I am just sitting in my Harrier here and am running some tests, but frankly I have no idea what is happening.
Selecting nozzles down, water injection on and going full power:
00:00 RPM 116.8; JPT 765
00:54 JPT starts to increase
00:57 JPT reaches and holds 780, now RPM starts to decrease
01:15 as RPM reaches 108, it suddenly jumps to 114; JPT remains 780
01:30 RPM is steadily decreasing again for 15 seconds and then jumps to 108 again. This 15s cycle repeats for as long as there is water left
02:50 water runs out; RPM stabilizes at 107, JPT remains 780
Here is a test trying to establish when damage occurs. We are airborne with nozzles forward at full throttle. This gives us 109 RPM at 710 JPT, which is the Maximum Thrust rating which we should be allowed to hold 15 minutes. So far so good.
Now as we are in combat we start to do some thrust vectoring. Setting nozzles down, we get 113.5 RPM at 760 JPT, which seems to be the Short Lift Dry rating which is allowed for 15 seconds. As far as I can tell, the engine starts to show certain signs of power loss after 2 minutes, so 1:45 over the time limit. Beware that without sufficient off time in between occasions, the effect seems to accumulating. Even though the aircraft limitations chart above does not indicate that pilot action is required to maintain this limit (Note 2), the DECS will not limit Short Lift Dry.
Now letās try to reduce power to Normal Lift Dry while VIFFing. We immediately reduce thrust from full power 113.5 RPM to 111 RPM. We should be allowed to use this for 2.5 minutes. As far as I can tell, damage starts to occur at 4.5 minutes, so about 2 minutes over the limit (its hard to tell at lower settings when the engine actually starts to degrade because of remaining power margins).
That is a very limited data set, so it is difficult to draw any conclusions yet. But I will formulate the theory that damage starts to occur after about 2 cumulative minutes over any limit, even with very small RPM exceeding. Considering the number of limits in place as per the limitations chart, this seems to be quite easy to collect unnoticeably during a mission.
Edit: Actually, as soon as you exceed your allowed 15 minutes at Maximum Thrust, damage to the engine occurs instantly!
0:00 the control law in effect of the DECS seems to be the Normal Lift Wet limit (116 % RPM). The DECS will try to maintain the maximum RPM allowable as long as JPT (Jet Pipe Temperature) does not exceed 780 deg C, which is currently the case.
0:54: JPT starts to increase due to engine degradation (youāre basically firewalling the throttle). Increase in JPT can be the way Razbam interpreted the engine degradation effects. Once the JPT limit of 780 deg C is hit (Normal Lift Wet limit enforced by the JPTL, Jet Pipe Temperature Limiter), a loss of RPM would typically be caused a few minutes after since the RPM cannot be maintained if it creates a JPT exceedance.
0:57: JPT seems to be controlled by the JPTL once again. Your normal lift wet limit is 780 deg C is exceeded since it should be roughly 15 seconds allowed max. Since the JPTL is seeing an increase in JPT, the engine controller will try to offset it by reducing RPM.
1:15: The Normal Lift Wet limit appears to have a combined time limit of 1.5 min. I would assume that once that limit is exceeded, the control law will control the engine to the next lower limit, which is Maximum Thrust (109 % RPM). Since JPT is increasing, the JPTL should control the RPM to decrease to maintain the max allowable JPT limit of 710 deg C.
For the rest, I donāt know why it decreases and jumps back again. I would assume it has something to do with the current logic implemented by Razbam (combined time limit A of 15 sec for Short Lift Wet limit). Maybe itās a mistake (counter that resets when it shouldnāt).
I know one thing, that when Iāve made continual passes on the range climbing out at full throttle without regarding JPT, the engine will degrade to the point that I can only make about 280 kts RTB. So, there is some sort of degradation going on if you donāt mind the temp.
Thank you for your analysis, but there are still things that donāt match up.
Why donāt I get Short Lift Wet first?
Since Normal Lift Wet has a limit of 1.5 minutes, why canāt it (116 RPM/780 JPT) be maintained after 57 seconds?
What I really donāt understand in the end is how such a computerized aircraft wonāt provide you with an indication on how much time you have left in the current power regime. This aircraft is packed with displays, flooding the pilot with all kind of useful and pointless information, but somehow McDonnell Douglas didnāt bother to add a thing on the HUD that says āHey, in 7 seconds you exceed the current rating. You donāt need to throttle down if you donāt want to but we will add it to the maintenance logā. The Power Margin indicator could have been such a thing, but it seems it is just a fancy RPM/JPT indicator, which there is already another one on the HUD and one on the front panel! I really donāt understand that piece, what is it good for? Is the RAZBAM implementation of the whole thing really correct?
Iām just interpreting what the engine is currently doingā¦ not what it SHOULD be doing. Big difference.
I donāt know how Razbam coded their DECS and JPTL. Typically for engines like that you have a bunch of control laws fighting each other to maintain adequate fuel scheduling and to respect engine limitations. There is no way in hell Razbam was able to gain access to 5000+ pages worth of wiring diagrams and logic SDDsā¦ so at that point I can only try to figure out how they implemented it. What Iād suggest is to record a video, note a couple of points (RPM & JPT) in relationship to time in an excel sheet and send that to Larry, Ron and CptSmiley directly so they can figure out if thereās something not working as intended. Itās highly possible. I canāt say Iāve seen many modern FADECs coded in DCS at all. Most use old HMUs (Hydro-mechenical units) as fuel flow governors, which is in a way a bit simpler to implement.
In a modern FADEC, you have lots and lots of different logics involved. Thousands of pages of documentation. Some of the engines also have a predictive model included (think: a second FADEC, yikes!) to determine what fuel scheduling you would need, which makes things even MORE complicated.
When you are conducting these tests in the air, what is the JPT? I think you may be getting longer time limits because depending on altitude and speed, that would lower the JPT at any given RPM versus sitting on the deck.
In watching many DCS streams on Twitch it sure seems like the Harrier is the new āgo toā ride for ground pounders now. Now that the A-10C has been out for YEARS Iām sure people are glad to have something new.
Like most of us who havenāt actually flown the thing, I havenāt a clue. But my intuition is that the DEC would not reduce power to protect the engine without a very clear and timely warning to the pilot. The temperature limits in the graph above are probably not trmperatures above which components begin to quickly degrade. They are temperature and time limits which if followed will allow the engine to provide a reliability and length of service that was promised to the customer. I seriously doubt that the DEC is going to start reducing the fuel on a guy whoās making an emergency return to the boat with a touch too much gas. The mechanic who downloads the exceedence might not be happy. But that is a completely different thing than an engine that starts tearing itself apart because a time/temp limit was exceded by two minutes.
Sorry my friend, but I signed an NDR last summer and have recused myself from commenting on it at all. Otherwise I would have been much more active in these discussions.
Pretty much anything I write from this point runs me down one road or another.
FYI, added a bit more meat to the Harrier guide, including Moving Map, radios & some other stuff. Still waiting for features like INS, mark points and waypoint input/edit, but itās getting there slowly but surely.
Your involvement is very good news for those of us with the Harrier module, even though you canāt talk to us about it!