Date: 201901
Local time of day: 1201-1800
Location: MEM.Airport, TN
Flight conditions: Marginal
Light: Daylight
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Initial Climb
Airspace: Class B MEM
ATC / advisory: Tower MEM
Second aircraft: Commercial Fixed Wing
Reporter organization: Government
Reporter function: Local
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control
Result: Air Traffic Control Issued New Clearance
Contributing factors: Airspace Structure; Chart Or Publication; Procedure
Primary problem: Procedure

Narrative: I was working Local Control during the outbound push. I was notified that there was a loss in separation between Aircraft X; RNAV to DLONG off Runway 36L; and Aircraft Y; vectored to a 010 heading off Runway 36R. The surface wind was 050 at 8 knots and there was a scattered layer of clouds at 3300 feet. This scattered layer was causing departing aircraft to be in the cloud layer prior to the turn on their RNAV departure. Due to the slight wind out of the east; I wanted to turn my departures to our standard off the field headings to get them in a turn sooner than an RNAV would turn them.Due to stronger wind at altitude; Aircraft Y drifted 4 degrees west of the 010 heading assigned. This caused a 60% separation loss and an RAE (Risk Analysis Event). What I was unaware of (and all of my coworkers whom I have asked); is that an aircraft on the RNAV SID Runway 36L flies due north to a 360 heading for about 5.5 miles prior to making a more westerly turn. I learned this after researching this event. Our training materials reflect that a gate receives a 350 heading off the field and our RNAV departure gate chart shows a radial of 355 to the JTEEE RNAV departure gate. Due to these materials; I thought the JTEEE RNAV departure flew 360 for about 1 mile and then turned left to at least 355 heading. This would be similar to our other RNAV departures that fly 360 for about 1 mile prior to turning towards the RNAV departure gate. Nowhere in our training materials does it state we cannot RNAV SID from Runway 36L and have a departure on a 010 heading off Runway 36R.When first told of the loss event; I did not understand why the loss was so much with a 4 degree drift. As I thought between a RNAV Runway 36R SID; on a 355 heading; and an aircraft on a 010 heading; the 4 degree drift would cause a 27% loss. This is a serious issue we have discovered. Other than wind; there are more reasons we use the 010 heading off the east side of the field when conducting simultaneous departures. As I said before; our ELVIS SIDs for jets are given a standard heading of 010 off the field. Also; sometimes an aircraft on a SELPH or CRSON RNAV departure tells the Local Controller they request a heading off the field instead of the RNAV to which the Local Controller would use a 010 heading. There needs to be training and guidance given that indicates we do not have 15 degree standard separation between an RNAV to DLONG departure and a 010 heading. This is at a minimum. Preferably; there needs to be a permanent adjustment made to the standard eastbound headings OR an adjustment made to the RNAV SID that will turn the aircraft to at least a 355 heading at 1 mile so that standard separation can be achieved between an RNAV SID and a standardized heading.




Synopsis: MEM Local Controller reported that a procedure for simultaneous departures off of parallel runways does not ensure the required degrees divergence for legal separation.