Hello folks,

I am not very experienced in regard to wide body A/Cs, so I see myself as a kind of beginner in that area.

It happend twice over the last two weeks that I "softly" slammed the T7F onto the runway (sim-in-sim mode for sure): :huh:
-first occurrence: ROD > 1000ft/m (fsx crash display)

-second occurrence "Tailstrike" (nicely displayed on the ECAM)

Both happened at flaps full, 148kts approach speed (as recommended by FMC), manual Thrust (with A/T arm switch off).

Reason of crash/tailstrike: airspeed too low because of insufficient thrust. Thrust levers set to full power too late by PF (me).

I know, that the operating manual underlines "A/T arm switch always on" because otherwise all the A/T functions like Auto-Idle on RAAS "Short-Runway-Alert" (!), Safe-Speed-Thrust on ECAM "Airspeed low" and so on will not function. (Surprisingly though TO/GA buttons nevertheless activate A/T even with "A/T arm switch" off. Only knew it as a FD only TO/GA trigger when A/T not armed.)

Well, I have done quite dozens of (FS-sim-)landings with B747 and A340 without even getting close to such situations. Perhaps it would have been a different story if I would have tried the A330 some times before switching to the T7?

 

And this leads me to the point:

IMO T7's GE90, a great and most reliable jet engine, might have a kind of "tricky" latency which is "beyond human intuitive expectation". Will say: in case of doubt only EEC can provide sufficient "pre roll". AND: only explicit (sim) training can implement respective foresight by the crew.

Theory:

Fan diameter seems to be at a critical size, compared to e.g. B747 standard engines. Even though the fan itself being more lightwight, the immense torque produced by the aerodynamics gives the N2 department "a very hard time". Small diameter fans, even when made of heavier material, have less latency, as their torque request is much smaller. This again would mean that migration from an A340-300 with small diameter fans (CFM 56!) to the T7 requires the biggest attention to latency changes and respective crew training.

 

Anyone making the same experience?

 

Always happy landings (and this is meant very serious!),

Claus


 

Not quite sure I am getting your post?? Are you talking about spool up times? It sounds like you have been using pretty basic addons in the past (I don't know of any A330 addon that correctly simulates spool times)

 

Jet engines take a long time to spool, on average 8 seconds from idle to TOGA, even with approach idle, spool up from idle will take some time.

 

Btw, I say it all the time here - Stick to stable approach criteria - 500 VMC 1000 IMC aircraft should be in landing config, within 5 knots of Vref and engines spooled. Follow that basic criteria and you will be fine. If not Go Around.

 

Regards

Btw, I say it all the time here - Stick to stable approach criteria - 500 VMC 1000 IMC aircraft should be in landing config, within 5 knots of Vref and engines spooled. Follow that basic criteria and you will be fine. If not Go Around.

 

Thanks for the good list of procedural items.

Regarding the sense of my post I should have mentioned that I made visual approaches with initial high speeds from relatively high altitudes and small RWY distances. (Which means high speed below 10000 ft and short distance visual approaches as  often cleared by ATC when traffic and WX allows.)

While "working the airspeed down" during final approach it is a T7 characteristic that flaps 25 and 30 (for deceleration) while still on idle is a bad idea. Therefore the A/T speed protection comes into play. I had turned it of intentionally to fly it all manually.

 

And as i was just "used to " B747-400 or A340-300, I noticed a much higher latency of the engines than on the other two wide body models.

 

Greetings,

Claus

It is definitely not beyond human intuitive expectations (well if I understand what you mean right) - you just need to get used to it a bit and "be in front of the plane". Bit of predictive flying instead of reactive.

 

FWIW this has been a genuine issue back when early jets were pushed into operation, as pilots were used to near-instantaneous power of the piston engine, and sometimes they would fail to appreciate the fact that jets will be slow to react.

Thanks for the replies. The pilot "rather flying in front of the plane than behind it" is a must, for sure. And in Comet's time pilots migrating from 2 or 4 prop engine A/Cs to jets often had huge problems with delayed thrust response (or stalling jet engines...).

