Why is GPS required in case of a SAARU failure on the 777?

[TheBusFlyer]

Hello everyone,

One of our flights got dispatched with a SAARU failure. MEL says at least one GPS must be serviceable and flight can not transit through known jamming areas. Why is this the case?

I presume GPS is being used in some way or another by both ADIRU and SAARU…and by having it inop or jammed would result in a loss of navigation information /integrity?

Any input would be appreciated.

Thanks

[TURIN]

If you lose the remaining ADIRU then GPS is your only primary nav aid is my guess.
GPS jamming and spoofing is a known problem that often causes at least one GPS to become inop for the remainder of the flight even when the aircraft has exited the jamming area.

[Amadis of Gaul]

Originally Posted by TheBusFlyer

Hello everyone,

One of our flights got dispatched with a SAARU failure. MEL says at least one GPS must be serviceable and flight can not transit through known jamming areas. Why is this the case?

Because why not?

[Noknoipobin]

In my view, since the ADIRU will no longer have the SAIRU as a backup, what would happen if the ADIRU failed during flight? What would the FMS use to update the aircraft’s position?There must be at least one GPS receiver available.As for NAVAIDs along the route, their availability may not be reliable.And if the aircraft enters a jamming or spoofing area, having GPS would be of no use anyway. That is why flight operations are not permitted in such areas.

[tdracer]

One fundamental concept of the MEL is that dispatch won't leave you one fault away from a Hazardous or potentially Catastrophic situation.

As others have noted, the ADIRU and SAARU back each other up - if one is failed, then the potential backup for navigation becomes the GPS.

[STBYRUD]

Well no, the SAARU is not a backup for the navigation function of the ADIRU - it only handles attitude and air data, hence the name. Without the ADIRU, GPS supplies position to the FMC, so you can at least still fly your way home using HDG SEL. I suppose its just a general precaution - you already lost a backup for your attitude and air data, you shouldn't also lose groundspeed, track and position while you're attempting to fly yourself home on the ISFD.

[TURIN]

The SAARU has its own solid state gyros and accelerometers built in for continued inertial reference.

[STBYRUD]

Yes, it has laser ring gyros as well as accelerometers, but so does the ISFD. The SAARU can only do three minutes of backup nav to keep the more advanced autopilot modes (APP, LOC, FPA, TRK SEL/HOLD...) available to allow for a graceful failure by supplying track, GS, cross and along track acceleration to the AFDS. Afterwards it does attitude and air data only - FMC position will be from the GPS only.

If the ADIRU data goes invalid, the SAARU does not follow the ADIRU outputs. The SAARU continues to calculate backup navigation data independent of the ADIRU. The ring laser gyros do not supply sufficiently accurate data for autopilot operation for extended periods of time, therefore, the backup navigation function goes invalid after 3 minutes.

Ref AMM SDS 34-20-00

[Noknoipobin]

Sorry ,I may not be communicating clearly in English; it could be a language issue on my part. What I am trying to say is that, in an aircraft, no system should fail to the point where almost nothing is left—there must always be something to back it up.(Navigation …) I think you understand what I mean.

In other words, when a system fails, there should be another system that can perform the same or a similar function. For example, when the ADIRU fails, there is the SAARU that can still provide limited functionality. But if there is no SAARU, and the ADIRU fails, then there should be another system to provide backup,(Navigation ,Initial&air data ….not back ADIRU up directly )such as GPS.

If I explain it in simple terms, I would call this a “backup.” However, you might say that GPS is not actually a backup for the ADIRU.
For example, if the light bulb in your house fails, but you have a flashlight, that is what I am trying to convey.
Another example: if you are a police officer and you have a gun, you also carry a knife as a backup. The knife cannot replace the gun or perform the same function as the gun, but it is still there as an alternative. That is what I am trying to say.

Or I have to say “redundancy “ instead of “back up”.