to determine if more or less brake pressure is needed to increase/decrease the deceleration rate.  So on a slippery runway yes it will help brake the aircraft faster.  But on a dry runway, Max reverser credit is really only going to spare you a few hundred feet.  This is why IDLE reverse is used in standard ops.  Its much cheaper to burn up the brakes with IDLE reverse than it is to ingest FOD doing max reverse and ruin an engine.

[member111333 8]

There's very likely an FSX limitation at play here - ever tried the reversers in the default aircraft? The FSX engine model (which all jet addons have to use at some base level, regardless of what's being shown on the cockpit displays) is crappy with this stuff. I know I've brought this up with our coders in previous development cycles and I'm basically positive we've done all we can with what FSX gives us. I will ask about the discrepancy vs. OP's video, but seriously guys - you're not buying a real plane here, it's not going to be 100% identical in every conceivable way. It's the best we can do operating on what's now a nearly 11 year old simulator.

 

I just checked btw and max reverse at that temp seems to be around 76% on our GE's at 25C in the current beta - 58 is definitely not our max reverse, wherever you saw that. Also - what really matters here is the aircraft's stopping distance and the wear placed on the brakes. Reversers simply save brake wear and life - they don't make the aircraft stop any faster. The combination of autobrake setting and reversers result in a preset deceleration rate, regardless of how much reverse is actually used. This means the airplane's going to stop in the same distance regardless of what combination of autobrake and reverser levels are actually used. If more reversers are used, less brakes are used to achieve the same stopping distance.

Thanks for the explanation. Really interesting. Now a "officially youtube proofed aviation expert" has learnt something very basic about reverse thrust and braking.

[Qavion 19]

RTO might not have a programmed deceleration rate.

 

 

Might not? :smile:

 

It's interesting to note that, according to the manuals, an IRUs is still required for RTO application. However, not it the way Autobrake 1~Max requires the IRUs.

Note that you can still arm RTO with the IRUs off. However, one of the requirements for RTO application (activation) is that the Brake System Control Unit ("computer") receive a valid (Captain's selected) IRU signal. RTO requires wheelspeed for determining if the aircraft speed is above 85 knots (RTO won't happen below this). However, the wheelspeed must agree with the IRU groundspeed for RTO to operate. The Brake System Control Unit computes an average wheelspeed of all 8 front (main gear) wheels, but then discards the two highest and two lowest wheelspeed values (Necessary complication?).

 

Another reason for not using reversers is the fragile nature of the reverser mechanism  and the fact that running an engine at high rpms will always put more strain on an engine generally. Reverser problems can be a real nightmare for engineers :wink:

 

Cheers

John H Watson

[thibodba57 462]

I would counter use of the IRU and having a programmed deceleration rate are quite different.  The is no letup on the braking system with RTO you can increase or decrease rate of deceleration by varying reverse, spoilers, encountering dry or any variation of contaminated runway.  So while the system might use the IRU to arm itself it doesn't use its input to vary brake pressure to maintain a predetermined deceleration rate.  

[mwilk 1,106]

This is exactly where i got the idea at first. Turned out, someone already made it. Here's the link. http://forum.simflight.com/topic/69388-throttle-manager-to-allow-axis-forwardreverse-toggle/

 

It's a bit messy to set it up at first, specially if you use multiple throttle axes. It's worth it at the end, though

Thank you. I'll give it a try.

[phil747fan 14]

to avoid FOD if my memory works it is up to 60 knots for GE and PW and 70 for RR engines ... we are talking reverse here.

 

on carbon brakes i think again memory only that autobrake works better on 2 and 3, 2 recommended was 2 for the two airlines i worked.

 

there is again from memory a difference between using no reverse on dry runway less than 20 to 100 meters and were poor braking conditions are met 500 to 1200 meters ... so suggestion for wet conditions was to use reverse and not at idlle unless MEL or maintenance issue or recommandation. actual QRH may answer some of this, PI i think.

[Qavion 19]

I would counter use of the IRU and having a programmed deceleration rate are quite different.  The is no letup on the braking system with RTO

 

 

Brian, you don't seem to be reading what I'm writing. You wrote that "RTO might not have a programmed deceleration rate."  I questioned your use of "might" because I know it definitely doesn't use the accelerometers in the IRUs. I then proceded to tell everyone how the IRU is used for RTO. Then you basically repeated what I said... But then you wrote this...

