Embraer Phenom 300 yaw damper fail due to loss of GPS signal [pdf]
faa.gov
faa.gov
Here's the problem. You have an Attitude and Heading Reference System (AHRS) with three rate gyros and three accelerometers, plus a magnetometer as a compass. From this information, you want to get aircraft orientation and heading. Integrating the rate gyros gives orientation, but because you're integrating a rate, there's cumulative error over time. The accelerometers give you a down reference. Not always a good one. Some AHRS units will lose their down reference if you fly in a circle for a while, and the accelerometers see a consistent but wrong "down" direction from centrifugal force.
So AHRS systems are prone to cumulative error accumulation. You also have a GPS system, which is prone to short-term noise but does not accumulate cumulative error. GPS is position only; there's no orientation info from GPS. (There are some multiple-antenna GPS systems that get orientation, but those are rare) So AHRS/GPS systems try to combine the two using various filters. Those filters embody assumptions about the error properties of each system. Fusion of the two sources provides position information, with the GPS fixes being augmented with short-term info from the AHRS. However, it's possible for GPS position to "jump" due to radio propagation problems. You see this on smartphones all the time. It's not as bad for aircraft, which usually have a clear view of the sky. (It's much harder for ground vehicles, which can't always see enough GPS satellites. We had a lot of trouble with this in 2005.)
The important outputs are attitude (pitch, roll, and yaw) which drive the artificial horizon ball (or, today, a graphic which looks like one). There's also heading, which drives the compass. The attitude outputs also drive the aircraft's stability control systems. The AHRS data can be used raw, or combined with GPS data to get not only attitude but position.
Usually, the AHRS outputs are used without GPS info to drive the aircraft systems that just need attitude. But the fused AHRS/GPS data is usually better, and dealing with inconstencies between the unaugmented AHRS info and the fused data is a pain. So there's a temptation to use the fused data for everything. Embraer apparently did this.
There's an argument for source integration. Air France 447 crashed because they lost airspeed and altitude data when the pitot tubes and static ports froze up. An integrated AHRS/GPS system would have given them accurate info on vertical speed and altitude even without air sensors. So integration isn't fundamentally a bad idea. If you integrate sources, though, you have to deal with sensor conflicts. That's a hard problem. Filters alone will not do it.
The integration used by Embraer (who did the avionics, by the way?) apparently doesn't handle GPS failures properly. The AHRS is still there, and that's all you need for aircraft stability. But if the fused outputs are used for stability augmentation, stall warning, and such, apparently they can fail when the GPS data does.
Garmin
These jets cannot fly without GPS.
When they (the military) knocked out GPS intentionally around China Lake NAS a few months back (for testing aircraft in GPS denied environments) -- all Embraers were told to avoid the area:
THIS NOTAM APPLIES TO ALL AIRCRAFT RELYING ON GPS. ADDITIONALLY, DUE TO GPS INTERFERENCE IMPACTS POTENTIALLY AFFECTING EMBRAER PHENOM 300 AIRCRAFT FLIGHT STABILITY CONTROLS, FAA RECOMMENDS EMB PHENOM PILOTS AVOID THE ABOVE TESTING AREA AND CLOSELY MONITOR FLIGHT CONTROL SYSTEMS DUE TO POTENTIAL LOSS OF GPS SIGNAL.
https://www.faasafety.gov/files/notices/2016/Jun/CHLK_16-08_...
My interest was more to the weapons system testing itself.
No need for FUD. Loss of Yaw Damper is not something that would endanger an aircraft
(the fact that it's a smaller aircraft might imply in having some certification differences as well)
That doesn't mean the plane can't fly. But it does mean that the pilot is going to have to fly it differently to avoid dutch roll, possibly flying at a lower altitude.
Keep in mind that this is aviation, which is very risk averse (by necessity). Even minor issues are treated as significant.
Loss of attitude info is a serious business, in the worst case the pilot may be left to fly only "raw data" and using the backup "steam gauge" instruments, because the displays and automation will stop working. Such event is no cake on a big jet, even during daylight, perfect weather and with the crew perfectly current and capable to fly like that.
Murphy will have it that it happens during night, in a storm and most crews today are trained to rely on the automation and rarely getting to hand fly, even less raw data, without any of the computers. The result will be another AF443 ...
And it's AF447
Damn.
Folks on the forums suspect the root cause is related to the Garmin GRS 77/GMU 44 AHRS part of the Garmin Prodigy G3000 package.[2,3]
0. https://www.faasafety.gov/files/notices/2016/Jun/CHLK_16-08_...
1. http://www.ainonline.com/aviation-news/business-aviation/201...
2. http://static.garmin.com/pumac/778_InstallationManual.pdf
3. http://static.garmin.com/pumac/1152_GRS77_GMU44Troubleshooti...
Modern planes aren't flown directly by people. They're flown by computers, with people using the controls in the cockpit to tell the computer what they want the plane to do (there is no direct link between the stick/yoke and the flight surfaces). The computer integrates all the information at its disposal and determines what movements of the flight control systems are necessary to meet the pilot's commands (this is known as fly by wire). In this case, it seems that computer is _really_ dependant on GPS.