 

But I thought of today's problems migrating from small or medium sized fan diameter engines to supersized ones like the GE90.

 

What contributed to my bad landing performance (on a T7F) was for sure the high landing weight of approx 250 t, which is less then 10 tons below the maximum. My ZFW was 230 t, FOB 20 t.

 

I would like to make just one more try to explain my "special landing experience" and the impression I got from the T7:

Comparing it with cars, the T7 and its GE90 engines is a modern car with 6 gear transmission which is the main reason for being more fuel sufficient. An A340-300 has 5 gears and a B747-400 has 4 gears. Always being in the 6th gear saves a lot of fuel but has one downside: as transmission is always high, torque forces grow immensely on sudden higher RPM requests. And for a fan propulsion system "transmission" means effective thrust, not only RPMs. If fan blades were able to change the AoA (like constant speed prop blades) the problem could be reduced. (I think they are working on it already.)

What also is interesting, that a GE90 can produce effective thrust 6 times of its own weight. But - if I remember correctly -  this was already the case for early B747 engines, but here are 4 engines, each one is lighter (smaller) but consumes much more fuel (something like 40% difference from 1969 until now). So overall thrust is delivered by 4 engines with the same weight/thrust-aspect ratio, but therefore running in lower gear with the downside of being infamous for fuel (and maintenance) costs...

O.K., sorry - it gets complicated again...

Even if some of you would think now: "this crazy German guy, what is his point?" I send out one more thought about "jet engine thrust application improvement":

Beside the mentioned "constant speed fan blade" it could be worth thinking about a (slightly) tiltable engine mount or carriage, that improves the angle between thrust vectors and flight trajectory.

(?? well: on TOs, GAs  and Climbs a part of engine thrust "gets lost" for "vertical take off ambitions" which is not economic for a fixed wing plane... :wacko:  )

 

Don't get me wrong,

Claus

I kinda get what you are saying. I don't remember if I saw it in the manuals or here on the forum, but the reduction from takeoff power to climb power appears slower than what I'm used to seeing due to the much larger blades and the amount of momentum they carry.

I kinda get what you are saying. I don't remember if I saw it in the manuals or here on the forum, but the reduction from takeoff power to climb power appears slower than what I'm used to seeing due to the much larger blades and the amount of momentum they carry.

 

Yes, that's part of what I mean, just the other way around: in the case of thrust reduction (from high to low RPM) the fan momentum delivers excess energy. From low to high (TO/GA) it "eats up" energy for a serious amount of precious GA-time...

And that serious amount of time is what I originally posted as "engine latency problematic?".

 

Thanks, Bryan!

Guy's I am probably still not getting it It doesn't matter if it is CFM56 on a little A320 or a GE90 on the 777, the FADEC and EEC takes care of things. Every engine has to undergo certification proving it can accelerate from approach Idle to TOGA within a specific time frame.

 

 

Every engine has to undergo certification proving it can accelerate from approach Idle to TOGA within a specific time frame.

 

Thanks, Rob

I don't have the numbers for APPR IDLE to TOGA time frames, but is it possible that this time frame is more widely spread for the T7 than for an e.g. A320? It definitely must be like that...

 

(BTW: time frame for a specific engine expands when powering a heavier A/C because of increased lift (=drag) and acceleration (=mass latency) requests.)

Hey Claus,

 

The weight of the aircraft is irrelevant, I don't have figures on me as I am away from home (will do some research)

 

From memory, maximum spool time (ISA conditions) is 8 seconds, CFM equipped A320 takes 7 seconds, even if the GE90 was slower it could only be a second slower, not exactly noticeable.

 

I don't even think the GE90 is slower, it could very well be faster than the CFM56.

 

EDIT - Ok I checked! I wasn't too far off,

 

For the A320 - AMM guideline

 

CFM56 7.5 seconds

V2500 6.2 seconds

 

So, let's say the GE90 was slower, it could only be half a second slower then the CFM56.