 

 

So while the system might use the IRU to arm itself it doesn't use its input to vary brake pressure to maintain a predetermined deceleration rate.

 

 

The system does NOT use the IRU to arm itself. My engineering training notes tell me this. That's why you can arm RTO with the IRUs OFF. There is a different set of logic for application (which does need the IRUs).

 

Please read my earlier messages, too... Especially the part which says... "I'd just like to point out that this is not true in all cases. ". I'm not disagreeing with everything you are saying, but you seem to be making out that I am or that what I am saying is wrong.

 

I apologise if this is not what you are intending to do.

 

Regards.

John H Watson (Former PMDG beta tester/tech advisor and 747 (-200, -300, -400, -400ER, -400F) engineer (for 40 years)... I even worked on C130s for a while as a lowly apprentice :wink: (ugh)

[JRBarrett 3,070]

I ran a quick test in Aerowinx PSX, using the CF6 GE engine. I parked at a sea level airport, (KSFO), and checked max reverse N1 on one engine. (Setting all 4 engines to max reverse thrust while sitting still would probably be a bad idea!)

 

As expected, the maximum N1 in reverse varies with air temperature. At 0C, it reached 87 percent. At 15C, it reached just under 93 percent.

 

Of course, PSX, being a fully stand-alone simulation of the 747-400, is not affected by limitations of the underlying FSX/P3D engine modeling, which may limit the maximum reverse N1 in the PMDG aircraft as Tabs explained.

[jcomm 5,925]

I ran a quick test in Aerowinx PSX, using the CF6 GE engine. I parked at a sea level airport, (KSFO), and checked max reverse N1 on one engine. (Setting all 4 engines to max reverse thrust while sitting still would probably be a bad idea!)

 

As expected, the maximum N1 in reverse varies with air temperature. At 0C, it reached 87 percent. At 15C, it reached just under 93 percent.

 

Of course, PSX, being a fully stand-alone simulation of the 747-400, is not affected by limitations of the underlying FSX/P3D engine modeling, which may limit the maximum reverse N1 in the PMDG aircraft as Tabs explained.

 

Aerowinx PSX is THE 747-400 simulator... That's about all that can be said, and yes, I had ran the tests too, and would probably run many others... but it's useless I believe... we're comparing apples to oranges.... Unless  fully external flight dynamics engine is used, which would then  be difficult to properly integrate with weather and other aspects of the underlying simulation platform ( fsx or p3d ) there's simply no way such a level of detail can be achieved, starting with the basic modelling of pitching moments due to thrust, only exception really being the visuals - that makes a huge difference indeed, and it's the only major limitation that an fsx version of the Queen will not impose... or even better, and x-Plane one ...

 

Maybe one day we can get the ultimate perfection of systems / flight dynamics modelling of Hardy Heinlin's masterpieces, and the great graphics of X-Plane or Aerofly FS 2 merged in a unique Boeing 747-400 simulation...

[walfob 10]

On the first video of this thread, of the 747 landing at Jo'burg, there are two orange balls on the centre column of the cockpit window. Can anyone tell me what these are? I've noticed them on Airbuses but never knew what they were.

 

Will they be modelled on 747v3?

 

Happy New Year!

[Flamin_Squirrel 71]

On the first video of this thread, of the 747 landing at Jo'burg, there are two orange balls on the centre column of the cockpit window. Can anyone tell me what these are? I've noticed them on Airbuses but never knew what they were.

 

Will they be modelled on 747v3?

 

Happy New Year!

 

IIRC they are there so the pilot can ensure their eye position is in the correct position for best visibility (both inside and outside).

[walfob 10]

 

 

IIRC they are there so the pilot can ensure their eye position is in the correct position for best visibility (both inside and outside).

 

Thank you.  IIRC meaning...?

 

So presumably an airline can fit them as an option and they don't need to be Boeing extras? 

[scandinavian13 5,192]

 

 

IIRC meaning...?

 

"If I Recall Correctly."

 

 

 

So presumably an airline can fit them as an option and they don't need to be Boeing extras? 

 

I've never seen them. All of Boeing's docs refer to existing flight deck infrastructure when setting an eyepoint. Airbus and Canadair use the ball method for sure.

[Flamin_Squirrel 71]

Ah sorry, IIRC means "if I recall correctly".

 

I can't see any reason why they wouldn't be on as standard.