OPINION OF THE COURT
GARTH, Circuit Judge:On essentially undisputed facts, the parties to this case dispute whether the Trustees’ of the Amalgamated Pension Fund (“the Fund”) interpretation of the Plan document to deny disability benefits to Edith *289Gaines is a permissible one. The district court found the Trustees’ interpretation to be arbitrary and capricious and directed payment of benefits to Gaines. Because we believe that the Trustees’ interpretation of the Plan is a reasonable one, and is therefore neither arbitrary nor capricious, we reverse the judgment of the district court.
I.
The Plan provision in question requires for eligibility, that a disability claimant:
(b) Has been Totally and Permanently Disabled, as defined in Section 2.11 hereof, such disability has continued for a period of at least 18 consecutive weeks, and is eligible to receive disability insurance benefits under Title II of the Federal Social Security Act within nine months of last date of Covered Employment.
Section 4.3(b). The Trustees interpret this provision to require for Plan eligibility that a claimant actually becomes eligible to receive benefits within nine months of termination of employment.2 Under the Trustees’ view, only those found to have become disabled within three months of termination are in fact eligible; the Trustees having interpreted the nine-month provision of the Plan to take into account the six-month waiting period prescribed by Social Security.
Gaines, on the other hand, interprets this provision to require only that the Social Security Administration find that the onset of the claimant’s disability, rather than the eligibility to receive Social Security benefits, occurred within nine months of termination. Since a claimant is not eligible to receive Social Security Disability benefits until six months after the date of onset of disability, this difference between the two interpretations is significant.
At trial, the parties stipulated that Gaines was a covered employee, had served the required number of years, and last worked on August 4, 1978. An Administrative Law Judge’s report3 found that Gaines’ disability commenced on April 23, 1979 (eight months and nineteen days after her termination). According to the April 23, 1979 date of onset of disability, Gaines’ Social Security benefits began in October, 1979, 14 months after termination, or five months beyond the period of time which would qualify Gaines under the Trustees’ interpretation of the Plan.
II.
Judicial review of interpretation of a pension plan document by the Trustees charged with administering the Plan is limited to whether the Trustees’ interpretation is arbitrary and capricious. Wolf v. National Shopmen Pension Fund, 728 F.2d 182, 187 (3d Cir.1984); Rosen v. Hotel and Restaurant Employees and Bartenders Union, 637 F.2d 592, 596 n. 5 (3d Cir.), cert. denied, 454 U.S. 898, 102 S.Ct. 398, 70 L.Ed.2d 213 (1981).
A plan interpretation should be upheld even if the court disagrees with it, so long as the interpretation is rationally related to a valid plan purpose and not contrary to the plain language of the plan. See Miles v. New York State Teamsters Conference Pension and Retirement Fund Employee Pension Benefit Plan, 698 F.2d 593, 601 (2d Cir.), cert. denied — U.S. —, 104 S.Ct. 105, 78 L.Ed.2d 108 (1983). Our review of the district court’s determination that the Trustees’ interpreta*290tion was arbitrary and capricious is plenary for application of legal principles. Findings of fact are measured by a clearly erroneous standard. The question of whether ambiguity exists is a question of law, subject to plenary review. See Fox v. U.S. Department of Housing, 680 F.2d 315, 319 (3d Cir.1982).
The terms of the Plan provide for eligibility for a disability pension if the claimant “is eligible to receive disability insurance benefits under Title II of the Federal Social Security Act within nine months of last date of Covered Employment.” Under the Social Security Act (SSA), 42 U.S.C. § 423, the time of entitlement for disability benefits is defined as “each month beginning with the first month after his waiting period____” Section 423(c)(2) defines this waiting period as “[t]he earliest period of five consecutive calendar months throughout which the individual with respect to whom such application is filed has been under a disabili-ty____” Thus, under the SSA, benefits do not begin, nor is a claimant entitled to benefits, until the sixth month after the onset of disability. Gaines’ eligibility certificate therefore bore a “date of entitlement” of “10/79”, or six months after April 23, 1979, the date of onset of disability-
The waiting period under the SSA is seen necessary to ensure that a claimant’s disability is in fact a permanent disability. See H.Rep. No. 1189, 84th Cong., 1st Sess. 6 (1955), U.S.Code Cong. & Admin.News 1956, p. 3877. The Trustees urge that the Social Security waiting period also serves to insure that the disability for which claim is made was the cause of the claimant’s termination of employment; that is, that a disability whose onset is closer in time to termination of employment is a disability more likely to be employment related.4
Given this background, we cannot agree with Gaines that the onset date must be the date which controls for qualification under the nine-month provision of the Plan. By its plain meaning, “eligible to receive” disability insurance benefits is clearly susceptible to an interpretation by the Trustees that the entitlement date for Social Security benefits must fall within the nine-month period prescribed by the Plan.5 Nor can we say that the Trustees’ interpretation of the Plan, which depends upon the “entitlement date” under the Social Security Act, is unrelated to any valid Plan purpose. We conclude that the interpretation by the Trustees is therefore neither arbi*291trary nor capricious, and, as a consequence, must be upheld.
III.
The judgment of the district court will be reversed, and the district court will be directed to enter judgment for the Amalgamated Insurance Fund.
. Section 2.1 provides in pertinent part:
(p) Total and Permanent Disability — Total disability by injury or disease which permanently prevents the employee from engaging in any occupation or employment for remuneration or profit as further evidenced by eligibility to receive disability insurance benefits under Title II of the Federal Social Security Act.
. A claimant need not actually receive the benefits within nine months to be eligible for Plan disability pension benefits. A determination by the Social Security Administration made even after nine months is sufficient, if the determination is that the claimant became eligible within the nine-month period after termination of employment.
. The report of the Administrative Law Judge was dated August 3, 1979.
. While the district court rejected testimony of this purpose as "self-serving," we are not bound by the Rule 52 "clearly erroneous” standard in our review of potential justification for the Trustee’s reading of the Plan. The Trustee’s subjective intent may be a question of fact; nevertheless, whether the Trustee’s interpretation is rationally related to valid Plan objectives is a legal question.
. Our research has not disclosed any other case interpreting such an eligibility clause. Music v. Western Conference of Teamsters Pension Fund, 712 F.2d 413 (9th Cir.1983) dealt with a similar interpretation of a plan to make eligibility depend on the date of entitlement after the completion of the waiting period. Music held that it was arbitrary and capricious to make computation of Music’s pension subject to a different plan in effect at the completion of the waiting period rather than the plan in effect at the time of termination of employment. The Music court did not decide whether eligibility under a plan might reasonably hinge on an entitlement date within nine months of termination of employment; in fact, in the Music case, benefits under the plan did not commence until after the waiting period expired.
The dissent, relying on the somewhat circular standard that "A rational and reasonable interpretation of a plan may still be arbitrary and capricious if contrary to the plain meaning of the plan," (Dennard v. Richards Group, Inc., 681 F.2d 306, 314 (5th Cir.1982)), would find that the plain meaning of the term "eligible” must necessarily be different from the meaning of the term "entitled.” Since "eligible" is not defined in the Plan, the dissent looks to the statutory definitions under the Social Security Act, 42 U.S.C. § 423(a)(1) to find this contrary "plain meaning.” However, the Social Security Act does not define “eligible,” but merely defines "entitled," thereby lending no substance to the distinction sought to be drawn by the dissent. We also observe that Websters Third New International Dictionary defines "eligible” in terms of "entitled”: "1. fitted or qualified to be chosen or used: entitled to something." (emphasis added).