Reverse thrust limits

[Hardy Heinlin]

Good morning,

two questions, please ...

1)
Different engine models obviously use different reverse thrust limits.
But do different airlines use different reverse thrust limits on the same engine model?
E.g. does Qantas and BA use the same reverse thrust limits?

2)
Where is the reverse thrust limit programmed? In the EEC or in the FMC?


Thank you,

|-|ardy

[G-CIVA]

1)
Different engine models obviously use different reverse thrust limits.
But do different airlines use different reverse thrust limits on the same engine model?
E.g. does Qantas and BA use the same reverse thrust limits?

Yes - see the illustrations & note the differing explanation regarding the Alternate EEC Mode for REV as depicted for RR engined aircraft within the QF fleet compared to the GE equipped aircraft.

2)
Where is the reverse thrust limit programmed? In the EEC or in the FMC?

It would appear from the FCOMs that ... in the case of the RR engine ...

"The EEC selects an unannunciated alternate mode when reverse thrust is used due to EPR sensing inaccuracies during reverser operation."

BAW



QFA



FYI PW engines appear to operate in the same manner as RR engines (source SIA):

[John H Watson]

But do different airlines use different reverse thrust limits on the same engine model?

What do you mean by same engine model? Do you consider an RB211-524G the same as an RB211-424H?

Or the RB211-524G2 the same as the RB211G3?

Or an RB211-524H2-T-19 the same as an RB211.72.C507?

Basically G and H should not be interchanged because of different thrust ratings. Depending on the engine, the engines can be modified from a G to an H and vice versa. RB211's can be fitted with Trent cores (which allow the engines to operate at higher temperatures), but I 'm not 100% sure if you can have a mix of Trent and non-Trent cores on the same aircraft. 

QANTAS Engine designation is RB211–524G2–19/15 or
RB211–524G2–T–19/15
• RB211 = Engine Type.
• –524 = Engine Series.
• G2 = Engine Mark No and Thrust Rating.
• –T = Trent HP system fitted.
• –19 = Boeing 747 Installation.
• /15 = Customer Number.

When changes are made to the engine, the FMC shows these differences (but only "G" and "H". It can't detect "T"). The CMC does show RB211-524G-T-19

CF6:

Reverse Control
Reverse control is active whenever the thrust reverser is not fully
stowed. The EEC calculates the reverse thrust demand based on the
thrust lever position. If the calculated reverse thrust N1 demand is
greater than 3,280 rpm, or if the thrust demand is calculated to be
greater than about 30,700 pounds, the fuel flow is reduced to ensure
that these limits are not exceeded.

Maximum reverse, set to a maximum N1 speed of 100% any time
the TLA position is at full reverse travel and both translating
cowls are deployed.

What prompted this question?

2)
Where is the reverse thrust limit programmed? In the EEC or in the FMC?

I'd say the EEC. Thrust derates, autothrottle, etc, won't affect reverse thrust.

[Hardy Heinlin]

Thanks, Steve. The EEC alternate mode stuff is clear and and is modelled in PSX. Also that EPR model engines use N1 for reverse control.

Do you consider an RB211-524G the same as an RB211-424H?

Not the same. Any different letter or number and I consider it not the same.

I've been told BA uses ca. 1.30 EPR / 70 91% N1 for the reverse thrust limit. I guess that's lower than in Qantas RR aircraft.

Whether FMC or EEC -- I'm asking because the FMC manages the forward thrust limits when the FMC is operative. Maybe it affects the reverse thrust limit as well when the FMC is operative. But I guess all reverse stuff is EEC stuff.


Regards,

|-|ardy

[Magoo]

Also there is no mention of a caution using reverse thrusters when you have a double FMC failure in the QRH.

[John H Watson]

I've been told BA uses ca. 1.30 EPR / 70% N1 for the reverse thrust limit. I guess that's lower than in Qantas RR aircraft.

Do BA mostly use the H model? If the H model has a higher thrust rating, the reverse may not have to be so strong (percentage-wise). Did someone official at BA tell you, or is this random data from sims and YouTube?

I think Qantas must have only had the G model. The FCOM only lists performance data for the "G" model and there were no instrument panel reminder placards like BA has for thrust rating.

[G-CIVA]

I've been told BA uses ca. 1.30 EPR / 70% N1 for the reverse thrust limit. I guess that's lower than in Qantas RR aircraft.

Sounds like one of those 'flight sim internet urban myths to me'.

From the BAW FCOM v1 ...



From the BAW FCTM ...



From an SOP point of view at BAW the pilot not flying control the selection & deployment of the pre-briefed level of reverse after the pilot flying has pulled the throttle levers back to the idle stop while the pilot flying continues with the landing manoeuvre, braking & subsequent roll out:

https://youtu.be/KPBp6qrqfVk?t=329

At Qantas the pilot flying selects & operates thrust reverse in the usual way:

https://youtu.be/LRUCnV3i25M?t=148

I see no reference to a particular EPR or N1 setting in any of the BAW reference material at my end.