 

Regards

The weight and complexity of a tiltable engine pylon mechanism would surely offset any small performance improvement gained, not to mention the safety concerns if it went wrong.

 

As for the high gearing analogy, it's important to remember that the HP and LP spools are not mechanically connected, so an HP spool does not labour like a vehicle engine trying to accelerate in high gear. It's more like a car with an automatic transmission. The HP compressor can stall (aerodynamically) or surge which is why gas turbine engine acceleration rates are limited. In fact the engine could accelerate faster, but needs to be limited to keep away from the surge region. The thrust response would indeed be quicker if the fan acted as a constant speed propeller, but do pilots really want to have to deal with rapidly changing forces and moments when the thrust is as high as the GE90-110/115.  This is one reason why aircraft like the T7 have automatic thrust asymmetry compensation. The sudden thrust asymmetry from an engine failure could be too great for a pilot to safely react to in time.

 

As well as increasing thrust response rates a constant speed fan would improve efficiency. However, like tiltable engine mountings, it would have a weight penalty would need to be offset against any improvements.

Hi folks,
thanks again for the replies and sharing thoughts about GE90 engine performance on the T7.
Yesterday I made again quite a few touch and goes with the T7F (with approx. 10 t below maximum landing weight) for further evaluation: I stick to my opinion that the T7 seems to be less forgiving than a B747-400 or A340-300.

maximum spool time (ISA conditions) is 8 seconds, CFM equipped A320 takes 7 seconds, even if the GE90 was slower it could only be a second slower, not exactly noticeable.

The PMDG T7F "in fact" needs about 8 seconds to change from APPR IDLE to TOGA (on the N1 display). So this time frame seems quite normal and is nothing pilots could be surprised of.
I noticed nevertheless two things I find remarkable:
1) commanded EEC TOGA is a typical engine safe value below 100%. O.K., that is normal but actually nothing a PF needs in case of emergency. Disconnecting both EECs on the overhead panel would be too distracting and time consuming...
So perhaps an "emergency power" switch, like it can be seen on e.g. turbo prop floating planes, could safe the day.
2) O.K. N1 is showing up in 8 seconds, but acceleration itself remains quite overdue. (Here - at latest - comes the point where it can be questioned how precise the T7 and its GE90 is actually simulated by PMDG.) N1 does not represent delivered mass acceleration. It is just a percentage of the max. allowed RPM of the fan. If the fan stalls (e.g. hypersonic airflow) the resulting thrust would be almost "nil" even at 110% N1...
So the lack of sufficient acceleration during the PMDG simulation could be based more on mass latency and high aerodynamic drag (flap setting and high pitch) than on "specialities" inside a GE90.

The HP compressor can stall (aerodynamically) or surge which is why gas turbine engine acceleration rates are limited.

Would be interesting to know if such engine internals are actually simulated. E.g. is it possible to intentionally stall engines running without EEC inside the T7X?

As for the high gearing analogy, it's important to remember that the HP and LP spools are not mechanically connected, so an HP spool does not labour like a vehicle engine trying to accelerate in high gear.

With the high gearing analogy I did not mean the ("viscous") connection of N2 to N1. I thought of the difference of transmission effort between small and large diameter fan blades. So a large diameter fan operates on lower RPMs producing the same amunt of thrust as a small diameter fan operating with higher RPMs. Actually like a large tire requires lower engine RPM than a small tire for the same speed on the road. The small tire requires higher engine RPM which results in higher fuel consumption. BUT the small tire can be accelerated easier, as it does not require more torque but higher engine rpm, which is easier to deliver. (reduction, tire on road=effective thrust)
Facit: the large fan diameter delivers fuel efficiency for the price of agility.
 