Almost true and oft-repeated. There are still means to somewhat control most FBW aircraft in the unlikely event all electrical control is lost; A320 can pitch/yaw with mechanical linkage to stabilizer trim and the rudder, for example (but, to your point, neither involve the sidestick). The flight is very likely headed for an unpleasant end but the intent is to buy time to restore computer control. Landing with trim, rudder, and differential thrust alone is possible (some have landed with less), but it's a big ask for a pilot accustomed to automation.
That scenario aside, when a whole lot of things fail an Airbus can also go into a mode called Direct Law, which basically tells the computer to take a hike and puts the pilot in direct control of surfaces proportional to airspeed (and puts structural integrity in jeopardy, because the computer isn't helping to obey design limits). The aircraft is extremely sensitive in this configuration.
(If you saw Sully recently, one of the reasons he quickly started the APU was to prevent the aircraft from leaving the relative safety of Normal Law in the absence of electrical power. This was described in the report, if I recall, and was one of the more critical decisions made.)
Boeing is entirely different in FBW philosophy from Airbus. A coarse summary might be that Airbus drivers are negotiating with a computer while Boeing drivers are generally flying the airplane. Both have merits and drawbacks and vocal camps.
Yeah, that's why I was specific about the stick/yoke connection. Though, after some research, it appears I was wrong about that as well (as least in the case of Embraer). In the 190, the ailerons are, effectively, directly controlled (there is a hydraulic system in line, but it is directly actuated by cables from the yoke). I was unable to find any conclusive details about the Phenom 300 after a cursory search, but it appears it is "fully" FBW.
> Boeing is entirely different in FBW philosophy from Airbus. A coarse summary might be that Airbus drivers are negotiating with a computer while Boeing drivers are generally flying the airplane. Both have merits and drawbacks and vocal camps.
Yeah, and both sides can point to major incidents that wouldn't have happened in the other cockpit.
What's a good example in the "negotiate with the computer" camp? Perhaps it's merely a tendency on my part, but I don't think I have run across any accident which militates as strongly in favor of Airbus's design as AF 447 seems to militate against it.
The references I've seen to advantages tend to be more subtle than the AF447 tragedy. Things like the Hudson river landing where in that strange situation where you have no power the workload was diminished by having the computer keep the airplane flying most efficiently and thus be able to reach the river instead of crashing by losing airspeed.
It also makes sense the advantages are less obvious as the flight envelope protection actively prohibits dangerous situations and you can't report on non-events. The stats also show Boeing and Airbus airplanes pretty evenly matched on safety.
The contributing factors were loss of outside air temperature and airspeed sensors because of icing, a dark moonless night above the ocean, and flying/falling outside of the operating range of the stall horn (when they pitched the nose down, they started recovering from the stall, airspeed went up, and the stall horn activated. when they pitched up, they went back outside of the operating range, and the horn muted).
A big contributing factor as well was that in an airbus, in normal operating conditions, pulling the stick all the way back will configure the aircraft for the best rate of climb (which can be different from just putting the elevator in full climb conditions). Due to the pitot tubes being iced up, the flight computers were in Direct mode (where movement of the sidestick is directly translated in movement of the flight controls, without computer intervention), which didn't help.
Almost that, they were in Alternate Law
1: https://en.wikipedia.org/wiki/American_Airlines_Flight_587
Your perspective will influence how you classify the accidents. Proponents of full FBW will argue AF 447 happened exactly because part of the FBW system was disabled.
My understanding is the GPS and Inertial Measuring Unit (IMU) are combined. If there is a loss of GPS, the Autopilot & Yaw Dampener considers the IMU from that channel unavailable.
Only the newer Embraer 450 & Embraer 500 models are FBW aircraft. They also have side-stick controls.
Scenes in movies of pilots physically struggling with the plane are not as unrealistic as they may seem.
http://www.vanityfair.com/news/business/2014/10/air-france-f...
Would you mind taking a moment to spell out what these statements mean to a non-pilot?
(For example, I expect that Airbus pilots still operate controls which affect the movement of the airplane; they aren't typing parameters into a computer. And I expect Boeing pilot's actions are still mediated by a computer.)
On all FBW systems there's a computer in between the pilot's appendages and the wiggly parts built into the airplane that control its orientation (roll, pitch, yaw, what have you).
Airbus systems position the pilot as a suggestive force. At any given time the computer has its idea of what's happening and the pilot influences this calculus within safety tolerances for various bad things that happen to airplanes. One example is a stall, which results from a variety of factors including excessive pitch of the wing against the oncoming air (called angle of attack -- think about tilting your hand back when holding it out of the car and it dropping back on you. that's a stall). Airbus computers typically prevent the pilot from introducing the situation by dampening, modifying, or outright ignoring control input to avoid critical angles of attack. That's one example.
Basically, without going alternate law, the Airbus computer will work to make it very hard (not impossible, obviously) to apply control input that would place the flight in jeopardy.
Boeing would allow the input and mostly permit the pilot to introduce the situation. That's the difference. They treat the pilot as the end of authority in command. That has compelling merits. So does Airbus. As JshWright said, both sides have incidents on the other they can point to as supporting data.
And it does work without GPS, as this link (which was posted in this page http://forums.jetcareers.com/threads/emb-300-phenom-yaw-damp... ) shows
What this is saying:
1) If you don't have a yaw damper, then you'll have a rough ride
2) because the autopilot will induce oscillation (dutch roll) after loss of GPS
I also think you misread the notice. The report is stating the loss of GPS apparently resulted in the subsequent loss of a number of other systems, including the yaw damper.