I suspect the SPD limitations for the use of anything greater than Idle Reverse are referenced for good reason:



An old discussion regarding RR RB211-524 G & H rated engines:

https://www.pprune.org/tech-log/166369-rb211-nomencalture.html

Cheers

Steve

[Mariano]

Hardy,

With respect to item two, looking at CF6 manuals, seems like the ECU deals with thrust reverser-related operations (it receives signals from TLA transducer and both LVDTs, excites both LVDTs and commands interlock actuator).

I see no reference to FMCs whatsoever.

I have a CF6 thrust reverser electrical schematic, but I'm sure you already have one for each engine type.

Best regards,

Mariano

[Hardy Heinlin]

OK, I think we can all agree that the FMC has no influence on the reverse thrust limit.

BA's 1.3 EPR REV limit has been confirmed by an active BA trainer and is applied to both G and H ratings.


Cheers,

|-|ardy

[John H Watson]

Sounds like one of those 'flight sim internet urban myths to me'.

Steve, we are not talking about crew operations, but engine logic. With the reverse levers fully up, max reverse thrust is limited to a certain N1 value by the EEC.

In my engineering training notes, the limit is stated (for "all" RB211 engines) as:

91.0%N1 for 40 seconds (100% = 3900rpm). During ground tests, we are not permitted to go above 30%N1. Idle reverse is restricted to 5 minutes (operationally).

I don't specifically know the relationship between N1 and EPR during reverse thrust operations. Note that there is different air control (VIVG/VBV values) through the engine during reverser ops.

We know that EPR can be unreliable during reverser ops, but I guess the big sims show steady readings.

[Hardy Heinlin]

I think I'll add an option for airline specific REV thrust limits ...

[mark744]

From the limited information available so far,
would one option for an RB211 of 91.0%N1 and 1.3 EPR  be a reasonable suggestion?

[Hardy Heinlin]

I'll just add an editable N1 value for each aircraft model.

[Hardy Heinlin]

Well, if the 91% N1 REV limit in the current PSX versions is OK, then there's no need to make an option. I'll just have to adjust the EPR model for reverse operations so that it indicates 1.3 EPR at 91% N1 REV.

Is the PW EPR limit OK at 91% N1 REV?

[John H Watson]

Well, if the 91% N1 REV limit in the current PSX versions is OK, then there's no need to make an option. I'll just have to adjust the EPR model for reverse operations so that it indicates 1.3 EPR at 91% N1 REV.

Doesn't that contradict your earlier statement?

I've been told BA uses ca. 1.30 EPR / 70% N1 for the reverse thrust limit.

Sorry, couldn't find any notes on N1 limits in my PW manuals.

[Hardy Heinlin]

Yes, a contradiction, sorry. I was confusing max reverse with partial reverse (intermediate lever stop).

Partial Reverse 1.2 EPR / 70%N1
Full Reverse     1.3 EPR / 91%N1

There is no intermediate stop for the mouse or keyboard. So it's just about the 91% limit and that seems to be OK in PSX; my RR model just needs a special N1-to-EPR translation when in reverse mode. This is just indication cosmetics because the physical model uses N1.


|-|ardy

[G-CIVA]

Steve, we are not talking about crew operations, but engine logic.

John

Fair enough, but Hardy is quoting a statement made by a current BA trainer (not an engineer) which is applicable to both G & H rated engines regarding 1.3 EPR/91%N1 REV Thrust.

It seems we are all at crossed purposes, therefore I shall respectfully bow out.

Best

Steve

[Hardy Heinlin]

:-) Sorry for the confusion. It's all adjusted now (on the RR) and will be available when PSX 10.61 is up.


Cheers,

|-|ardy

[Hardy Heinlin]

I just noticed my REV EPR modifications are variable as the relation between EPR and N1 depends on OAT and pressure altitude; my forward thrust model takes these parameters into account, of course. And so does the reverse thrust model too.

Is there any OAT/pressure altitude reference -- e.g. ISA at sea level -- for the RB211 reverse thrust values mentioned above? I mean these values:

1.1 EPR --> REV Idle
1.2 EPR / 70% N1 --> REV half up
1.3 EPR / 91% N1 --> REV max

When the OAT varies from -80 to 50°C, the N1 varies by ca. 30% when the EPR remains constant.


Regards,

|-|ardy

[John H Watson]

Nothing in the AMM. I don't see how there would be any hangar maintenance data for 70% or 91%, because engineers are not allowed to take the engine past 30%N1.

I doubt pilots would be looking at N1's during rollout.

Can you get an approximation of EPR from this blurry Qantas video? (From 1:50minutes).  It certainly looks like it's 1.3 or more at max reverse.

https://www.youtube.com/watch?v=IEwqymZWaM4