As well as increasing thrust response rates a constant speed fan would improve efficiency. However, like tiltable engine mountings, it would have a weight penalty would need to be offset against any improvements

O.K. That was only imagination. Although constant speed blades for high bypass fan engines are in development or evaluation phase, I think. Isn't the UDF (unducted fan) project a constant speed fan? Unfortunately it seems to be too loud?!
Tiltable engine mountings are probably developmentally further away, but it would make sense to produce propulsion vectors parallel to the trajectory rather than "somewhere else"...
BTW there are at least two more advantages: intake airflow would be less bended and lower nacelle air would be less turbulent.

Always happy landings,
Claus

Hey Claus,

 

What A340 and 747 are you referring to? I wouldn't look too deeply into this regarding FS addons, best place for your questions would be an actual real aviation & engineering forum http://www.airmech.co.uk/forums/

 

Don't expect these addons to behave like the real aircraft. PMDG and other high end addons are great study tools for procedures and understanding system logic, in no way are they certified or accurate to the level you seem to be enquiring about.

 

Regards

Hey Rob!

What A340 and 747 are you referring to?

It is all only FS based "talk shop". As I said, I tried wide bodies not so often. Autolanding them on 4000+ meter RWYs is actually not so interesting in the long term. So I like very much "practicing" short visual approaches on short runways with a relatively high level of corrections (crosswinds etc.) or late decision TOGAs. Doing so with the A340-300 by PSS in fs2002 or 747-400 by PMDG (fs9 on a friend's pc) was never as demanding as with the T7, what surprised me. I just got the impression that there is a tricky "lower left coffin corner" surrounding this A/C.
The most logical culprit would be the power plant. But it turned out to be questionable if this could be investigated on such - even though elaborated - sim...

best place for your questions would be an actual real aviation & engineering forum http://www.airmech.co.uk/forums/

I have just checked the page. I find it very interesting, but the "romance" of the sim world of actually being "surreal" gets lost on most RW occasions. That is why I prefer vr-piloting to a RW pilot's life, as the heroism and adventure spirit got somewhere else in the meantime. I have a friend working at a Lufthansa engine shop. He is a CFM56 gear box expert. I asked him one day about the location of the two ignitors in the CFM56 engine (>ignition selector switch L/R B737NGX). He asked me: which ignitors? No idea... And that is what I sometimes find so disappointing about RW aviation. The fascination gets lost.

high end addons are great study tools for procedures and understanding system logic, in no way are they certified or accurate to the level you seem to be enquiring about.

I am aware of that. In the background there are thousands of "if.. then..." command lines calculating with variables or fixed values whose origin my be questioned. But devoting oneself to that sim world is a great hobby. Thinking about all technical correlations is a good brain trainer, flying around the globe educates geological knowledge and so on.
Last but not least writing about all that stuff in English is a good training for me too, as this is not my mother tongue.


BTW: one more example of aviation romance vs. RW matters. Today afternoon, when I was at work, I suddenly saw the A350 (AIB101, MSN003) on an official "fly by" inbound final RWY23 EDHI, Hamburg Finkenwerder. I called my colleagues and they all looked up, out of the windows, fascinated, although not all are aviation enthusiasts... I looked on the dataflight24 web page to follow its flight track and was suprised that it seemed to go inbound Toulouse directly after that. Later in the evening on my way home I heard on the radio news that just today hundreds of workers were on strike at the Airbus factories due to planned lay offs of up to 8.000 workers inside the EADS group (Civil Aviation, Aerospace and Military). Gone was all of the aviation romance...

http://www.flightradar24.com/2013-11-28/14:06/12x/AIB101
(Funny to follow the rest of their test flight, at 1547Z (over Rodez, France) they changed call sign from AIB101 to AIB102)

In this context I like that video very much (sorry, Mr. Boeing):
https://www.youtube.com/watch?v=HRT5H375bgQ

Always happy (virtual) landings,
Claus

Alright, I'm going to weigh in here with some real world flying knowledge regarding the performance of turbofan engines.

