queryFlights(body: FlightBoardApiCallBody): Observable<any>{
    if (this.getFlightsURL){
      return this.queryFlightsHelper(body, this.getFlightsURL);
    } else {
      this.sls.getServices()
        .map(s => s[GET_FLIGTHS_SERVICE_NAME])
        .switchMap(url => {
          if (url != null){
            return this.queryFlightsHelper(body, url);
          } else {
            return Observable.empty();
          }
        });
    }
  }

private queryFlightsHelper(b: FlightBoardApiCallBody, url: string): Observable<any>{...}

this.flightSubscription = this.f.queryFlights(body).subscribe((res) => {...});