Vr-pilot, I like the points you're bringing up, and hopefully I can shed some light on why 340s and 747s, despite similar total engine thrust output, accelerate differently. "Latency" is a good word to use here, and it happens as a result of 2 main characteristics inherent to turbine engines.

1 - Spool time: It's been discussed here lots, but there are actual requirements that mean that all engines will have to achieve high power when the controls are essentially "slammed". I'm Canadian, but here's the FAR regulation for you on this. Note this sucker's been around since March 1971, so it's nothing new:
 

FAR Part 33, Subpart E, Section 73

The design and construction of the engine must enable an increase --

(a) We don't care about what 'a' said here. Look it up of you want.

( B) From the fixed minimum flight idle power lever position when provided, or if not provided, from not more than 15 percent of the rated takeoff power or thrust available to 95 percent rated takeoff power or thrust in not over 5 seconds. The 5-second power or thrust response must occur from a stabilized static condition using only the bleed air and accessories loads necessary to run the engine. This takeoff rating is specified by the applicant and need not include thrust augmentation.

[Amdt. 33-1, 36 FR 5493, Mar. 24, 1971]

Now this requirement is critical for large aircraft go-arounds, but also applies to setting takeoff power on your takeoff roll. But 95% of your full thrust can be there in 5 seconds. It's the law.

And now for the other half of this explanation:

2 - Shear force: Here's where more of your "latency" shows up: A small amount of air moving really fast shears through the static air around it without having a large effect on it. Moving a larger amount of air a slower amount decreases the shearing effect the fast-moving wind has, making the air that's been moved more efficient, and far quieter. That's why engines have gotten much larger since the turbojet age. Turbojets created incredible shearing effects. Turbofans were brought in to change that, and have. That being said, a static turbofan, when cranked up to 11 (takeoff thrust), will still shear a lot, which is inefficient

So your latency will still be present, and more so in a 777 than in a 340 or 747. Why in those cases? Simply put, number of engines. 4 engines, despite maybe having less max thrust, have cumulatively more push-power for the initial stages of the takeoff roll because of simply having everything spread out. Not to mention the shear forces on engines 1&2 as well as 3&4 will actually assist each other to some degree.

This shear effect on the GE90s is why your FCTM and FCOMs (if you read them) state that a static start vs a rolling start doesn't make a measurable difference in ground roll for your takeoff. Doing a static start will just put your thrust at it's worst possible efficiency because of the highest possible shear on those engines. Rolling starts allow for the engines to push while they're also spooling up. So for 5 seconds of spool-up, you're taking advantage of relatively lower shear (low shear = high efficiency). Add the fact that you have to do your 50-55% N1 primer before setting takeoff thrust, and you're making good use of low shear. And, last but not least, aircraft inertia and shear forces from engines don't like each other at all. (where the car references don't quite work in these comparisons)

 

Think of the Dyson bladeless fan. That sucker is packed with aviation technology at it's finest. You feel lots of air, but it's not moving all that fast. It uses shear air to actually boost fan air. Dyson claims that almost 1 part of 16 of the air the fan blows actually comes from shear from nearby air. It makes use of it by blowing things slowly so static air can latch on and add to the total, not fast air what would just skip by it.

If you're having trouble understanding shear, grab a nice book on turbine engines, it'll spit out a bit more info.

I could spit out more info on the debate about multi-shaft turbines if you want, or reductor gears, but the essence of those designs is that your compressors need to spin far faster than your fan (not prop, but fan. All modern commercial jet engines are turbofans). A 2 stage compressor has a LP stage that spins as fast as your fan (some argue that's bad). Rolls Royce Trent engines are 3 shaft beasties, where there's an IT (intermediate turbine) that essentially allows the fan to do it's own thing, independent of the compressors (huzzah!). GE is looking at putting a gear between the LP and the fan, slowing the fan down (good), keeping the LP stage speed high (also good), yet the two would be mechanically linked, and therefore proportionate (unlike the Trent engines).

I really hope this shear info helps you understand why the 777 with GE90s has what you call "latency". (i like the term, I really do!)